CASE STUDY 2 LAKE SEDIMENTS 1 Basics about lakes

Lakes are very complex natural systems that interact with the climate and their catchment areas The continuous sedimentation of minerals at the bottmon of lakes offer a great opportunity to study the recent climatic hystory of the earth The sedi-mentation rate is generally much higher than in oceans and lake sediments provides a continuous high-resolution archive of the past 20000 years There are three main sources of minerals in lake sediments (Fig 1) (1) detrital minerals from the catch-ment which are transported into the lake by rivers and streams (2) dust deposition from the atmosphere (loess and volcanic ash) and (3) minerals that from directly in the lake (so-called authigenic minerals) The sources (1) and (2) depend on the cli-mate and the geology of the catchment while the formation of authigenic minerals reflect the biogeochemical processes occurring in the lake These processes depend on the catchment and the climate as well though the relationship is very complex

+++ +

+

+ ++ +

++

+ ++ +++

++ ++

+

wind-blown (loess)

soil erosiondetritalminerals

flyash from volcanic eruptions pollution

water transport (rivers)

authigenic

Fe3+Fe2+

iron cycle

Fig 1 A schematic representation of the various sources of magnetic minerals in lake sediments Detrital magnetic minerals are transported by wind (loess) or by water (drainage system) in the catchment Occasionally layers of volcanic ash form after a large volcanic eruption Authigenic magnetic minerals are forming directly in the lake through the redox cycle of iron

1

To understand how a lake works it is necessary to introduce some basic concepts The water column is divided into the uppermost epilimnion and the underlying hypo-limnion The epilymnion is constantly mixed and is where photosynthesis occur (pho-tic zone) The photosynthetic activity of algae provides the primary production of organic matter in the lake This production is limited by the availability of elements that are necessary for the photosyntheses The limiting elements are generally phos-phor (P) nitrogen (N) or iron (Fe) Depending on the amount of primary production with respect to the total water volume a lake is said to be oligotrophic (low primary production) or eutrophic (high primary production) The intermediate situation is called mesotrophic The transition from oligotrophic to eutrophic conditions is called eutrophication you have probably heard this term in connection to the human indu-ced deterioration of many lakes In the hypolimnion organic compounds are broken down by bacteria and other aquatic organisms The water in the hypolimnion is stra-tified its temperature does almost not depend on the depth and the transport of dis-solved ions and gases occurs mainly by diffusion The temperature profile of the wa-ter column changes seasonally and so does the boundary between epilimnion and hypolimnion Major events of complete mixing of the entire water column may occur in spring andor in autumn especially in lakes that are covered by ice during the winter A lake is said to be amictic if the hypolimnion is never mixed meromictic if major mixing events occur only sporadically (for example after a storm) monomictic or dimictic if major mixing events occur once or twice a year respectively

The combination of primary production and water mixing has very profund effects on the biology and the chemistry of lakes and lake sediments Iron minerals participate in complex redox reactions in the water the sediment and the sedimentwater inter-face We give a brief description of these processes considering the example of a lake that forms after the retreat of a glacier Immeadiately after the retreat of the glacier the climate is still cold and windy the catchmet is not covered by vegetation and physical weathering dominates over chemical weathering As a consequence the water column is mixed very often (by strong winds and ice melting) and the supply of nutrients for the photosynthesis is low The lake is oligotrophic and sediments de-posited during this period contain predominantly detrital minerals from the catch-ment and atmospheric dust As the climate gets warmer during the Holocene che-mical weathering in the catchment releases nutrients that sustain primary production in the lake Water mixing is less frequent and degradation of organic matter occurs in the hypolymnion The respiration of organic matter consumes oxygen which is trans-ported into deep waters by diffusion and mixing Autigenic minerals are produced especially during summer If the climate becomes more warm and humid chemical erosion in the soils of the catchment is enhanced Rivers and streams contain more dissolved minerals that en-hance algal growth Algae may grow explosively in spring giving rise to so-called algal blooms The flux of orgainc matter to the deep water increases drastically and so does the degradation of this matter by bacteria and other organisms Oxygen is consumed at a high rate by the respiration of organic matter and becomes progres-sively depleted The water in the hypolimnion becomes anoxic and the thickness of

2

the epilimnion may decrease drastically to only few meters This process is called eutrophication During the last centrury eutrophication was often induced by the discharge of phosporous-rich seawage waters produced by human activities During eutrophication events life in lakes is restricted more and more to organisms that can survive in the absence of oxygen fishes and other animals die The perverse aspect of europhication is a positive feedback process anoxic waters contain more dissolved mi-nerals and are more dense This prevents the dense anoxic water from mixin with the epilimnion and effectively maintains eutrophication even after the nutrient input de-creases to values that are typical for a mesotrophic or oligotrophic lake Eutrophica-tion is easy to recognize in the lake sediments During oligotrophic conditions a com-munitiy of small animals lives on the sedimentwater interface and produces a mixing of the uppermost 15-20 cm of the sediment (a process called bioturbation) Biotruba-tion prevents the preservation of annual stratifications in the sediments When the deep waters become anoxic the biological community that produced bioturbation dies out As a consequence the annual sediment lamination is preserved The seaso-nal change of primary orgainc matter production generates an alternation of light and dark sediment layers called biochemical varves During this workshop you will measure sediment samples collected from lake Baldeg-gersee a small Swiss lake that became eutrophied during the last century because of intense land use in the catchment Baldeggersee (httpwwwbaldegger-hallwilersee chbaldeggerseeinformationenhtml) is located on the Swiss plateau in the northern of the Alps and was formed after the retreat of the Reuss glacier about 16000 years ago The lake has a surface of 252 km and drains a catchment area of 268 km The maximum water depth is 66 m and the mean residence time of the water in the lake is 42 years The lake is surronded by hills that protect it from the dominant wind direction and obstacolate wind-induced water mixing (meromixis) This fact makes the lake naturally prone to eutrophication as recorded by occasionally occurring packets of varved sediments during the Holocene A restoration program was started in 1983 to bring the lake to its natural conditions before the onset of eutrophication during the last century Since 1983 oxygen is pumped into the hipolymnium during summer Despite this high artificial input of oxygen the lake is surprisingly still anoxic However some early signs of a change have been detected with magnetic measurements 2 The iron cycle in lakes

The iron cycle in lakes is driven by the different redox conditions in the sediment or the water column The so-called oxic to anoxic transizion zone (OATZ) separates oxic waters or sediments from an underlying anoxic layer (Fig 2) Most chemical reac-tions that involve iron minerals occurs near the OATZ Above the OATZ the organic matter is degraded by the aerobic respiration

2 106 3 16 3 4 2 2 3 3 4 2(CH O) (NH ) (H PO ) 138O 106CO 16HNO H PO 122H O+ rarr + + +

3

To understand how a lake works it is necessary to introduce some basic concepts The water column is divided into the uppermost epilimnion and the underlying hypo-limnion The epilymnion is constantly mixed and is where photosynthesis occur (pho-tic zone) The photosynthetic activity of algae provides the primary production of organic matter in the lake This production is limited by the availability of elements that are necessary for the photosyntheses The limiting elements are generally phos-phor (P) nitrogen (N) or iron (Fe) Depending on the amount of primary production with respect to the total water volume a lake is said to be oligotrophic (low primary production) or eutrophic (high primary production) The intermediate situation is called mesotrophic The transition from oligotrophic to eutrophic conditions is called eutrophication you have probably heard this term in connection to the human indu-ced deterioration of many lakes In the hypolimnion organic compounds are broken down by bacteria and other aquatic organisms The water in the hypolimnion is stra-tified its temperature does almost not depend on the depth and the transport of dis-solved ions and gases occurs mainly by diffusion The temperature profile of the wa-ter column changes seasonally and so does the boundary between epilimnion and hypolimnion Major events of complete mixing of the entire water column may occur in spring andor in autumn especially in lakes that are covered by ice during the winter A lake is said to be amictic if the hypolimnion is never mixed meromictic if major mixing events occur only sporadically (for example after a storm) monomictic or dimictic if major mixing events occur once or twice a year respectively

The combination of primary production and water mixing has very profund effects on the biology and the chemistry of lakes and lake sediments Iron minerals participate in complex redox reactions in the water the sediment and the sedimentwater inter-face We give a brief description of these processes considering the example of a lake that forms after the retreat of a glacier Immeadiately after the retreat of the glacier the climate is still cold and windy the catchmet is not covered by vegetation and physical weathering dominates over chemical weathering As a consequence the water column is mixed very often (by strong winds and ice melting) and the supply of nutrients for the photosynthesis is low The lake is oligotrophic and sediments de-posited during this period contain predominantly detrital minerals from the catch-ment and atmospheric dust As the climate gets warmer during the Holocene che-mical weathering in the catchment releases nutrients that sustain primary production in the lake Water mixing is less frequent and degradation of organic matter occurs in the hypolymnion The respiration of organic matter consumes oxygen which is trans-ported into deep waters by diffusion and mixing Autigenic minerals are produced especially during summer If the climate becomes more warm and humid chemical erosion in the soils of the catchment is enhanced Rivers and streams contain more dissolved minerals that en-hance algal growth Algae may grow explosively in spring giving rise to so-called algal blooms The flux of orgainc matter to the deep water increases drastically and so does the degradation of this matter by bacteria and other organisms Oxygen is consumed at a high rate by the respiration of organic matter and becomes progres-sively depleted The water in the hypolimnion becomes anoxic and the thickness of

2

the epilimnion may decrease drastically to only few meters This process is called eutrophication During the last centrury eutrophication was often induced by the discharge of phosporous-rich seawage waters produced by human activities During eutrophication events life in lakes is restricted more and more to organisms that can survive in the absence of oxygen fishes and other animals die The perverse aspect of europhication is a positive feedback process anoxic waters contain more dissolved mi-nerals and are more dense This prevents the dense anoxic water from mixin with the epilimnion and effectively maintains eutrophication even after the nutrient input de-creases to values that are typical for a mesotrophic or oligotrophic lake Eutrophica-tion is easy to recognize in the lake sediments During oligotrophic conditions a com-munitiy of small animals lives on the sedimentwater interface and produces a mixing of the uppermost 15-20 cm of the sediment (a process called bioturbation) Biotruba-tion prevents the preservation of annual stratifications in the sediments When the deep waters become anoxic the biological community that produced bioturbation dies out As a consequence the annual sediment lamination is preserved The seaso-nal change of primary orgainc matter production generates an alternation of light and dark sediment layers called biochemical varves During this workshop you will measure sediment samples collected from lake Baldeg-gersee a small Swiss lake that became eutrophied during the last century because of intense land use in the catchment Baldeggersee (httpwwwbaldegger-hallwilersee chbaldeggerseeinformationenhtml) is located on the Swiss plateau in the northern of the Alps and was formed after the retreat of the Reuss glacier about 16000 years ago The lake has a surface of 252 km and drains a catchment area of 268 km The maximum water depth is 66 m and the mean residence time of the water in the lake is 42 years The lake is surronded by hills that protect it from the dominant wind direction and obstacolate wind-induced water mixing (meromixis) This fact makes the lake naturally prone to eutrophication as recorded by occasionally occurring packets of varved sediments during the Holocene A restoration program was started in 1983 to bring the lake to its natural conditions before the onset of eutrophication during the last century Since 1983 oxygen is pumped into the hipolymnium during summer Despite this high artificial input of oxygen the lake is surprisingly still anoxic However some early signs of a change have been detected with magnetic measurements 2 The iron cycle in lakes

The iron cycle in lakes is driven by the different redox conditions in the sediment or the water column The so-called oxic to anoxic transizion zone (OATZ) separates oxic waters or sediments from an underlying anoxic layer (Fig 2) Most chemical reac-tions that involve iron minerals occurs near the OATZ Above the OATZ the organic matter is degraded by the aerobic respiration

2 106 3 16 3 4 2 2 3 3 4 2(CH O) (NH ) (H PO ) 138O 106CO 16HNO H PO 122H O+ rarr + + +

3

the epilimnion may decrease drastically to only few meters This process is called eutrophication During the last centrury eutrophication was often induced by the discharge of phosporous-rich seawage waters produced by human activities During eutrophication events life in lakes is restricted more and more to organisms that can survive in the absence of oxygen fishes and other animals die The perverse aspect of europhication is a positive feedback process anoxic waters contain more dissolved mi-nerals and are more dense This prevents the dense anoxic water from mixin with the epilimnion and effectively maintains eutrophication even after the nutrient input de-creases to values that are typical for a mesotrophic or oligotrophic lake Eutrophica-tion is easy to recognize in the lake sediments During oligotrophic conditions a com-munitiy of small animals lives on the sedimentwater interface and produces a mixing of the uppermost 15-20 cm of the sediment (a process called bioturbation) Biotruba-tion prevents the preservation of annual stratifications in the sediments When the deep waters become anoxic the biological community that produced bioturbation dies out As a consequence the annual sediment lamination is preserved The seaso-nal change of primary orgainc matter production generates an alternation of light and dark sediment layers called biochemical varves During this workshop you will measure sediment samples collected from lake Baldeg-gersee a small Swiss lake that became eutrophied during the last century because of intense land use in the catchment Baldeggersee (httpwwwbaldegger-hallwilersee chbaldeggerseeinformationenhtml) is located on the Swiss plateau in the northern of the Alps and was formed after the retreat of the Reuss glacier about 16000 years ago The lake has a surface of 252 km and drains a catchment area of 268 km The maximum water depth is 66 m and the mean residence time of the water in the lake is 42 years The lake is surronded by hills that protect it from the dominant wind direction and obstacolate wind-induced water mixing (meromixis) This fact makes the lake naturally prone to eutrophication as recorded by occasionally occurring packets of varved sediments during the Holocene A restoration program was started in 1983 to bring the lake to its natural conditions before the onset of eutrophication during the last century Since 1983 oxygen is pumped into the hipolymnium during summer Despite this high artificial input of oxygen the lake is surprisingly still anoxic However some early signs of a change have been detected with magnetic measurements 2 The iron cycle in lakes

The iron cycle in lakes is driven by the different redox conditions in the sediment or the water column The so-called oxic to anoxic transizion zone (OATZ) separates oxic waters or sediments from an underlying anoxic layer (Fig 2) Most chemical reac-tions that involve iron minerals occurs near the OATZ Above the OATZ the organic matter is degraded by the aerobic respiration

2 106 3 16 3 4 2 2 3 3 4 2(CH O) (NH ) (H PO ) 138O 106CO 16HNO H PO 122H O+ rarr + + +

3

Fig 2 Schematic representation of the chemical evrironments (a) and bacterial habitats (b) in the upper marine sediment column [after P Hesse Evidence for bac-terial paleoecological origin of mineral magnetic cycles in oxic and sub-oxic Tasman Sea sediments Marine Geology 117 1-17 1994] A similar schema apply to lake se-diments Equant and elongated magnetosome-producing bacteria apparently prefer high- and low-oxygen concentrations respectively Low-cases letters indicate the oc-currence of different bacteria (a-c) laboratory cultures of magnetotactic bacteria (d-e) inferred bacterial concentration in marine sediments (f) laboratory culture of sul-fate-reducing freshwater strain

When oxygen is no longer available some bacteria use nitrate to oxidize organic mat-ter instead of oxygen

2 106 3 16 3 4 3 2 2 3 4 2(CH O) (NH ) (H PO ) 944HNO 106CO 552N H PO 1772H O+ rarr + + +

Near the OATZ reduction of iron is carried out by two groups of bacteria iron dissimilatory reducers and magnetotactic bacteria The chemical reaction is

2 106 3 16 3 4

2+2 3 3 4 2

(CH O) (NH ) (H PO ) 424FeOOH+848H

424Fe 106CO 16NH H PO 742H O

++ rarr

+ + + +

Iron dissimilatory reducers such as Shewanella induce the precipitation of very fine (10-100 nm) magnetite grains around their cells Magnetotactic bacteria grow chains

4

of magnetite crystals inside their cells Magntotactic strains produce almost perfect magnetite particles (called magnetosomes) with a specific size ( nm) and shape (Fig 3) They use these magnetite chains to orient along the Earth magnetic field and swim along straight lines Magnetosomes have unique magnetic properties that allow their identification with magntic measurements

100asymp

Fig 3 (a) The magnetotactic bacterium Aquaspirillum Magnetotacticum with a chain of magnetosomes (b) Electron micrograph of an intact chain of elongated magnetosomes and (c) electron migrograph of bullet-shaped magnetosomes both found in the sediments of Baldeggersee (Switzerland)

Below the OATZ the degradation of organic matter involves the reduction of sulfate 2 2

2 106 3 16 3 4 4 2 3 3 4 2(CH O) (NH ) (H PO ) 53SO 106CO 16NH 8S +H PO 106H Ominus minus+ rarr + + +

This reaction releases sulfphide ions that may react with existing iron minerals or with to form the iron sulphide greigite ( ) Some magnetotactic bacteria produce greigite magnetosomes The last step of organic degradation that occur under the most reducing conditions is fermentation

2Fe +3 4Fe S

In a oligotrophic lake the primary production of organic matter is not sufficiently high to sustain the entire chain of redox reactions describred above since all degra-dable organic matter is respired before iron andor sulphate reduction can take place The OATZ may therefore not exist or is located deep in the sediments As the availability of organic matter increases oxygen is consumed more rapidly than it can diffuse into the sediments and the OATZ raises to the watersediment interface The particular physical properties of the watersediment interface enhance the iron cycle The ferrous iron produced by the reduction of ferric iron diffuses out of the sediment into the water column where it oxidizes again Ferric iron precipitates as iron oxy-hydroxides (ferrihydrite or goetite FeOOH) that settle on the sediment where the iron is reduced again (Fig 3) The enhanced cycling of iron is accompained by the abundant production of autigenic magntite When the lake becomes eutrophic the OATZ eventually migrates into the water column

5

Fig 4 Conceptual diagram of the iron cycle when the OATZ coincides with the wa-tersediment interface Nor-mal arrows indicate chemical reactions (ox=oxidation red= reduction) double arrows the settling of particles and da-shed arrows the diffusion of ions Above the OATZ iron oxyhydroxides precipitate af-

ter the soluble ions that diffused from the sediment oxidizes to the insoluble These oxyhydroxides settle and may become reduced again in the sediment The

magnetic end product of the iron cycle in the sediment is magnetite eventually produced by magnetotactic bacteria Below the OATZ sulphide ions react with and iron sulphides form Later on if the lake becomes more oxygenated the OATZ eventually migrates down in the sediment column and iron sulphides are distroyed

2Fe +

3Fe +

3 4Fe O2Fe +

Fe3+ Fe2+

Fe2+FeOOH

Fe3O4

Fe2+ Fe3S4

water

sedimentOATZ

ox

red

sulphidicsediment

3 Characterization of lake sediments with magnetic measurements

The production and dissolution of authigenic magnetite is the main feature that control the magnetic properties of lake sediments in temperate climates Authigenic magnetite occurs in much smaller grain sizes (10-100 nm) than detritic magnetic mi-nerals This difference in grain size is reflected by the magnetic properties This workshop is intended to give you a brief (and necessarily incomplete) overview of the different magnetic measurements that can be used to characterize a lake sedi-ment Low-field susceptibility Low-field susceptibility measurements are fast and unexpensive and can be perfor-med on collected samples as well as directly on sediment cores Magnetic suscepti-bilty is sometimes difficult to interpret since all kinds of magnetism (diamagnetism paramagnetism and ferrimagnetism) are sensed

dia para ferχ χ χ χ= + + ri

Diamagnetism is very weak and is almost always negligible The paramagnetic su-sceptibility can be calculated from the slope of a hysteresis loop at high fields Frequency dependence of susceptibility Some susceptometers can measure the susceptibility at two different frequencies and and allow the calculation of the so-called frequency dependence of suscepti-bility

1f

2f

fdχ

6

1 2fd

1 2

( ) ( )100

( )log( )f f

f f fχ χ

χχ

minus=

1

2

85

with The frequency dependence is simply the relative difference between two susceptibilities measured at different frequencies expressed in percent This parame-ter is sensitive to the presence of very fine (suparparamagnetic) ferromagnetic parti-cles and is therefore particularly suitable for the identitfication of fine magnetite grains produced by dissimilatory iron reducing bacteria The theoretical range of values for is between 0 and 50 However a maximum value of 30 is observed in natural samples

1f flt

fdχ

minus04 minus02 02 04

minus2

2

minus04 minus02 02 04

minus10

minus05

minus05 minus04 minus03 minus02 minus01

minus02

00

02

χpara

s

rs

c

cr

M

M

H

H

(a)

(b)

(c) rsM

rsminusM

Fig 5 (a) Hysteresis loop of a lake sediment sample from Bal-deggersee Switzerland (b) The same hysteresis loop after sub-traction of the paramagnetic contribution (c) Backfield curve of the same sample All mea-surements were performed with a Micromag Vibrating Sample Magnetometer The hysteresis parameters deduced from the measurements are 04 and 455 mT The remanence ratio close to 05 suggests that the sample contains a large amount of single-domain grains Elec-trom microscope observations of this sample revealed the presen-ce of intact chains of magneto-somes (see Fig 3b) The back-field curve is not completely sa-turated at 300 mT indicating the presence of some high-coer-civity minerals

rs sM M =

cr c 1H H = crH =

7

Hysteresis parameters and the Day plot The measurement of hysteresis loops and so-called backfield curves provides useful information about the magnetic mineralogy and granulometry (Fig 5) The slope of the hysteresis loop at high fields originates from the paramagntic minerals The interpretation of the hysteresis loop is facilitated after the linear contribution of the paramagnetic minerals is subtracted by the so-called paramagnetic correction Most software provided with measuring instruments can perform the paramagnetic cor-rection automatically The corrected loop comprises the magnetic contribution of all ferrimagnetic grains The maximum magnetization reached by the hysteresis loop is called saturation magnetization it is the highest possible magnetization carried by all magnetic grains The intercept of the hysteresis loop with the magnetization axis is the saturation remanence it is the permanent magnetization of all grains in a zero field Superparamagnetic grains are too small to retain a permanent magne-tization and in this case An assemblage of randomly oriented single-domain grains has Very large (multidomain) grains are characterized by

The remanence ratio is a grain size indicator higher values of indicate the presence of single-domain grains such as magnetosomes The

coercivity is the field at which in a hysteresis loop The coercivity of remanence is the negative field that must be applied to cancel the saturation remanence acquired in a large positive field A combination of the ratios and

is used in the so-called Day plot (Fig 6) Mixtures of sigle-doman and super-paramagnetic or multidomain grains produce a characteristic grouping of the measu-rement along a mixing trend line in the Day plot Theoretical mixing models can be used under some circumstances to infer the proportion of sigle-doman grains in a sample

sM

rsM

r 0M =

s rs05 1M Mle lt

rM Ms

s

rs sM M

rM M

cH 0M =

crH

rs sM M

cr cH H

Fig 6 The so-called Day plot is an effective way to represent the hysteresis para-meters In this plot the effect of a binary mixture of single-domain (SD) and multido-main (MD) particle is shown (line) Open circles are lake sediments from Minnesota the dot is the sample used as example in Fig 5 [Modified from D Dunlop Theory and application of the Day plot 2 Applications to data for rocks sediments and soils Journal of Geophysical research 107 DOI 101292001JB000487 2002b]

8

Isothermal remanent magnetization (IRM) and magntization curves The IRM is the permanent magnetization acquired by an assemblage of ferrimagnetic grains after a (strong) magnetic field was applied Each magnetic grain is a miniature compass needle with a permanent dipole moment The minimum field required to re-verse the direction of such dipole is called the switching field Each magnetic grain has a switching field that depends on its size mineralogy and other physical pro-perties An assemblage of magnetic grains all with a different switching field changes its remanent magnetization when a magnetic field is applied The application of suc-cessively increasing fields is used to measure a so-called magnetization curve The derivative of such curve is the statistical distribution of switching fields also called coercivity distribution (Fig 7)

1 10 1000

80

120

40

D+EX

BS

Fig 7 (a) Distribtuion of switching fields measured on a lake sediment from Bal-deggersee (black curve) This distribution has been modelled with a liear combination of different model curves (coloured lines) each representing the magnetic contribution of a specific set of magnetic particles BS and BH are two groups of magnetosomes the difference between them is probably due to their shape D and EX are detrital par-ticles and extracellular magnetite which could not be modeled separately in this sample [redrawn from R Egli Characterization of individual rock magnetic compo-nents by analysis of remanence curves 1 Unmixing natural sediments Sudia Geo-physica et Geodaetica 48 391-446 2004] (b) Distribution of switching fields for de-trital extracellular and biogenic particles (same abbreviations as in (a)) inferred from the measurement of 122 sediments from different lakes [redrawn from R Egli Characterization of individual rock magnetic components by analysis of remanence curves 3 Bacterial magnetite and natural processes in lakes Physics and Chemistry of the Earth 29 869-884 2004] Minerals that can display switching fields above 300 mT are called high-coercivity minerals On the other hand magnetite and maghemite are low-coercivity minerals A semiquantitative parameter that indicates the relative amount of low coercivity minerals in a sample is the so-called S-ratio S (S stay for saturation) defined as

rs 300

rs

M IRMS

Mminus

=

where is the remanent magnetization acquired in a 300 mT field indicates that there are no high-coercivity minerals

300IRM 1S =

BH

H0

1

2

3

4

1 10 100

switching field mT

mag

net

ic c

on

trib

uti

on

BS

DEX

mag

net

ic c

on

trib

uti

on

switching field mT

(a) (b) BH

9

Anhysteretic remanent magnetization (ARM) and the King plot ARM is a particular kind of magntization acquired in an alternating field whose amplitude slowly decreases to zero If the field is perfectly symmetric the remanent magnetization of the sample after this treatement is zero (it is said that the sample has been AF demagnetized) However if a small constant bias field is added to the alternating field the sample acquires a magnetization called ARM This magnetization is proportional to the bias field and is typically much smaller than the saturation remanence An interesting property of the ARM is its extreme grain size sensitivity the highest ARMs are acquired by single-domain grains (which correspond to 30-100 nm grain size in magnetite) The efficiency of the ARM is evaluated by normalizing it with the saturation remanence or with the susceptibility Hence the parameters and (hereafter called ARM ratios) are sometimes used as rough grains size estimators High ARM ratios ( and ) are characteristic for fine-grained (single-domain) particles and magnetosome chains The so-called King plot combines the parameters χ and and a modified version is used to track the formation of authigenic magnetite in lake sediments (Fig 8)

DCH

DC rs( )ARM H M DC( )ARM H χ

3DC rs( ) 10 mAARM H M minus minusgt 1

DC( ) 5ARM H χ gt

DC( )ARM H χ

Fig 8 (a) The King plot is the susceptibility of the ferrimagnetic grains

is the so-called ARM susceptibility Dots are measurements of 01 microm magnetite particles at different concetrations Particles with a given grain size plot along lines whose slope is the ARM ratio Steeper lines correspond to smaller grain sizes (b) The ldquoOldfield plotrdquo is a modified version of the King plot where the ARM is normalized by the susceptibility (x-axis) and frequency dependence of suscep-tibility (y-axis) The shaded areas indicate typical values for detrital magnetite (A) and magnetosomes (B) [redrawn from F Oldfield Toward the discrimination of fine grained ferrimagnets by magnetic measurements in lake and near-shore marine sediments Journal of Geophysical Research 99 9045-9050 1994]

ferriχARM DCARM Hχ =

FDχ

10

Thermomagnetic curves Low temperature measurements can be used to identify mineral-specific transition temperatures as well as for the identification of superparamagnetic particles that become permanently magnetized below room temperature High temperature mea-surements of lake sediments rich in organic matter are difficult because the high surface-to-mass ratio of the authigenic minerals and the presence of organic matter accelerates the irreversible alteration of many magntic parameters above 200degC 4 Characterization of an eutrophication event

The goal of this case study is the characterization of the last eutrophication event of Baldeggersee which began in around 1900 For this purpose you will measure seven sediment samples labelled BAL-GR BA9-75 G101 G044 G010 AR23 and MV1 BAL-GR is a sample of sediment collected in a zone of the catchment area that was eroded by a small stream that ends in Baldeggersee This sample is expected to re-present the actual detrital input of the lake BA9-75 was collected near the bottom of the sediment core This sediment was deposited about 14000 years ago soon after the formation of the lake G101 G044 and G010 are sediment samples collected at depths of 101 44 and 10 cm respectively They represent three stages of the last eutrophication event G101 was deposited at the time of the first human settlements around the lake G044 marks the onset of eutrophication and G010 is totally anoxic Bot G044 and G010 display chemical varves (no bioturbation) AR23 is a sediment taken from the Aral sea (UzbekistanKazakstan) MV1 is a cultured sample of the magnetotactic bacterium Magetotactic Vibrium 1 and provides a reference for the magnetic properties of magnetosomes The results of low-field susceptibilty hysteresis and ARM measurememts are summarized in Tab 1 and Fig 9 Sample χ

3 1nm kgminus paraχ

3 1nm kgminus ferriχ

3 1nm kgminus ARMχ

3 1nm kgminus rs sM M cr cH H

BAL-GR BA9-75 G101 G044 G010 AR23 MV1

1195 510

1307 951 497

1580 158

326184594297207827

00

869326713654291753158

629 253 2140 3440 301 4250 3330

0208 0330 0237 0340 0249 0373 0482

3352 2158 1992 1857 2474 1743 1161

Tab 1 Summary of the measurements The low-field susceptibility χ has been mea-sured with a Kappabridge KLY-2 The paramagnetic susceptibility and the Day plot parameters and were obtained from hysteresis loops measured with a Micromag Vibrating Sample Magnetometer The ARM was imparted with a D-TECH D2000 AF system (maximum AF field 200 mT DC field 01 mT) and measured with a 2G cryogenic magnetometer The susceptibility of ARM was calculated by dividing the ARM magnetization by the DC field

paraχ

rs sM M cr cH H

ARMχ

11

00

02

04

06

BAL-GR

MV-1

AR23G044 BA9-75

G010G101

1 2 3 4 5 60

1

2

3

4

5

BAL-GR

AR23

MV-1 G044

G101

G010 BA9-75

0 20 40 60 80 100

(a) (b)

ferri3

AR

M3

nm kg

nm

kg

χ

χ

minus

minus

1

1

SD

MD

MM

H H

rss

cr c

Fig 9 (a) Representation of the measurements (open dots) on a Day plot The solid line is a model for the binary mixture of single-domain (SD) and multidomain (MD) magnetite (b) Representation of the measurements (open dots) on a King plot The solid line refers to the ARM ratio of 20 microm magnetite grains All lake sediments show a clear trend indicated by the dashed line to the right This line does not intersect the origin indicating a different composition of the samples Samples G010 BA9-75 and BAL-GR are also compatible with the line given by 20 microm magnetite The dashed line to the left is the calculated trend for magnetosomes [calculated after R Egli and W Lowrie Anhysteretic remanent magnetization of fine magnetic particles Journal of Geophysical Research 107 doi 1010292001JB000671 2002]

Figure 9 shows some interesting features that help us with the interpretation of the magnetic composition of the sediments The Day plot shows that the sediments are a mixture of relatively coarse magnetic grains probably of detrital origin and fine SD authigenic magnetite One endmember of this binary mixture are obviously the mag-netosomes in MV1 The other endmember is BAL-GR the material trasported into the lake from the catchment area The binary mixing trend is less steep than the mo-del curve in Fig 9a probably because BAL-GR does not contain only magnetite Mixtures of coarse magnetite maghemite or hematite will not necessary fall into the MD range predicted by the Day plot More detailed informations can be obtained from the King plot in Fig 9b Samples with the same magnetic composition but dif-ferent concentration of magnetic minerals would plot along lines that intercept the origin This is the case for G010 BA9-75 and BAL-GR BAL-GR and BA9-75 are expected to be similar in composition because during the early hystory of the lake the primary productivity was low and the production of authigenic minerals in the sediment is negligible The only difference between BAL-GR and BA9-75 is given by the concentration of ferrimagnetic minerals which is lower by a factor 3 in the lake sediment Why this difference Baldeggersee is a hard water lake and the sediments contain more than 50 calcium carbonate that precipitated from the water The lower concentration of magnetic minerals is simply produced by a dilution effect in

12

the nonmagnetic carbonate All lake sediment samples plot along a trend defined by a dashed line in Fig 9b This line does not intercept the origin demonstrating the variable composition of the lake sediments The second dashed line indicates the trend predicted for magnetosomes which is confirmed by the measurement of MV1 If we assume that lake sediments are binary mixtures of coarse detrital magnetite with a roughtly constant concentration and single-domain authigenic magnetite with a variable concentration the resulting mixing trend is a line whose slope is idetical to that of the single-domain grains The actual slope is slightly smaller than that pre-dicted by this model The reason of this discrepancy is that our simple model did not consider the production of extracellular magnetite which is in part superparamag-netic Superparamagntic grains are characterized by and lower the slope of the mixing trend predicted by the model

ARM 0χ =

One last feature to explain is the fact that despite G010 was sedimented during a period of very high primary productivity (around 1970 triggered by phosphor rich sewage waters) its properties are similar to those of BA9-75 (very low productivity) How is this possible To explain this apparent contradiction more detailed measure-ments are necessary The precise measurement of the distribution of switching field (Fig 7) allows the quantification of the different groups of magnetic grains that occur in the sediment With this method it was possible to dempnstrate that the autigenic magnetie was dissolved under anoxic conditions Fig 10 show the results of these measurements

Fig 10 Concentration of the two groups of magnetosomes BS and BH shown in Fig 7 as a function of depth for the uppermost 70 cm of sediments in Baldeggersee (Switzerland) The magnetic contributions have been normalized by the magnetization of the detrital plarticles Notice that the concentration of magnetosomes peaks at 44 cm and dorps to 13 of the original value at 10 cm This depth corresponds to the period of most severe eutrophication when the hypolimnion was completely anoxic The drop in the concentration of magneto-somes can be explained by ther dissolution The dissolution is more rapid for small particles therefore detrital magnetite was less affected The lake restoration program started in 1982 is clearly marked by an increase in the concentra-tion of magnetosomes The decreasing trend af-ter 1982 is due to the fact that the amout of oxygen pumped into the hypolimnium was de-creased [from R Egli Characterization of individual rock magnetic components by analy-sis of remanence curves 3 Bacterial magnetite and natural processes in lakes Physics and

Chemistry of the Earth 29 869-884 2004]

0 1

0

20

40

60

2

dept

h c

m

IRMBS IRMD+EX IRMBH IRMD+EX

1982

1942

13

5 Possible applications

We have seen how it is possible to characterize the iron cycle in lake sediments by quantifying the production of authigenic magnetite A lake is an ecosystem in equi-librion with the surrounding environment Its geochemical conditions depend on many factors that include the climate and local geological events in the catchment area These factors are more or less indirectly reflected in the properties of the sedi-ments Magnetic measurements of lake sediments have been used to roughly recon-struct the climate during the Holocene A fully quantitatie paleoclimatic reconstruc-tion needs to be based on a precise model of the interaction between lakes and en-vironments Recent studies pointed out that many naturals system and even the entire Earth are governed by nonlinear laws The profund consequences of this fact is that these system can display a cahotic behaviuor Nonliearity has been confirmed in the case of Baldeggersee where the answer of the lake to a nutrient input cycling was studied on the magnetic properties of the sediments (Fig 11)

50 100 time

Ncrit

min

Ncritmax

nutr

ient

s(b)

oxic

sub-

oxic

anoxic

AR

M r

atio

magnetization

oligotrophic

eutrophic

0

1

2

3

0 1 2 3

kA

RM

IR

M o

f BS

+B

H m

mA

minus1

IRMBS+BH IRMD+EX

1972

1982

1984

1999

1942

1910

1885

lt1700 (a)

Fig 11 (a) Cycling of Baldeggersee sediments through an eutrophication event The x-axis describes the total concentration of magnetosomes the y-axis shows a grain size parameter that becomes small if the particles are dissolved Before the industrializa-tion the lake was oligotrophic as shown by the low concentration of magnetosomes For a certain period the amount of magntosomes increased along with the primary production until reductive dissolution started in 1910 The concentration of magnto-somes was relatively constant until 1942 and decreases abruptly until a restoration program was started in 1982 Notice that recovering occurs along a different path as expected from a nonlinear system (b) A model for the magnetic properties of the sediments as a function of the nutrient imput simulated by a sinusoidal curve with some random spikes due to extreme local events (storms landslides) The model reproduces accurately the features of (a) [from the same source of Fig 10]

14

of magnetite crystals inside their cells Magntotactic strains produce almost perfect magnetite particles (called magnetosomes) with a specific size ( nm) and shape (Fig 3) They use these magnetite chains to orient along the Earth magnetic field and swim along straight lines Magnetosomes have unique magnetic properties that allow their identification with magntic measurements

100asymp

Fig 3 (a) The magnetotactic bacterium Aquaspirillum Magnetotacticum with a chain of magnetosomes (b) Electron micrograph of an intact chain of elongated magnetosomes and (c) electron migrograph of bullet-shaped magnetosomes both found in the sediments of Baldeggersee (Switzerland)

Below the OATZ the degradation of organic matter involves the reduction of sulfate 2 2

2 106 3 16 3 4 4 2 3 3 4 2(CH O) (NH ) (H PO ) 53SO 106CO 16NH 8S +H PO 106H Ominus minus+ rarr + + +

This reaction releases sulfphide ions that may react with existing iron minerals or with to form the iron sulphide greigite ( ) Some magnetotactic bacteria produce greigite magnetosomes The last step of organic degradation that occur under the most reducing conditions is fermentation

2Fe +3 4Fe S

In a oligotrophic lake the primary production of organic matter is not sufficiently high to sustain the entire chain of redox reactions describred above since all degra-dable organic matter is respired before iron andor sulphate reduction can take place The OATZ may therefore not exist or is located deep in the sediments As the availability of organic matter increases oxygen is consumed more rapidly than it can diffuse into the sediments and the OATZ raises to the watersediment interface The particular physical properties of the watersediment interface enhance the iron cycle The ferrous iron produced by the reduction of ferric iron diffuses out of the sediment into the water column where it oxidizes again Ferric iron precipitates as iron oxy-hydroxides (ferrihydrite or goetite FeOOH) that settle on the sediment where the iron is reduced again (Fig 3) The enhanced cycling of iron is accompained by the abundant production of autigenic magntite When the lake becomes eutrophic the OATZ eventually migrates into the water column

5

Fig 4 Conceptual diagram of the iron cycle when the OATZ coincides with the wa-tersediment interface Nor-mal arrows indicate chemical reactions (ox=oxidation red= reduction) double arrows the settling of particles and da-shed arrows the diffusion of ions Above the OATZ iron oxyhydroxides precipitate af-

ter the soluble ions that diffused from the sediment oxidizes to the insoluble These oxyhydroxides settle and may become reduced again in the sediment The

magnetic end product of the iron cycle in the sediment is magnetite eventually produced by magnetotactic bacteria Below the OATZ sulphide ions react with and iron sulphides form Later on if the lake becomes more oxygenated the OATZ eventually migrates down in the sediment column and iron sulphides are distroyed

2Fe +

3Fe +

3 4Fe O2Fe +

Fe3+ Fe2+

Fe2+FeOOH

Fe3O4

Fe2+ Fe3S4

water

sedimentOATZ

ox

red

sulphidicsediment

3 Characterization of lake sediments with magnetic measurements

The production and dissolution of authigenic magnetite is the main feature that control the magnetic properties of lake sediments in temperate climates Authigenic magnetite occurs in much smaller grain sizes (10-100 nm) than detritic magnetic mi-nerals This difference in grain size is reflected by the magnetic properties This workshop is intended to give you a brief (and necessarily incomplete) overview of the different magnetic measurements that can be used to characterize a lake sedi-ment Low-field susceptibility Low-field susceptibility measurements are fast and unexpensive and can be perfor-med on collected samples as well as directly on sediment cores Magnetic suscepti-bilty is sometimes difficult to interpret since all kinds of magnetism (diamagnetism paramagnetism and ferrimagnetism) are sensed

dia para ferχ χ χ χ= + + ri

Diamagnetism is very weak and is almost always negligible The paramagnetic su-sceptibility can be calculated from the slope of a hysteresis loop at high fields Frequency dependence of susceptibility Some susceptometers can measure the susceptibility at two different frequencies and and allow the calculation of the so-called frequency dependence of suscepti-bility

1f

2f

fdχ

6

1 2fd

1 2

( ) ( )100

( )log( )f f

f f fχ χ

χχ

minus=

1

2

85

with The frequency dependence is simply the relative difference between two susceptibilities measured at different frequencies expressed in percent This parame-ter is sensitive to the presence of very fine (suparparamagnetic) ferromagnetic parti-cles and is therefore particularly suitable for the identitfication of fine magnetite grains produced by dissimilatory iron reducing bacteria The theoretical range of values for is between 0 and 50 However a maximum value of 30 is observed in natural samples

1f flt

fdχ

minus04 minus02 02 04

minus2

2

minus04 minus02 02 04

minus10

minus05

minus05 minus04 minus03 minus02 minus01

minus02

00

02

χpara

s

rs

c

cr

M

M

H

H

(a)

(b)

(c) rsM

rsminusM

Fig 5 (a) Hysteresis loop of a lake sediment sample from Bal-deggersee Switzerland (b) The same hysteresis loop after sub-traction of the paramagnetic contribution (c) Backfield curve of the same sample All mea-surements were performed with a Micromag Vibrating Sample Magnetometer The hysteresis parameters deduced from the measurements are 04 and 455 mT The remanence ratio close to 05 suggests that the sample contains a large amount of single-domain grains Elec-trom microscope observations of this sample revealed the presen-ce of intact chains of magneto-somes (see Fig 3b) The back-field curve is not completely sa-turated at 300 mT indicating the presence of some high-coer-civity minerals

rs sM M =

cr c 1H H = crH =

7

Hysteresis parameters and the Day plot The measurement of hysteresis loops and so-called backfield curves provides useful information about the magnetic mineralogy and granulometry (Fig 5) The slope of the hysteresis loop at high fields originates from the paramagntic minerals The interpretation of the hysteresis loop is facilitated after the linear contribution of the paramagnetic minerals is subtracted by the so-called paramagnetic correction Most software provided with measuring instruments can perform the paramagnetic cor-rection automatically The corrected loop comprises the magnetic contribution of all ferrimagnetic grains The maximum magnetization reached by the hysteresis loop is called saturation magnetization it is the highest possible magnetization carried by all magnetic grains The intercept of the hysteresis loop with the magnetization axis is the saturation remanence it is the permanent magnetization of all grains in a zero field Superparamagnetic grains are too small to retain a permanent magne-tization and in this case An assemblage of randomly oriented single-domain grains has Very large (multidomain) grains are characterized by

The remanence ratio is a grain size indicator higher values of indicate the presence of single-domain grains such as magnetosomes The

coercivity is the field at which in a hysteresis loop The coercivity of remanence is the negative field that must be applied to cancel the saturation remanence acquired in a large positive field A combination of the ratios and

is used in the so-called Day plot (Fig 6) Mixtures of sigle-doman and super-paramagnetic or multidomain grains produce a characteristic grouping of the measu-rement along a mixing trend line in the Day plot Theoretical mixing models can be used under some circumstances to infer the proportion of sigle-doman grains in a sample

sM

rsM

r 0M =

s rs05 1M Mle lt

rM Ms

s

rs sM M

rM M

cH 0M =

crH

rs sM M

cr cH H

Fig 6 The so-called Day plot is an effective way to represent the hysteresis para-meters In this plot the effect of a binary mixture of single-domain (SD) and multido-main (MD) particle is shown (line) Open circles are lake sediments from Minnesota the dot is the sample used as example in Fig 5 [Modified from D Dunlop Theory and application of the Day plot 2 Applications to data for rocks sediments and soils Journal of Geophysical research 107 DOI 101292001JB000487 2002b]

8

Isothermal remanent magnetization (IRM) and magntization curves The IRM is the permanent magnetization acquired by an assemblage of ferrimagnetic grains after a (strong) magnetic field was applied Each magnetic grain is a miniature compass needle with a permanent dipole moment The minimum field required to re-verse the direction of such dipole is called the switching field Each magnetic grain has a switching field that depends on its size mineralogy and other physical pro-perties An assemblage of magnetic grains all with a different switching field changes its remanent magnetization when a magnetic field is applied The application of suc-cessively increasing fields is used to measure a so-called magnetization curve The derivative of such curve is the statistical distribution of switching fields also called coercivity distribution (Fig 7)

1 10 1000

80

120

40

D+EX

BS

Fig 7 (a) Distribtuion of switching fields measured on a lake sediment from Bal-deggersee (black curve) This distribution has been modelled with a liear combination of different model curves (coloured lines) each representing the magnetic contribution of a specific set of magnetic particles BS and BH are two groups of magnetosomes the difference between them is probably due to their shape D and EX are detrital par-ticles and extracellular magnetite which could not be modeled separately in this sample [redrawn from R Egli Characterization of individual rock magnetic compo-nents by analysis of remanence curves 1 Unmixing natural sediments Sudia Geo-physica et Geodaetica 48 391-446 2004] (b) Distribution of switching fields for de-trital extracellular and biogenic particles (same abbreviations as in (a)) inferred from the measurement of 122 sediments from different lakes [redrawn from R Egli Characterization of individual rock magnetic components by analysis of remanence curves 3 Bacterial magnetite and natural processes in lakes Physics and Chemistry of the Earth 29 869-884 2004] Minerals that can display switching fields above 300 mT are called high-coercivity minerals On the other hand magnetite and maghemite are low-coercivity minerals A semiquantitative parameter that indicates the relative amount of low coercivity minerals in a sample is the so-called S-ratio S (S stay for saturation) defined as

rs 300

rs

M IRMS

Mminus

=

where is the remanent magnetization acquired in a 300 mT field indicates that there are no high-coercivity minerals

300IRM 1S =

BH

H0

1

2

3

4

1 10 100

switching field mT

mag

net

ic c

on

trib

uti

on

BS

DEX

mag

net

ic c

on

trib

uti

on

switching field mT

(a) (b) BH

9

Anhysteretic remanent magnetization (ARM) and the King plot ARM is a particular kind of magntization acquired in an alternating field whose amplitude slowly decreases to zero If the field is perfectly symmetric the remanent magnetization of the sample after this treatement is zero (it is said that the sample has been AF demagnetized) However if a small constant bias field is added to the alternating field the sample acquires a magnetization called ARM This magnetization is proportional to the bias field and is typically much smaller than the saturation remanence An interesting property of the ARM is its extreme grain size sensitivity the highest ARMs are acquired by single-domain grains (which correspond to 30-100 nm grain size in magnetite) The efficiency of the ARM is evaluated by normalizing it with the saturation remanence or with the susceptibility Hence the parameters and (hereafter called ARM ratios) are sometimes used as rough grains size estimators High ARM ratios ( and ) are characteristic for fine-grained (single-domain) particles and magnetosome chains The so-called King plot combines the parameters χ and and a modified version is used to track the formation of authigenic magnetite in lake sediments (Fig 8)

DCH

DC rs( )ARM H M DC( )ARM H χ

3DC rs( ) 10 mAARM H M minus minusgt 1

DC( ) 5ARM H χ gt

DC( )ARM H χ

Fig 8 (a) The King plot is the susceptibility of the ferrimagnetic grains

is the so-called ARM susceptibility Dots are measurements of 01 microm magnetite particles at different concetrations Particles with a given grain size plot along lines whose slope is the ARM ratio Steeper lines correspond to smaller grain sizes (b) The ldquoOldfield plotrdquo is a modified version of the King plot where the ARM is normalized by the susceptibility (x-axis) and frequency dependence of suscep-tibility (y-axis) The shaded areas indicate typical values for detrital magnetite (A) and magnetosomes (B) [redrawn from F Oldfield Toward the discrimination of fine grained ferrimagnets by magnetic measurements in lake and near-shore marine sediments Journal of Geophysical Research 99 9045-9050 1994]

ferriχARM DCARM Hχ =

FDχ

10

Thermomagnetic curves Low temperature measurements can be used to identify mineral-specific transition temperatures as well as for the identification of superparamagnetic particles that become permanently magnetized below room temperature High temperature mea-surements of lake sediments rich in organic matter are difficult because the high surface-to-mass ratio of the authigenic minerals and the presence of organic matter accelerates the irreversible alteration of many magntic parameters above 200degC 4 Characterization of an eutrophication event

The goal of this case study is the characterization of the last eutrophication event of Baldeggersee which began in around 1900 For this purpose you will measure seven sediment samples labelled BAL-GR BA9-75 G101 G044 G010 AR23 and MV1 BAL-GR is a sample of sediment collected in a zone of the catchment area that was eroded by a small stream that ends in Baldeggersee This sample is expected to re-present the actual detrital input of the lake BA9-75 was collected near the bottom of the sediment core This sediment was deposited about 14000 years ago soon after the formation of the lake G101 G044 and G010 are sediment samples collected at depths of 101 44 and 10 cm respectively They represent three stages of the last eutrophication event G101 was deposited at the time of the first human settlements around the lake G044 marks the onset of eutrophication and G010 is totally anoxic Bot G044 and G010 display chemical varves (no bioturbation) AR23 is a sediment taken from the Aral sea (UzbekistanKazakstan) MV1 is a cultured sample of the magnetotactic bacterium Magetotactic Vibrium 1 and provides a reference for the magnetic properties of magnetosomes The results of low-field susceptibilty hysteresis and ARM measurememts are summarized in Tab 1 and Fig 9 Sample χ

3 1nm kgminus paraχ

3 1nm kgminus ferriχ

3 1nm kgminus ARMχ

3 1nm kgminus rs sM M cr cH H

BAL-GR BA9-75 G101 G044 G010 AR23 MV1

1195 510

1307 951 497

1580 158

326184594297207827

00

869326713654291753158

629 253 2140 3440 301 4250 3330

0208 0330 0237 0340 0249 0373 0482

3352 2158 1992 1857 2474 1743 1161

Tab 1 Summary of the measurements The low-field susceptibility χ has been mea-sured with a Kappabridge KLY-2 The paramagnetic susceptibility and the Day plot parameters and were obtained from hysteresis loops measured with a Micromag Vibrating Sample Magnetometer The ARM was imparted with a D-TECH D2000 AF system (maximum AF field 200 mT DC field 01 mT) and measured with a 2G cryogenic magnetometer The susceptibility of ARM was calculated by dividing the ARM magnetization by the DC field

paraχ

rs sM M cr cH H

ARMχ

11

00

02

04

06

BAL-GR

MV-1

AR23G044 BA9-75

G010G101

1 2 3 4 5 60

1

2

3

4

5

BAL-GR

AR23

MV-1 G044

G101

G010 BA9-75

0 20 40 60 80 100

(a) (b)

ferri3

AR

M3

nm kg

nm

kg

χ

χ

minus

minus

1

1

SD

MD

MM

H H

rss

cr c

Fig 9 (a) Representation of the measurements (open dots) on a Day plot The solid line is a model for the binary mixture of single-domain (SD) and multidomain (MD) magnetite (b) Representation of the measurements (open dots) on a King plot The solid line refers to the ARM ratio of 20 microm magnetite grains All lake sediments show a clear trend indicated by the dashed line to the right This line does not intersect the origin indicating a different composition of the samples Samples G010 BA9-75 and BAL-GR are also compatible with the line given by 20 microm magnetite The dashed line to the left is the calculated trend for magnetosomes [calculated after R Egli and W Lowrie Anhysteretic remanent magnetization of fine magnetic particles Journal of Geophysical Research 107 doi 1010292001JB000671 2002]

Figure 9 shows some interesting features that help us with the interpretation of the magnetic composition of the sediments The Day plot shows that the sediments are a mixture of relatively coarse magnetic grains probably of detrital origin and fine SD authigenic magnetite One endmember of this binary mixture are obviously the mag-netosomes in MV1 The other endmember is BAL-GR the material trasported into the lake from the catchment area The binary mixing trend is less steep than the mo-del curve in Fig 9a probably because BAL-GR does not contain only magnetite Mixtures of coarse magnetite maghemite or hematite will not necessary fall into the MD range predicted by the Day plot More detailed informations can be obtained from the King plot in Fig 9b Samples with the same magnetic composition but dif-ferent concentration of magnetic minerals would plot along lines that intercept the origin This is the case for G010 BA9-75 and BAL-GR BAL-GR and BA9-75 are expected to be similar in composition because during the early hystory of the lake the primary productivity was low and the production of authigenic minerals in the sediment is negligible The only difference between BAL-GR and BA9-75 is given by the concentration of ferrimagnetic minerals which is lower by a factor 3 in the lake sediment Why this difference Baldeggersee is a hard water lake and the sediments contain more than 50 calcium carbonate that precipitated from the water The lower concentration of magnetic minerals is simply produced by a dilution effect in

12

the nonmagnetic carbonate All lake sediment samples plot along a trend defined by a dashed line in Fig 9b This line does not intercept the origin demonstrating the variable composition of the lake sediments The second dashed line indicates the trend predicted for magnetosomes which is confirmed by the measurement of MV1 If we assume that lake sediments are binary mixtures of coarse detrital magnetite with a roughtly constant concentration and single-domain authigenic magnetite with a variable concentration the resulting mixing trend is a line whose slope is idetical to that of the single-domain grains The actual slope is slightly smaller than that pre-dicted by this model The reason of this discrepancy is that our simple model did not consider the production of extracellular magnetite which is in part superparamag-netic Superparamagntic grains are characterized by and lower the slope of the mixing trend predicted by the model

ARM 0χ =

One last feature to explain is the fact that despite G010 was sedimented during a period of very high primary productivity (around 1970 triggered by phosphor rich sewage waters) its properties are similar to those of BA9-75 (very low productivity) How is this possible To explain this apparent contradiction more detailed measure-ments are necessary The precise measurement of the distribution of switching field (Fig 7) allows the quantification of the different groups of magnetic grains that occur in the sediment With this method it was possible to dempnstrate that the autigenic magnetie was dissolved under anoxic conditions Fig 10 show the results of these measurements

Fig 10 Concentration of the two groups of magnetosomes BS and BH shown in Fig 7 as a function of depth for the uppermost 70 cm of sediments in Baldeggersee (Switzerland) The magnetic contributions have been normalized by the magnetization of the detrital plarticles Notice that the concentration of magnetosomes peaks at 44 cm and dorps to 13 of the original value at 10 cm This depth corresponds to the period of most severe eutrophication when the hypolimnion was completely anoxic The drop in the concentration of magneto-somes can be explained by ther dissolution The dissolution is more rapid for small particles therefore detrital magnetite was less affected The lake restoration program started in 1982 is clearly marked by an increase in the concentra-tion of magnetosomes The decreasing trend af-ter 1982 is due to the fact that the amout of oxygen pumped into the hypolimnium was de-creased [from R Egli Characterization of individual rock magnetic components by analy-sis of remanence curves 3 Bacterial magnetite and natural processes in lakes Physics and

Chemistry of the Earth 29 869-884 2004]

0 1

0

20

40

60

2

dept

h c

m

IRMBS IRMD+EX IRMBH IRMD+EX

1982

1942

13

5 Possible applications

We have seen how it is possible to characterize the iron cycle in lake sediments by quantifying the production of authigenic magnetite A lake is an ecosystem in equi-librion with the surrounding environment Its geochemical conditions depend on many factors that include the climate and local geological events in the catchment area These factors are more or less indirectly reflected in the properties of the sedi-ments Magnetic measurements of lake sediments have been used to roughly recon-struct the climate during the Holocene A fully quantitatie paleoclimatic reconstruc-tion needs to be based on a precise model of the interaction between lakes and en-vironments Recent studies pointed out that many naturals system and even the entire Earth are governed by nonlinear laws The profund consequences of this fact is that these system can display a cahotic behaviuor Nonliearity has been confirmed in the case of Baldeggersee where the answer of the lake to a nutrient input cycling was studied on the magnetic properties of the sediments (Fig 11)

50 100 time

Ncrit

min

Ncritmax

nutr

ient

s(b)

oxic

sub-

oxic

anoxic

AR

M r

atio

magnetization

oligotrophic

eutrophic

0

1

2

3

0 1 2 3

kA

RM

IR

M o

f BS

+B

H m

mA

minus1

IRMBS+BH IRMD+EX

1972

1982

1984

1999

1942

1910

1885

lt1700 (a)

Fig 11 (a) Cycling of Baldeggersee sediments through an eutrophication event The x-axis describes the total concentration of magnetosomes the y-axis shows a grain size parameter that becomes small if the particles are dissolved Before the industrializa-tion the lake was oligotrophic as shown by the low concentration of magnetosomes For a certain period the amount of magntosomes increased along with the primary production until reductive dissolution started in 1910 The concentration of magnto-somes was relatively constant until 1942 and decreases abruptly until a restoration program was started in 1982 Notice that recovering occurs along a different path as expected from a nonlinear system (b) A model for the magnetic properties of the sediments as a function of the nutrient imput simulated by a sinusoidal curve with some random spikes due to extreme local events (storms landslides) The model reproduces accurately the features of (a) [from the same source of Fig 10]

14

Fig 4 Conceptual diagram of the iron cycle when the OATZ coincides with the wa-tersediment interface Nor-mal arrows indicate chemical reactions (ox=oxidation red= reduction) double arrows the settling of particles and da-shed arrows the diffusion of ions Above the OATZ iron oxyhydroxides precipitate af-

ter the soluble ions that diffused from the sediment oxidizes to the insoluble These oxyhydroxides settle and may become reduced again in the sediment The

magnetic end product of the iron cycle in the sediment is magnetite eventually produced by magnetotactic bacteria Below the OATZ sulphide ions react with and iron sulphides form Later on if the lake becomes more oxygenated the OATZ eventually migrates down in the sediment column and iron sulphides are distroyed

2Fe +

3Fe +

3 4Fe O2Fe +

Fe3+ Fe2+

Fe2+FeOOH

Fe3O4

Fe2+ Fe3S4

water

sedimentOATZ

ox

red

sulphidicsediment

3 Characterization of lake sediments with magnetic measurements

The production and dissolution of authigenic magnetite is the main feature that control the magnetic properties of lake sediments in temperate climates Authigenic magnetite occurs in much smaller grain sizes (10-100 nm) than detritic magnetic mi-nerals This difference in grain size is reflected by the magnetic properties This workshop is intended to give you a brief (and necessarily incomplete) overview of the different magnetic measurements that can be used to characterize a lake sedi-ment Low-field susceptibility Low-field susceptibility measurements are fast and unexpensive and can be perfor-med on collected samples as well as directly on sediment cores Magnetic suscepti-bilty is sometimes difficult to interpret since all kinds of magnetism (diamagnetism paramagnetism and ferrimagnetism) are sensed

dia para ferχ χ χ χ= + + ri

Diamagnetism is very weak and is almost always negligible The paramagnetic su-sceptibility can be calculated from the slope of a hysteresis loop at high fields Frequency dependence of susceptibility Some susceptometers can measure the susceptibility at two different frequencies and and allow the calculation of the so-called frequency dependence of suscepti-bility

1f

2f

fdχ

6

1 2fd

1 2

( ) ( )100

( )log( )f f

f f fχ χ

χχ

minus=

1

2

85

with The frequency dependence is simply the relative difference between two susceptibilities measured at different frequencies expressed in percent This parame-ter is sensitive to the presence of very fine (suparparamagnetic) ferromagnetic parti-cles and is therefore particularly suitable for the identitfication of fine magnetite grains produced by dissimilatory iron reducing bacteria The theoretical range of values for is between 0 and 50 However a maximum value of 30 is observed in natural samples

1f flt

fdχ

minus04 minus02 02 04

minus2

2

minus04 minus02 02 04

minus10

minus05

minus05 minus04 minus03 minus02 minus01

minus02

00

02

χpara

s

rs

c

cr

M

M

H

H

(a)

(b)

(c) rsM

rsminusM

Fig 5 (a) Hysteresis loop of a lake sediment sample from Bal-deggersee Switzerland (b) The same hysteresis loop after sub-traction of the paramagnetic contribution (c) Backfield curve of the same sample All mea-surements were performed with a Micromag Vibrating Sample Magnetometer The hysteresis parameters deduced from the measurements are 04 and 455 mT The remanence ratio close to 05 suggests that the sample contains a large amount of single-domain grains Elec-trom microscope observations of this sample revealed the presen-ce of intact chains of magneto-somes (see Fig 3b) The back-field curve is not completely sa-turated at 300 mT indicating the presence of some high-coer-civity minerals

rs sM M =

cr c 1H H = crH =

7

Hysteresis parameters and the Day plot The measurement of hysteresis loops and so-called backfield curves provides useful information about the magnetic mineralogy and granulometry (Fig 5) The slope of the hysteresis loop at high fields originates from the paramagntic minerals The interpretation of the hysteresis loop is facilitated after the linear contribution of the paramagnetic minerals is subtracted by the so-called paramagnetic correction Most software provided with measuring instruments can perform the paramagnetic cor-rection automatically The corrected loop comprises the magnetic contribution of all ferrimagnetic grains The maximum magnetization reached by the hysteresis loop is called saturation magnetization it is the highest possible magnetization carried by all magnetic grains The intercept of the hysteresis loop with the magnetization axis is the saturation remanence it is the permanent magnetization of all grains in a zero field Superparamagnetic grains are too small to retain a permanent magne-tization and in this case An assemblage of randomly oriented single-domain grains has Very large (multidomain) grains are characterized by

The remanence ratio is a grain size indicator higher values of indicate the presence of single-domain grains such as magnetosomes The

coercivity is the field at which in a hysteresis loop The coercivity of remanence is the negative field that must be applied to cancel the saturation remanence acquired in a large positive field A combination of the ratios and

is used in the so-called Day plot (Fig 6) Mixtures of sigle-doman and super-paramagnetic or multidomain grains produce a characteristic grouping of the measu-rement along a mixing trend line in the Day plot Theoretical mixing models can be used under some circumstances to infer the proportion of sigle-doman grains in a sample

sM

rsM

r 0M =

s rs05 1M Mle lt

rM Ms

s

rs sM M

rM M

cH 0M =

crH

rs sM M

cr cH H

Fig 6 The so-called Day plot is an effective way to represent the hysteresis para-meters In this plot the effect of a binary mixture of single-domain (SD) and multido-main (MD) particle is shown (line) Open circles are lake sediments from Minnesota the dot is the sample used as example in Fig 5 [Modified from D Dunlop Theory and application of the Day plot 2 Applications to data for rocks sediments and soils Journal of Geophysical research 107 DOI 101292001JB000487 2002b]

8

Isothermal remanent magnetization (IRM) and magntization curves The IRM is the permanent magnetization acquired by an assemblage of ferrimagnetic grains after a (strong) magnetic field was applied Each magnetic grain is a miniature compass needle with a permanent dipole moment The minimum field required to re-verse the direction of such dipole is called the switching field Each magnetic grain has a switching field that depends on its size mineralogy and other physical pro-perties An assemblage of magnetic grains all with a different switching field changes its remanent magnetization when a magnetic field is applied The application of suc-cessively increasing fields is used to measure a so-called magnetization curve The derivative of such curve is the statistical distribution of switching fields also called coercivity distribution (Fig 7)

1 10 1000

80

120

40

D+EX

BS

Fig 7 (a) Distribtuion of switching fields measured on a lake sediment from Bal-deggersee (black curve) This distribution has been modelled with a liear combination of different model curves (coloured lines) each representing the magnetic contribution of a specific set of magnetic particles BS and BH are two groups of magnetosomes the difference between them is probably due to their shape D and EX are detrital par-ticles and extracellular magnetite which could not be modeled separately in this sample [redrawn from R Egli Characterization of individual rock magnetic compo-nents by analysis of remanence curves 1 Unmixing natural sediments Sudia Geo-physica et Geodaetica 48 391-446 2004] (b) Distribution of switching fields for de-trital extracellular and biogenic particles (same abbreviations as in (a)) inferred from the measurement of 122 sediments from different lakes [redrawn from R Egli Characterization of individual rock magnetic components by analysis of remanence curves 3 Bacterial magnetite and natural processes in lakes Physics and Chemistry of the Earth 29 869-884 2004] Minerals that can display switching fields above 300 mT are called high-coercivity minerals On the other hand magnetite and maghemite are low-coercivity minerals A semiquantitative parameter that indicates the relative amount of low coercivity minerals in a sample is the so-called S-ratio S (S stay for saturation) defined as

rs 300

rs

M IRMS

Mminus

=

where is the remanent magnetization acquired in a 300 mT field indicates that there are no high-coercivity minerals

300IRM 1S =

BH

H0

1

2

3

4

1 10 100

switching field mT

mag

net

ic c

on

trib

uti

on

BS

DEX

mag

net

ic c

on

trib

uti

on

switching field mT

(a) (b) BH

9

Anhysteretic remanent magnetization (ARM) and the King plot ARM is a particular kind of magntization acquired in an alternating field whose amplitude slowly decreases to zero If the field is perfectly symmetric the remanent magnetization of the sample after this treatement is zero (it is said that the sample has been AF demagnetized) However if a small constant bias field is added to the alternating field the sample acquires a magnetization called ARM This magnetization is proportional to the bias field and is typically much smaller than the saturation remanence An interesting property of the ARM is its extreme grain size sensitivity the highest ARMs are acquired by single-domain grains (which correspond to 30-100 nm grain size in magnetite) The efficiency of the ARM is evaluated by normalizing it with the saturation remanence or with the susceptibility Hence the parameters and (hereafter called ARM ratios) are sometimes used as rough grains size estimators High ARM ratios ( and ) are characteristic for fine-grained (single-domain) particles and magnetosome chains The so-called King plot combines the parameters χ and and a modified version is used to track the formation of authigenic magnetite in lake sediments (Fig 8)

DCH

DC rs( )ARM H M DC( )ARM H χ

3DC rs( ) 10 mAARM H M minus minusgt 1

DC( ) 5ARM H χ gt

DC( )ARM H χ

Fig 8 (a) The King plot is the susceptibility of the ferrimagnetic grains

is the so-called ARM susceptibility Dots are measurements of 01 microm magnetite particles at different concetrations Particles with a given grain size plot along lines whose slope is the ARM ratio Steeper lines correspond to smaller grain sizes (b) The ldquoOldfield plotrdquo is a modified version of the King plot where the ARM is normalized by the susceptibility (x-axis) and frequency dependence of suscep-tibility (y-axis) The shaded areas indicate typical values for detrital magnetite (A) and magnetosomes (B) [redrawn from F Oldfield Toward the discrimination of fine grained ferrimagnets by magnetic measurements in lake and near-shore marine sediments Journal of Geophysical Research 99 9045-9050 1994]

ferriχARM DCARM Hχ =

FDχ

10

Thermomagnetic curves Low temperature measurements can be used to identify mineral-specific transition temperatures as well as for the identification of superparamagnetic particles that become permanently magnetized below room temperature High temperature mea-surements of lake sediments rich in organic matter are difficult because the high surface-to-mass ratio of the authigenic minerals and the presence of organic matter accelerates the irreversible alteration of many magntic parameters above 200degC 4 Characterization of an eutrophication event

The goal of this case study is the characterization of the last eutrophication event of Baldeggersee which began in around 1900 For this purpose you will measure seven sediment samples labelled BAL-GR BA9-75 G101 G044 G010 AR23 and MV1 BAL-GR is a sample of sediment collected in a zone of the catchment area that was eroded by a small stream that ends in Baldeggersee This sample is expected to re-present the actual detrital input of the lake BA9-75 was collected near the bottom of the sediment core This sediment was deposited about 14000 years ago soon after the formation of the lake G101 G044 and G010 are sediment samples collected at depths of 101 44 and 10 cm respectively They represent three stages of the last eutrophication event G101 was deposited at the time of the first human settlements around the lake G044 marks the onset of eutrophication and G010 is totally anoxic Bot G044 and G010 display chemical varves (no bioturbation) AR23 is a sediment taken from the Aral sea (UzbekistanKazakstan) MV1 is a cultured sample of the magnetotactic bacterium Magetotactic Vibrium 1 and provides a reference for the magnetic properties of magnetosomes The results of low-field susceptibilty hysteresis and ARM measurememts are summarized in Tab 1 and Fig 9 Sample χ

3 1nm kgminus paraχ

3 1nm kgminus ferriχ

3 1nm kgminus ARMχ

3 1nm kgminus rs sM M cr cH H

BAL-GR BA9-75 G101 G044 G010 AR23 MV1

1195 510

1307 951 497

1580 158

326184594297207827

00

869326713654291753158

629 253 2140 3440 301 4250 3330

0208 0330 0237 0340 0249 0373 0482

3352 2158 1992 1857 2474 1743 1161

Tab 1 Summary of the measurements The low-field susceptibility χ has been mea-sured with a Kappabridge KLY-2 The paramagnetic susceptibility and the Day plot parameters and were obtained from hysteresis loops measured with a Micromag Vibrating Sample Magnetometer The ARM was imparted with a D-TECH D2000 AF system (maximum AF field 200 mT DC field 01 mT) and measured with a 2G cryogenic magnetometer The susceptibility of ARM was calculated by dividing the ARM magnetization by the DC field

paraχ

rs sM M cr cH H

ARMχ

11

00

02

04

06

BAL-GR

MV-1

AR23G044 BA9-75

G010G101

1 2 3 4 5 60

1

2

3

4

5

BAL-GR

AR23

MV-1 G044

G101

G010 BA9-75

0 20 40 60 80 100

(a) (b)

ferri3

AR

M3

nm kg

nm

kg

χ

χ

minus

minus

1

1

SD

MD

MM

H H

rss

cr c

Fig 9 (a) Representation of the measurements (open dots) on a Day plot The solid line is a model for the binary mixture of single-domain (SD) and multidomain (MD) magnetite (b) Representation of the measurements (open dots) on a King plot The solid line refers to the ARM ratio of 20 microm magnetite grains All lake sediments show a clear trend indicated by the dashed line to the right This line does not intersect the origin indicating a different composition of the samples Samples G010 BA9-75 and BAL-GR are also compatible with the line given by 20 microm magnetite The dashed line to the left is the calculated trend for magnetosomes [calculated after R Egli and W Lowrie Anhysteretic remanent magnetization of fine magnetic particles Journal of Geophysical Research 107 doi 1010292001JB000671 2002]

Figure 9 shows some interesting features that help us with the interpretation of the magnetic composition of the sediments The Day plot shows that the sediments are a mixture of relatively coarse magnetic grains probably of detrital origin and fine SD authigenic magnetite One endmember of this binary mixture are obviously the mag-netosomes in MV1 The other endmember is BAL-GR the material trasported into the lake from the catchment area The binary mixing trend is less steep than the mo-del curve in Fig 9a probably because BAL-GR does not contain only magnetite Mixtures of coarse magnetite maghemite or hematite will not necessary fall into the MD range predicted by the Day plot More detailed informations can be obtained from the King plot in Fig 9b Samples with the same magnetic composition but dif-ferent concentration of magnetic minerals would plot along lines that intercept the origin This is the case for G010 BA9-75 and BAL-GR BAL-GR and BA9-75 are expected to be similar in composition because during the early hystory of the lake the primary productivity was low and the production of authigenic minerals in the sediment is negligible The only difference between BAL-GR and BA9-75 is given by the concentration of ferrimagnetic minerals which is lower by a factor 3 in the lake sediment Why this difference Baldeggersee is a hard water lake and the sediments contain more than 50 calcium carbonate that precipitated from the water The lower concentration of magnetic minerals is simply produced by a dilution effect in

12

the nonmagnetic carbonate All lake sediment samples plot along a trend defined by a dashed line in Fig 9b This line does not intercept the origin demonstrating the variable composition of the lake sediments The second dashed line indicates the trend predicted for magnetosomes which is confirmed by the measurement of MV1 If we assume that lake sediments are binary mixtures of coarse detrital magnetite with a roughtly constant concentration and single-domain authigenic magnetite with a variable concentration the resulting mixing trend is a line whose slope is idetical to that of the single-domain grains The actual slope is slightly smaller than that pre-dicted by this model The reason of this discrepancy is that our simple model did not consider the production of extracellular magnetite which is in part superparamag-netic Superparamagntic grains are characterized by and lower the slope of the mixing trend predicted by the model

ARM 0χ =

One last feature to explain is the fact that despite G010 was sedimented during a period of very high primary productivity (around 1970 triggered by phosphor rich sewage waters) its properties are similar to those of BA9-75 (very low productivity) How is this possible To explain this apparent contradiction more detailed measure-ments are necessary The precise measurement of the distribution of switching field (Fig 7) allows the quantification of the different groups of magnetic grains that occur in the sediment With this method it was possible to dempnstrate that the autigenic magnetie was dissolved under anoxic conditions Fig 10 show the results of these measurements

Fig 10 Concentration of the two groups of magnetosomes BS and BH shown in Fig 7 as a function of depth for the uppermost 70 cm of sediments in Baldeggersee (Switzerland) The magnetic contributions have been normalized by the magnetization of the detrital plarticles Notice that the concentration of magnetosomes peaks at 44 cm and dorps to 13 of the original value at 10 cm This depth corresponds to the period of most severe eutrophication when the hypolimnion was completely anoxic The drop in the concentration of magneto-somes can be explained by ther dissolution The dissolution is more rapid for small particles therefore detrital magnetite was less affected The lake restoration program started in 1982 is clearly marked by an increase in the concentra-tion of magnetosomes The decreasing trend af-ter 1982 is due to the fact that the amout of oxygen pumped into the hypolimnium was de-creased [from R Egli Characterization of individual rock magnetic components by analy-sis of remanence curves 3 Bacterial magnetite and natural processes in lakes Physics and

Chemistry of the Earth 29 869-884 2004]

0 1

0

20

40

60

2

dept

h c

m

IRMBS IRMD+EX IRMBH IRMD+EX

1982

1942

13

5 Possible applications

We have seen how it is possible to characterize the iron cycle in lake sediments by quantifying the production of authigenic magnetite A lake is an ecosystem in equi-librion with the surrounding environment Its geochemical conditions depend on many factors that include the climate and local geological events in the catchment area These factors are more or less indirectly reflected in the properties of the sedi-ments Magnetic measurements of lake sediments have been used to roughly recon-struct the climate during the Holocene A fully quantitatie paleoclimatic reconstruc-tion needs to be based on a precise model of the interaction between lakes and en-vironments Recent studies pointed out that many naturals system and even the entire Earth are governed by nonlinear laws The profund consequences of this fact is that these system can display a cahotic behaviuor Nonliearity has been confirmed in the case of Baldeggersee where the answer of the lake to a nutrient input cycling was studied on the magnetic properties of the sediments (Fig 11)

50 100 time

Ncrit

min

Ncritmax

nutr

ient

s(b)

oxic

sub-

oxic

anoxic

AR

M r

atio

magnetization

oligotrophic

eutrophic

0

1

2

3

0 1 2 3

kA

RM

IR

M o

f BS

+B

H m

mA

minus1

IRMBS+BH IRMD+EX

1972

1982

1984

1999

1942

1910

1885

lt1700 (a)

Fig 11 (a) Cycling of Baldeggersee sediments through an eutrophication event The x-axis describes the total concentration of magnetosomes the y-axis shows a grain size parameter that becomes small if the particles are dissolved Before the industrializa-tion the lake was oligotrophic as shown by the low concentration of magnetosomes For a certain period the amount of magntosomes increased along with the primary production until reductive dissolution started in 1910 The concentration of magnto-somes was relatively constant until 1942 and decreases abruptly until a restoration program was started in 1982 Notice that recovering occurs along a different path as expected from a nonlinear system (b) A model for the magnetic properties of the sediments as a function of the nutrient imput simulated by a sinusoidal curve with some random spikes due to extreme local events (storms landslides) The model reproduces accurately the features of (a) [from the same source of Fig 10]

14

1 2fd

1 2

( ) ( )100

( )log( )f f

f f fχ χ

χχ

minus=

1

2

85

with The frequency dependence is simply the relative difference between two susceptibilities measured at different frequencies expressed in percent This parame-ter is sensitive to the presence of very fine (suparparamagnetic) ferromagnetic parti-cles and is therefore particularly suitable for the identitfication of fine magnetite grains produced by dissimilatory iron reducing bacteria The theoretical range of values for is between 0 and 50 However a maximum value of 30 is observed in natural samples

1f flt

fdχ

minus04 minus02 02 04

minus2

2

minus04 minus02 02 04

minus10

minus05

minus05 minus04 minus03 minus02 minus01

minus02

00

02

χpara

s

rs

c

cr

M

M

H

H

(a)

(b)

(c) rsM

rsminusM

Fig 5 (a) Hysteresis loop of a lake sediment sample from Bal-deggersee Switzerland (b) The same hysteresis loop after sub-traction of the paramagnetic contribution (c) Backfield curve of the same sample All mea-surements were performed with a Micromag Vibrating Sample Magnetometer The hysteresis parameters deduced from the measurements are 04 and 455 mT The remanence ratio close to 05 suggests that the sample contains a large amount of single-domain grains Elec-trom microscope observations of this sample revealed the presen-ce of intact chains of magneto-somes (see Fig 3b) The back-field curve is not completely sa-turated at 300 mT indicating the presence of some high-coer-civity minerals

rs sM M =

cr c 1H H = crH =

7

Hysteresis parameters and the Day plot The measurement of hysteresis loops and so-called backfield curves provides useful information about the magnetic mineralogy and granulometry (Fig 5) The slope of the hysteresis loop at high fields originates from the paramagntic minerals The interpretation of the hysteresis loop is facilitated after the linear contribution of the paramagnetic minerals is subtracted by the so-called paramagnetic correction Most software provided with measuring instruments can perform the paramagnetic cor-rection automatically The corrected loop comprises the magnetic contribution of all ferrimagnetic grains The maximum magnetization reached by the hysteresis loop is called saturation magnetization it is the highest possible magnetization carried by all magnetic grains The intercept of the hysteresis loop with the magnetization axis is the saturation remanence it is the permanent magnetization of all grains in a zero field Superparamagnetic grains are too small to retain a permanent magne-tization and in this case An assemblage of randomly oriented single-domain grains has Very large (multidomain) grains are characterized by

The remanence ratio is a grain size indicator higher values of indicate the presence of single-domain grains such as magnetosomes The

coercivity is the field at which in a hysteresis loop The coercivity of remanence is the negative field that must be applied to cancel the saturation remanence acquired in a large positive field A combination of the ratios and

is used in the so-called Day plot (Fig 6) Mixtures of sigle-doman and super-paramagnetic or multidomain grains produce a characteristic grouping of the measu-rement along a mixing trend line in the Day plot Theoretical mixing models can be used under some circumstances to infer the proportion of sigle-doman grains in a sample

sM

rsM

r 0M =

s rs05 1M Mle lt

rM Ms

s

rs sM M

rM M

cH 0M =

crH

rs sM M

cr cH H

Fig 6 The so-called Day plot is an effective way to represent the hysteresis para-meters In this plot the effect of a binary mixture of single-domain (SD) and multido-main (MD) particle is shown (line) Open circles are lake sediments from Minnesota the dot is the sample used as example in Fig 5 [Modified from D Dunlop Theory and application of the Day plot 2 Applications to data for rocks sediments and soils Journal of Geophysical research 107 DOI 101292001JB000487 2002b]

8

Isothermal remanent magnetization (IRM) and magntization curves The IRM is the permanent magnetization acquired by an assemblage of ferrimagnetic grains after a (strong) magnetic field was applied Each magnetic grain is a miniature compass needle with a permanent dipole moment The minimum field required to re-verse the direction of such dipole is called the switching field Each magnetic grain has a switching field that depends on its size mineralogy and other physical pro-perties An assemblage of magnetic grains all with a different switching field changes its remanent magnetization when a magnetic field is applied The application of suc-cessively increasing fields is used to measure a so-called magnetization curve The derivative of such curve is the statistical distribution of switching fields also called coercivity distribution (Fig 7)

1 10 1000

80

120

40

D+EX

BS

Fig 7 (a) Distribtuion of switching fields measured on a lake sediment from Bal-deggersee (black curve) This distribution has been modelled with a liear combination of different model curves (coloured lines) each representing the magnetic contribution of a specific set of magnetic particles BS and BH are two groups of magnetosomes the difference between them is probably due to their shape D and EX are detrital par-ticles and extracellular magnetite which could not be modeled separately in this sample [redrawn from R Egli Characterization of individual rock magnetic compo-nents by analysis of remanence curves 1 Unmixing natural sediments Sudia Geo-physica et Geodaetica 48 391-446 2004] (b) Distribution of switching fields for de-trital extracellular and biogenic particles (same abbreviations as in (a)) inferred from the measurement of 122 sediments from different lakes [redrawn from R Egli Characterization of individual rock magnetic components by analysis of remanence curves 3 Bacterial magnetite and natural processes in lakes Physics and Chemistry of the Earth 29 869-884 2004] Minerals that can display switching fields above 300 mT are called high-coercivity minerals On the other hand magnetite and maghemite are low-coercivity minerals A semiquantitative parameter that indicates the relative amount of low coercivity minerals in a sample is the so-called S-ratio S (S stay for saturation) defined as

rs 300

rs

M IRMS

Mminus

=

where is the remanent magnetization acquired in a 300 mT field indicates that there are no high-coercivity minerals

300IRM 1S =

BH

H0

1

2

3

4

1 10 100

switching field mT

mag

net

ic c

on

trib

uti

on

BS

DEX

mag

net

ic c

on

trib

uti

on

switching field mT

(a) (b) BH

9

Anhysteretic remanent magnetization (ARM) and the King plot ARM is a particular kind of magntization acquired in an alternating field whose amplitude slowly decreases to zero If the field is perfectly symmetric the remanent magnetization of the sample after this treatement is zero (it is said that the sample has been AF demagnetized) However if a small constant bias field is added to the alternating field the sample acquires a magnetization called ARM This magnetization is proportional to the bias field and is typically much smaller than the saturation remanence An interesting property of the ARM is its extreme grain size sensitivity the highest ARMs are acquired by single-domain grains (which correspond to 30-100 nm grain size in magnetite) The efficiency of the ARM is evaluated by normalizing it with the saturation remanence or with the susceptibility Hence the parameters and (hereafter called ARM ratios) are sometimes used as rough grains size estimators High ARM ratios ( and ) are characteristic for fine-grained (single-domain) particles and magnetosome chains The so-called King plot combines the parameters χ and and a modified version is used to track the formation of authigenic magnetite in lake sediments (Fig 8)

DCH

DC rs( )ARM H M DC( )ARM H χ

3DC rs( ) 10 mAARM H M minus minusgt 1

DC( ) 5ARM H χ gt

DC( )ARM H χ

Fig 8 (a) The King plot is the susceptibility of the ferrimagnetic grains

is the so-called ARM susceptibility Dots are measurements of 01 microm magnetite particles at different concetrations Particles with a given grain size plot along lines whose slope is the ARM ratio Steeper lines correspond to smaller grain sizes (b) The ldquoOldfield plotrdquo is a modified version of the King plot where the ARM is normalized by the susceptibility (x-axis) and frequency dependence of suscep-tibility (y-axis) The shaded areas indicate typical values for detrital magnetite (A) and magnetosomes (B) [redrawn from F Oldfield Toward the discrimination of fine grained ferrimagnets by magnetic measurements in lake and near-shore marine sediments Journal of Geophysical Research 99 9045-9050 1994]

ferriχARM DCARM Hχ =

FDχ

10

Thermomagnetic curves Low temperature measurements can be used to identify mineral-specific transition temperatures as well as for the identification of superparamagnetic particles that become permanently magnetized below room temperature High temperature mea-surements of lake sediments rich in organic matter are difficult because the high surface-to-mass ratio of the authigenic minerals and the presence of organic matter accelerates the irreversible alteration of many magntic parameters above 200degC 4 Characterization of an eutrophication event

The goal of this case study is the characterization of the last eutrophication event of Baldeggersee which began in around 1900 For this purpose you will measure seven sediment samples labelled BAL-GR BA9-75 G101 G044 G010 AR23 and MV1 BAL-GR is a sample of sediment collected in a zone of the catchment area that was eroded by a small stream that ends in Baldeggersee This sample is expected to re-present the actual detrital input of the lake BA9-75 was collected near the bottom of the sediment core This sediment was deposited about 14000 years ago soon after the formation of the lake G101 G044 and G010 are sediment samples collected at depths of 101 44 and 10 cm respectively They represent three stages of the last eutrophication event G101 was deposited at the time of the first human settlements around the lake G044 marks the onset of eutrophication and G010 is totally anoxic Bot G044 and G010 display chemical varves (no bioturbation) AR23 is a sediment taken from the Aral sea (UzbekistanKazakstan) MV1 is a cultured sample of the magnetotactic bacterium Magetotactic Vibrium 1 and provides a reference for the magnetic properties of magnetosomes The results of low-field susceptibilty hysteresis and ARM measurememts are summarized in Tab 1 and Fig 9 Sample χ

3 1nm kgminus paraχ

3 1nm kgminus ferriχ

3 1nm kgminus ARMχ

3 1nm kgminus rs sM M cr cH H

BAL-GR BA9-75 G101 G044 G010 AR23 MV1

1195 510

1307 951 497

1580 158

326184594297207827

00

869326713654291753158

629 253 2140 3440 301 4250 3330

0208 0330 0237 0340 0249 0373 0482

3352 2158 1992 1857 2474 1743 1161

Tab 1 Summary of the measurements The low-field susceptibility χ has been mea-sured with a Kappabridge KLY-2 The paramagnetic susceptibility and the Day plot parameters and were obtained from hysteresis loops measured with a Micromag Vibrating Sample Magnetometer The ARM was imparted with a D-TECH D2000 AF system (maximum AF field 200 mT DC field 01 mT) and measured with a 2G cryogenic magnetometer The susceptibility of ARM was calculated by dividing the ARM magnetization by the DC field

paraχ

rs sM M cr cH H

ARMχ

11

00

02

04

06

BAL-GR

MV-1

AR23G044 BA9-75

G010G101

1 2 3 4 5 60

1

2

3

4

5

BAL-GR

AR23

MV-1 G044

G101

G010 BA9-75

0 20 40 60 80 100

(a) (b)

ferri3

AR

M3

nm kg

nm

kg

χ

χ

minus

minus

1

1

SD

MD

MM

H H

rss

cr c

Fig 9 (a) Representation of the measurements (open dots) on a Day plot The solid line is a model for the binary mixture of single-domain (SD) and multidomain (MD) magnetite (b) Representation of the measurements (open dots) on a King plot The solid line refers to the ARM ratio of 20 microm magnetite grains All lake sediments show a clear trend indicated by the dashed line to the right This line does not intersect the origin indicating a different composition of the samples Samples G010 BA9-75 and BAL-GR are also compatible with the line given by 20 microm magnetite The dashed line to the left is the calculated trend for magnetosomes [calculated after R Egli and W Lowrie Anhysteretic remanent magnetization of fine magnetic particles Journal of Geophysical Research 107 doi 1010292001JB000671 2002]

Figure 9 shows some interesting features that help us with the interpretation of the magnetic composition of the sediments The Day plot shows that the sediments are a mixture of relatively coarse magnetic grains probably of detrital origin and fine SD authigenic magnetite One endmember of this binary mixture are obviously the mag-netosomes in MV1 The other endmember is BAL-GR the material trasported into the lake from the catchment area The binary mixing trend is less steep than the mo-del curve in Fig 9a probably because BAL-GR does not contain only magnetite Mixtures of coarse magnetite maghemite or hematite will not necessary fall into the MD range predicted by the Day plot More detailed informations can be obtained from the King plot in Fig 9b Samples with the same magnetic composition but dif-ferent concentration of magnetic minerals would plot along lines that intercept the origin This is the case for G010 BA9-75 and BAL-GR BAL-GR and BA9-75 are expected to be similar in composition because during the early hystory of the lake the primary productivity was low and the production of authigenic minerals in the sediment is negligible The only difference between BAL-GR and BA9-75 is given by the concentration of ferrimagnetic minerals which is lower by a factor 3 in the lake sediment Why this difference Baldeggersee is a hard water lake and the sediments contain more than 50 calcium carbonate that precipitated from the water The lower concentration of magnetic minerals is simply produced by a dilution effect in

12

the nonmagnetic carbonate All lake sediment samples plot along a trend defined by a dashed line in Fig 9b This line does not intercept the origin demonstrating the variable composition of the lake sediments The second dashed line indicates the trend predicted for magnetosomes which is confirmed by the measurement of MV1 If we assume that lake sediments are binary mixtures of coarse detrital magnetite with a roughtly constant concentration and single-domain authigenic magnetite with a variable concentration the resulting mixing trend is a line whose slope is idetical to that of the single-domain grains The actual slope is slightly smaller than that pre-dicted by this model The reason of this discrepancy is that our simple model did not consider the production of extracellular magnetite which is in part superparamag-netic Superparamagntic grains are characterized by and lower the slope of the mixing trend predicted by the model

ARM 0χ =

One last feature to explain is the fact that despite G010 was sedimented during a period of very high primary productivity (around 1970 triggered by phosphor rich sewage waters) its properties are similar to those of BA9-75 (very low productivity) How is this possible To explain this apparent contradiction more detailed measure-ments are necessary The precise measurement of the distribution of switching field (Fig 7) allows the quantification of the different groups of magnetic grains that occur in the sediment With this method it was possible to dempnstrate that the autigenic magnetie was dissolved under anoxic conditions Fig 10 show the results of these measurements

Fig 10 Concentration of the two groups of magnetosomes BS and BH shown in Fig 7 as a function of depth for the uppermost 70 cm of sediments in Baldeggersee (Switzerland) The magnetic contributions have been normalized by the magnetization of the detrital plarticles Notice that the concentration of magnetosomes peaks at 44 cm and dorps to 13 of the original value at 10 cm This depth corresponds to the period of most severe eutrophication when the hypolimnion was completely anoxic The drop in the concentration of magneto-somes can be explained by ther dissolution The dissolution is more rapid for small particles therefore detrital magnetite was less affected The lake restoration program started in 1982 is clearly marked by an increase in the concentra-tion of magnetosomes The decreasing trend af-ter 1982 is due to the fact that the amout of oxygen pumped into the hypolimnium was de-creased [from R Egli Characterization of individual rock magnetic components by analy-sis of remanence curves 3 Bacterial magnetite and natural processes in lakes Physics and

Chemistry of the Earth 29 869-884 2004]

0 1

0

20

40

60

2

dept

h c

m

IRMBS IRMD+EX IRMBH IRMD+EX

1982

1942

13

5 Possible applications

We have seen how it is possible to characterize the iron cycle in lake sediments by quantifying the production of authigenic magnetite A lake is an ecosystem in equi-librion with the surrounding environment Its geochemical conditions depend on many factors that include the climate and local geological events in the catchment area These factors are more or less indirectly reflected in the properties of the sedi-ments Magnetic measurements of lake sediments have been used to roughly recon-struct the climate during the Holocene A fully quantitatie paleoclimatic reconstruc-tion needs to be based on a precise model of the interaction between lakes and en-vironments Recent studies pointed out that many naturals system and even the entire Earth are governed by nonlinear laws The profund consequences of this fact is that these system can display a cahotic behaviuor Nonliearity has been confirmed in the case of Baldeggersee where the answer of the lake to a nutrient input cycling was studied on the magnetic properties of the sediments (Fig 11)

50 100 time

Ncrit

min

Ncritmax

nutr

ient

s(b)

oxic

sub-

oxic

anoxic

AR

M r

atio

magnetization

oligotrophic

eutrophic

0

1

2

3

0 1 2 3

kA

RM

IR

M o

f BS

+B

H m

mA

minus1

IRMBS+BH IRMD+EX

1972

1982

1984

1999

1942

1910

1885

lt1700 (a)

Fig 11 (a) Cycling of Baldeggersee sediments through an eutrophication event The x-axis describes the total concentration of magnetosomes the y-axis shows a grain size parameter that becomes small if the particles are dissolved Before the industrializa-tion the lake was oligotrophic as shown by the low concentration of magnetosomes For a certain period the amount of magntosomes increased along with the primary production until reductive dissolution started in 1910 The concentration of magnto-somes was relatively constant until 1942 and decreases abruptly until a restoration program was started in 1982 Notice that recovering occurs along a different path as expected from a nonlinear system (b) A model for the magnetic properties of the sediments as a function of the nutrient imput simulated by a sinusoidal curve with some random spikes due to extreme local events (storms landslides) The model reproduces accurately the features of (a) [from the same source of Fig 10]

14

Hysteresis parameters and the Day plot The measurement of hysteresis loops and so-called backfield curves provides useful information about the magnetic mineralogy and granulometry (Fig 5) The slope of the hysteresis loop at high fields originates from the paramagntic minerals The interpretation of the hysteresis loop is facilitated after the linear contribution of the paramagnetic minerals is subtracted by the so-called paramagnetic correction Most software provided with measuring instruments can perform the paramagnetic cor-rection automatically The corrected loop comprises the magnetic contribution of all ferrimagnetic grains The maximum magnetization reached by the hysteresis loop is called saturation magnetization it is the highest possible magnetization carried by all magnetic grains The intercept of the hysteresis loop with the magnetization axis is the saturation remanence it is the permanent magnetization of all grains in a zero field Superparamagnetic grains are too small to retain a permanent magne-tization and in this case An assemblage of randomly oriented single-domain grains has Very large (multidomain) grains are characterized by

The remanence ratio is a grain size indicator higher values of indicate the presence of single-domain grains such as magnetosomes The

coercivity is the field at which in a hysteresis loop The coercivity of remanence is the negative field that must be applied to cancel the saturation remanence acquired in a large positive field A combination of the ratios and

is used in the so-called Day plot (Fig 6) Mixtures of sigle-doman and super-paramagnetic or multidomain grains produce a characteristic grouping of the measu-rement along a mixing trend line in the Day plot Theoretical mixing models can be used under some circumstances to infer the proportion of sigle-doman grains in a sample

sM

rsM

r 0M =

s rs05 1M Mle lt

rM Ms

s

rs sM M

rM M

cH 0M =

crH

rs sM M

cr cH H

Fig 6 The so-called Day plot is an effective way to represent the hysteresis para-meters In this plot the effect of a binary mixture of single-domain (SD) and multido-main (MD) particle is shown (line) Open circles are lake sediments from Minnesota the dot is the sample used as example in Fig 5 [Modified from D Dunlop Theory and application of the Day plot 2 Applications to data for rocks sediments and soils Journal of Geophysical research 107 DOI 101292001JB000487 2002b]

8

Isothermal remanent magnetization (IRM) and magntization curves The IRM is the permanent magnetization acquired by an assemblage of ferrimagnetic grains after a (strong) magnetic field was applied Each magnetic grain is a miniature compass needle with a permanent dipole moment The minimum field required to re-verse the direction of such dipole is called the switching field Each magnetic grain has a switching field that depends on its size mineralogy and other physical pro-perties An assemblage of magnetic grains all with a different switching field changes its remanent magnetization when a magnetic field is applied The application of suc-cessively increasing fields is used to measure a so-called magnetization curve The derivative of such curve is the statistical distribution of switching fields also called coercivity distribution (Fig 7)

1 10 1000

80

120

40

D+EX

BS

Fig 7 (a) Distribtuion of switching fields measured on a lake sediment from Bal-deggersee (black curve) This distribution has been modelled with a liear combination of different model curves (coloured lines) each representing the magnetic contribution of a specific set of magnetic particles BS and BH are two groups of magnetosomes the difference between them is probably due to their shape D and EX are detrital par-ticles and extracellular magnetite which could not be modeled separately in this sample [redrawn from R Egli Characterization of individual rock magnetic compo-nents by analysis of remanence curves 1 Unmixing natural sediments Sudia Geo-physica et Geodaetica 48 391-446 2004] (b) Distribution of switching fields for de-trital extracellular and biogenic particles (same abbreviations as in (a)) inferred from the measurement of 122 sediments from different lakes [redrawn from R Egli Characterization of individual rock magnetic components by analysis of remanence curves 3 Bacterial magnetite and natural processes in lakes Physics and Chemistry of the Earth 29 869-884 2004] Minerals that can display switching fields above 300 mT are called high-coercivity minerals On the other hand magnetite and maghemite are low-coercivity minerals A semiquantitative parameter that indicates the relative amount of low coercivity minerals in a sample is the so-called S-ratio S (S stay for saturation) defined as

rs 300

rs

M IRMS

Mminus

=

where is the remanent magnetization acquired in a 300 mT field indicates that there are no high-coercivity minerals

300IRM 1S =

BH

H0

1

2

3

4

1 10 100

switching field mT

mag

net

ic c

on

trib

uti

on

BS

DEX

mag

net

ic c

on

trib

uti

on

switching field mT

(a) (b) BH

9

Anhysteretic remanent magnetization (ARM) and the King plot ARM is a particular kind of magntization acquired in an alternating field whose amplitude slowly decreases to zero If the field is perfectly symmetric the remanent magnetization of the sample after this treatement is zero (it is said that the sample has been AF demagnetized) However if a small constant bias field is added to the alternating field the sample acquires a magnetization called ARM This magnetization is proportional to the bias field and is typically much smaller than the saturation remanence An interesting property of the ARM is its extreme grain size sensitivity the highest ARMs are acquired by single-domain grains (which correspond to 30-100 nm grain size in magnetite) The efficiency of the ARM is evaluated by normalizing it with the saturation remanence or with the susceptibility Hence the parameters and (hereafter called ARM ratios) are sometimes used as rough grains size estimators High ARM ratios ( and ) are characteristic for fine-grained (single-domain) particles and magnetosome chains The so-called King plot combines the parameters χ and and a modified version is used to track the formation of authigenic magnetite in lake sediments (Fig 8)

DCH

DC rs( )ARM H M DC( )ARM H χ

3DC rs( ) 10 mAARM H M minus minusgt 1

DC( ) 5ARM H χ gt

DC( )ARM H χ

Fig 8 (a) The King plot is the susceptibility of the ferrimagnetic grains

is the so-called ARM susceptibility Dots are measurements of 01 microm magnetite particles at different concetrations Particles with a given grain size plot along lines whose slope is the ARM ratio Steeper lines correspond to smaller grain sizes (b) The ldquoOldfield plotrdquo is a modified version of the King plot where the ARM is normalized by the susceptibility (x-axis) and frequency dependence of suscep-tibility (y-axis) The shaded areas indicate typical values for detrital magnetite (A) and magnetosomes (B) [redrawn from F Oldfield Toward the discrimination of fine grained ferrimagnets by magnetic measurements in lake and near-shore marine sediments Journal of Geophysical Research 99 9045-9050 1994]

ferriχARM DCARM Hχ =

FDχ

10

Thermomagnetic curves Low temperature measurements can be used to identify mineral-specific transition temperatures as well as for the identification of superparamagnetic particles that become permanently magnetized below room temperature High temperature mea-surements of lake sediments rich in organic matter are difficult because the high surface-to-mass ratio of the authigenic minerals and the presence of organic matter accelerates the irreversible alteration of many magntic parameters above 200degC 4 Characterization of an eutrophication event

The goal of this case study is the characterization of the last eutrophication event of Baldeggersee which began in around 1900 For this purpose you will measure seven sediment samples labelled BAL-GR BA9-75 G101 G044 G010 AR23 and MV1 BAL-GR is a sample of sediment collected in a zone of the catchment area that was eroded by a small stream that ends in Baldeggersee This sample is expected to re-present the actual detrital input of the lake BA9-75 was collected near the bottom of the sediment core This sediment was deposited about 14000 years ago soon after the formation of the lake G101 G044 and G010 are sediment samples collected at depths of 101 44 and 10 cm respectively They represent three stages of the last eutrophication event G101 was deposited at the time of the first human settlements around the lake G044 marks the onset of eutrophication and G010 is totally anoxic Bot G044 and G010 display chemical varves (no bioturbation) AR23 is a sediment taken from the Aral sea (UzbekistanKazakstan) MV1 is a cultured sample of the magnetotactic bacterium Magetotactic Vibrium 1 and provides a reference for the magnetic properties of magnetosomes The results of low-field susceptibilty hysteresis and ARM measurememts are summarized in Tab 1 and Fig 9 Sample χ

3 1nm kgminus paraχ

3 1nm kgminus ferriχ

3 1nm kgminus ARMχ

3 1nm kgminus rs sM M cr cH H

BAL-GR BA9-75 G101 G044 G010 AR23 MV1

1195 510

1307 951 497

1580 158

326184594297207827

00

869326713654291753158

629 253 2140 3440 301 4250 3330

0208 0330 0237 0340 0249 0373 0482

3352 2158 1992 1857 2474 1743 1161

Tab 1 Summary of the measurements The low-field susceptibility χ has been mea-sured with a Kappabridge KLY-2 The paramagnetic susceptibility and the Day plot parameters and were obtained from hysteresis loops measured with a Micromag Vibrating Sample Magnetometer The ARM was imparted with a D-TECH D2000 AF system (maximum AF field 200 mT DC field 01 mT) and measured with a 2G cryogenic magnetometer The susceptibility of ARM was calculated by dividing the ARM magnetization by the DC field

paraχ

rs sM M cr cH H

ARMχ

11

00

02

04

06

BAL-GR

MV-1

AR23G044 BA9-75

G010G101

1 2 3 4 5 60

1

2

3

4

5

BAL-GR

AR23

MV-1 G044

G101

G010 BA9-75

0 20 40 60 80 100

(a) (b)

ferri3

AR

M3

nm kg

nm

kg

χ

χ

minus

minus

1

1

SD

MD

MM

H H

rss

cr c

Fig 9 (a) Representation of the measurements (open dots) on a Day plot The solid line is a model for the binary mixture of single-domain (SD) and multidomain (MD) magnetite (b) Representation of the measurements (open dots) on a King plot The solid line refers to the ARM ratio of 20 microm magnetite grains All lake sediments show a clear trend indicated by the dashed line to the right This line does not intersect the origin indicating a different composition of the samples Samples G010 BA9-75 and BAL-GR are also compatible with the line given by 20 microm magnetite The dashed line to the left is the calculated trend for magnetosomes [calculated after R Egli and W Lowrie Anhysteretic remanent magnetization of fine magnetic particles Journal of Geophysical Research 107 doi 1010292001JB000671 2002]

Figure 9 shows some interesting features that help us with the interpretation of the magnetic composition of the sediments The Day plot shows that the sediments are a mixture of relatively coarse magnetic grains probably of detrital origin and fine SD authigenic magnetite One endmember of this binary mixture are obviously the mag-netosomes in MV1 The other endmember is BAL-GR the material trasported into the lake from the catchment area The binary mixing trend is less steep than the mo-del curve in Fig 9a probably because BAL-GR does not contain only magnetite Mixtures of coarse magnetite maghemite or hematite will not necessary fall into the MD range predicted by the Day plot More detailed informations can be obtained from the King plot in Fig 9b Samples with the same magnetic composition but dif-ferent concentration of magnetic minerals would plot along lines that intercept the origin This is the case for G010 BA9-75 and BAL-GR BAL-GR and BA9-75 are expected to be similar in composition because during the early hystory of the lake the primary productivity was low and the production of authigenic minerals in the sediment is negligible The only difference between BAL-GR and BA9-75 is given by the concentration of ferrimagnetic minerals which is lower by a factor 3 in the lake sediment Why this difference Baldeggersee is a hard water lake and the sediments contain more than 50 calcium carbonate that precipitated from the water The lower concentration of magnetic minerals is simply produced by a dilution effect in

12

the nonmagnetic carbonate All lake sediment samples plot along a trend defined by a dashed line in Fig 9b This line does not intercept the origin demonstrating the variable composition of the lake sediments The second dashed line indicates the trend predicted for magnetosomes which is confirmed by the measurement of MV1 If we assume that lake sediments are binary mixtures of coarse detrital magnetite with a roughtly constant concentration and single-domain authigenic magnetite with a variable concentration the resulting mixing trend is a line whose slope is idetical to that of the single-domain grains The actual slope is slightly smaller than that pre-dicted by this model The reason of this discrepancy is that our simple model did not consider the production of extracellular magnetite which is in part superparamag-netic Superparamagntic grains are characterized by and lower the slope of the mixing trend predicted by the model

ARM 0χ =

One last feature to explain is the fact that despite G010 was sedimented during a period of very high primary productivity (around 1970 triggered by phosphor rich sewage waters) its properties are similar to those of BA9-75 (very low productivity) How is this possible To explain this apparent contradiction more detailed measure-ments are necessary The precise measurement of the distribution of switching field (Fig 7) allows the quantification of the different groups of magnetic grains that occur in the sediment With this method it was possible to dempnstrate that the autigenic magnetie was dissolved under anoxic conditions Fig 10 show the results of these measurements

Fig 10 Concentration of the two groups of magnetosomes BS and BH shown in Fig 7 as a function of depth for the uppermost 70 cm of sediments in Baldeggersee (Switzerland) The magnetic contributions have been normalized by the magnetization of the detrital plarticles Notice that the concentration of magnetosomes peaks at 44 cm and dorps to 13 of the original value at 10 cm This depth corresponds to the period of most severe eutrophication when the hypolimnion was completely anoxic The drop in the concentration of magneto-somes can be explained by ther dissolution The dissolution is more rapid for small particles therefore detrital magnetite was less affected The lake restoration program started in 1982 is clearly marked by an increase in the concentra-tion of magnetosomes The decreasing trend af-ter 1982 is due to the fact that the amout of oxygen pumped into the hypolimnium was de-creased [from R Egli Characterization of individual rock magnetic components by analy-sis of remanence curves 3 Bacterial magnetite and natural processes in lakes Physics and

Chemistry of the Earth 29 869-884 2004]

0 1

0

20

40

60

2

dept

h c

m

IRMBS IRMD+EX IRMBH IRMD+EX

1982

1942

13

5 Possible applications

We have seen how it is possible to characterize the iron cycle in lake sediments by quantifying the production of authigenic magnetite A lake is an ecosystem in equi-librion with the surrounding environment Its geochemical conditions depend on many factors that include the climate and local geological events in the catchment area These factors are more or less indirectly reflected in the properties of the sedi-ments Magnetic measurements of lake sediments have been used to roughly recon-struct the climate during the Holocene A fully quantitatie paleoclimatic reconstruc-tion needs to be based on a precise model of the interaction between lakes and en-vironments Recent studies pointed out that many naturals system and even the entire Earth are governed by nonlinear laws The profund consequences of this fact is that these system can display a cahotic behaviuor Nonliearity has been confirmed in the case of Baldeggersee where the answer of the lake to a nutrient input cycling was studied on the magnetic properties of the sediments (Fig 11)

50 100 time

Ncrit

min

Ncritmax

nutr

ient

s(b)

oxic

sub-

oxic

anoxic

AR

M r

atio

magnetization

oligotrophic

eutrophic

0

1

2

3

0 1 2 3

kA

RM

IR

M o

f BS

+B

H m

mA

minus1

IRMBS+BH IRMD+EX

1972

1982

1984

1999

1942

1910

1885

lt1700 (a)

Fig 11 (a) Cycling of Baldeggersee sediments through an eutrophication event The x-axis describes the total concentration of magnetosomes the y-axis shows a grain size parameter that becomes small if the particles are dissolved Before the industrializa-tion the lake was oligotrophic as shown by the low concentration of magnetosomes For a certain period the amount of magntosomes increased along with the primary production until reductive dissolution started in 1910 The concentration of magnto-somes was relatively constant until 1942 and decreases abruptly until a restoration program was started in 1982 Notice that recovering occurs along a different path as expected from a nonlinear system (b) A model for the magnetic properties of the sediments as a function of the nutrient imput simulated by a sinusoidal curve with some random spikes due to extreme local events (storms landslides) The model reproduces accurately the features of (a) [from the same source of Fig 10]

14

Isothermal remanent magnetization (IRM) and magntization curves The IRM is the permanent magnetization acquired by an assemblage of ferrimagnetic grains after a (strong) magnetic field was applied Each magnetic grain is a miniature compass needle with a permanent dipole moment The minimum field required to re-verse the direction of such dipole is called the switching field Each magnetic grain has a switching field that depends on its size mineralogy and other physical pro-perties An assemblage of magnetic grains all with a different switching field changes its remanent magnetization when a magnetic field is applied The application of suc-cessively increasing fields is used to measure a so-called magnetization curve The derivative of such curve is the statistical distribution of switching fields also called coercivity distribution (Fig 7)

1 10 1000

80

120

40

D+EX

BS

Fig 7 (a) Distribtuion of switching fields measured on a lake sediment from Bal-deggersee (black curve) This distribution has been modelled with a liear combination of different model curves (coloured lines) each representing the magnetic contribution of a specific set of magnetic particles BS and BH are two groups of magnetosomes the difference between them is probably due to their shape D and EX are detrital par-ticles and extracellular magnetite which could not be modeled separately in this sample [redrawn from R Egli Characterization of individual rock magnetic compo-nents by analysis of remanence curves 1 Unmixing natural sediments Sudia Geo-physica et Geodaetica 48 391-446 2004] (b) Distribution of switching fields for de-trital extracellular and biogenic particles (same abbreviations as in (a)) inferred from the measurement of 122 sediments from different lakes [redrawn from R Egli Characterization of individual rock magnetic components by analysis of remanence curves 3 Bacterial magnetite and natural processes in lakes Physics and Chemistry of the Earth 29 869-884 2004] Minerals that can display switching fields above 300 mT are called high-coercivity minerals On the other hand magnetite and maghemite are low-coercivity minerals A semiquantitative parameter that indicates the relative amount of low coercivity minerals in a sample is the so-called S-ratio S (S stay for saturation) defined as

rs 300

rs

M IRMS

Mminus

=

where is the remanent magnetization acquired in a 300 mT field indicates that there are no high-coercivity minerals

300IRM 1S =

BH

H0

1

2

3

4

1 10 100

switching field mT

mag

net

ic c

on

trib

uti

on

BS

DEX

mag

net

ic c

on

trib

uti

on

switching field mT

(a) (b) BH

9

Anhysteretic remanent magnetization (ARM) and the King plot ARM is a particular kind of magntization acquired in an alternating field whose amplitude slowly decreases to zero If the field is perfectly symmetric the remanent magnetization of the sample after this treatement is zero (it is said that the sample has been AF demagnetized) However if a small constant bias field is added to the alternating field the sample acquires a magnetization called ARM This magnetization is proportional to the bias field and is typically much smaller than the saturation remanence An interesting property of the ARM is its extreme grain size sensitivity the highest ARMs are acquired by single-domain grains (which correspond to 30-100 nm grain size in magnetite) The efficiency of the ARM is evaluated by normalizing it with the saturation remanence or with the susceptibility Hence the parameters and (hereafter called ARM ratios) are sometimes used as rough grains size estimators High ARM ratios ( and ) are characteristic for fine-grained (single-domain) particles and magnetosome chains The so-called King plot combines the parameters χ and and a modified version is used to track the formation of authigenic magnetite in lake sediments (Fig 8)

DCH

DC rs( )ARM H M DC( )ARM H χ

3DC rs( ) 10 mAARM H M minus minusgt 1

DC( ) 5ARM H χ gt

DC( )ARM H χ

Fig 8 (a) The King plot is the susceptibility of the ferrimagnetic grains

is the so-called ARM susceptibility Dots are measurements of 01 microm magnetite particles at different concetrations Particles with a given grain size plot along lines whose slope is the ARM ratio Steeper lines correspond to smaller grain sizes (b) The ldquoOldfield plotrdquo is a modified version of the King plot where the ARM is normalized by the susceptibility (x-axis) and frequency dependence of suscep-tibility (y-axis) The shaded areas indicate typical values for detrital magnetite (A) and magnetosomes (B) [redrawn from F Oldfield Toward the discrimination of fine grained ferrimagnets by magnetic measurements in lake and near-shore marine sediments Journal of Geophysical Research 99 9045-9050 1994]

ferriχARM DCARM Hχ =

FDχ

10

Thermomagnetic curves Low temperature measurements can be used to identify mineral-specific transition temperatures as well as for the identification of superparamagnetic particles that become permanently magnetized below room temperature High temperature mea-surements of lake sediments rich in organic matter are difficult because the high surface-to-mass ratio of the authigenic minerals and the presence of organic matter accelerates the irreversible alteration of many magntic parameters above 200degC 4 Characterization of an eutrophication event

The goal of this case study is the characterization of the last eutrophication event of Baldeggersee which began in around 1900 For this purpose you will measure seven sediment samples labelled BAL-GR BA9-75 G101 G044 G010 AR23 and MV1 BAL-GR is a sample of sediment collected in a zone of the catchment area that was eroded by a small stream that ends in Baldeggersee This sample is expected to re-present the actual detrital input of the lake BA9-75 was collected near the bottom of the sediment core This sediment was deposited about 14000 years ago soon after the formation of the lake G101 G044 and G010 are sediment samples collected at depths of 101 44 and 10 cm respectively They represent three stages of the last eutrophication event G101 was deposited at the time of the first human settlements around the lake G044 marks the onset of eutrophication and G010 is totally anoxic Bot G044 and G010 display chemical varves (no bioturbation) AR23 is a sediment taken from the Aral sea (UzbekistanKazakstan) MV1 is a cultured sample of the magnetotactic bacterium Magetotactic Vibrium 1 and provides a reference for the magnetic properties of magnetosomes The results of low-field susceptibilty hysteresis and ARM measurememts are summarized in Tab 1 and Fig 9 Sample χ

3 1nm kgminus paraχ

3 1nm kgminus ferriχ

3 1nm kgminus ARMχ

3 1nm kgminus rs sM M cr cH H

BAL-GR BA9-75 G101 G044 G010 AR23 MV1

1195 510

1307 951 497

1580 158

326184594297207827

00

869326713654291753158

629 253 2140 3440 301 4250 3330

0208 0330 0237 0340 0249 0373 0482

3352 2158 1992 1857 2474 1743 1161

Tab 1 Summary of the measurements The low-field susceptibility χ has been mea-sured with a Kappabridge KLY-2 The paramagnetic susceptibility and the Day plot parameters and were obtained from hysteresis loops measured with a Micromag Vibrating Sample Magnetometer The ARM was imparted with a D-TECH D2000 AF system (maximum AF field 200 mT DC field 01 mT) and measured with a 2G cryogenic magnetometer The susceptibility of ARM was calculated by dividing the ARM magnetization by the DC field

paraχ

rs sM M cr cH H

ARMχ

11

00

02

04

06

BAL-GR

MV-1

AR23G044 BA9-75

G010G101

1 2 3 4 5 60

1

2

3

4

5

BAL-GR

AR23

MV-1 G044

G101

G010 BA9-75

0 20 40 60 80 100

(a) (b)

ferri3

AR

M3

nm kg

nm

kg

χ

χ

minus

minus

1

1

SD

MD

MM

H H

rss

cr c

Fig 9 (a) Representation of the measurements (open dots) on a Day plot The solid line is a model for the binary mixture of single-domain (SD) and multidomain (MD) magnetite (b) Representation of the measurements (open dots) on a King plot The solid line refers to the ARM ratio of 20 microm magnetite grains All lake sediments show a clear trend indicated by the dashed line to the right This line does not intersect the origin indicating a different composition of the samples Samples G010 BA9-75 and BAL-GR are also compatible with the line given by 20 microm magnetite The dashed line to the left is the calculated trend for magnetosomes [calculated after R Egli and W Lowrie Anhysteretic remanent magnetization of fine magnetic particles Journal of Geophysical Research 107 doi 1010292001JB000671 2002]

Figure 9 shows some interesting features that help us with the interpretation of the magnetic composition of the sediments The Day plot shows that the sediments are a mixture of relatively coarse magnetic grains probably of detrital origin and fine SD authigenic magnetite One endmember of this binary mixture are obviously the mag-netosomes in MV1 The other endmember is BAL-GR the material trasported into the lake from the catchment area The binary mixing trend is less steep than the mo-del curve in Fig 9a probably because BAL-GR does not contain only magnetite Mixtures of coarse magnetite maghemite or hematite will not necessary fall into the MD range predicted by the Day plot More detailed informations can be obtained from the King plot in Fig 9b Samples with the same magnetic composition but dif-ferent concentration of magnetic minerals would plot along lines that intercept the origin This is the case for G010 BA9-75 and BAL-GR BAL-GR and BA9-75 are expected to be similar in composition because during the early hystory of the lake the primary productivity was low and the production of authigenic minerals in the sediment is negligible The only difference between BAL-GR and BA9-75 is given by the concentration of ferrimagnetic minerals which is lower by a factor 3 in the lake sediment Why this difference Baldeggersee is a hard water lake and the sediments contain more than 50 calcium carbonate that precipitated from the water The lower concentration of magnetic minerals is simply produced by a dilution effect in

12

the nonmagnetic carbonate All lake sediment samples plot along a trend defined by a dashed line in Fig 9b This line does not intercept the origin demonstrating the variable composition of the lake sediments The second dashed line indicates the trend predicted for magnetosomes which is confirmed by the measurement of MV1 If we assume that lake sediments are binary mixtures of coarse detrital magnetite with a roughtly constant concentration and single-domain authigenic magnetite with a variable concentration the resulting mixing trend is a line whose slope is idetical to that of the single-domain grains The actual slope is slightly smaller than that pre-dicted by this model The reason of this discrepancy is that our simple model did not consider the production of extracellular magnetite which is in part superparamag-netic Superparamagntic grains are characterized by and lower the slope of the mixing trend predicted by the model

ARM 0χ =

One last feature to explain is the fact that despite G010 was sedimented during a period of very high primary productivity (around 1970 triggered by phosphor rich sewage waters) its properties are similar to those of BA9-75 (very low productivity) How is this possible To explain this apparent contradiction more detailed measure-ments are necessary The precise measurement of the distribution of switching field (Fig 7) allows the quantification of the different groups of magnetic grains that occur in the sediment With this method it was possible to dempnstrate that the autigenic magnetie was dissolved under anoxic conditions Fig 10 show the results of these measurements

Fig 10 Concentration of the two groups of magnetosomes BS and BH shown in Fig 7 as a function of depth for the uppermost 70 cm of sediments in Baldeggersee (Switzerland) The magnetic contributions have been normalized by the magnetization of the detrital plarticles Notice that the concentration of magnetosomes peaks at 44 cm and dorps to 13 of the original value at 10 cm This depth corresponds to the period of most severe eutrophication when the hypolimnion was completely anoxic The drop in the concentration of magneto-somes can be explained by ther dissolution The dissolution is more rapid for small particles therefore detrital magnetite was less affected The lake restoration program started in 1982 is clearly marked by an increase in the concentra-tion of magnetosomes The decreasing trend af-ter 1982 is due to the fact that the amout of oxygen pumped into the hypolimnium was de-creased [from R Egli Characterization of individual rock magnetic components by analy-sis of remanence curves 3 Bacterial magnetite and natural processes in lakes Physics and

Chemistry of the Earth 29 869-884 2004]

0 1

0

20

40

60

2

dept

h c

m

IRMBS IRMD+EX IRMBH IRMD+EX

1982

1942

13

5 Possible applications

We have seen how it is possible to characterize the iron cycle in lake sediments by quantifying the production of authigenic magnetite A lake is an ecosystem in equi-librion with the surrounding environment Its geochemical conditions depend on many factors that include the climate and local geological events in the catchment area These factors are more or less indirectly reflected in the properties of the sedi-ments Magnetic measurements of lake sediments have been used to roughly recon-struct the climate during the Holocene A fully quantitatie paleoclimatic reconstruc-tion needs to be based on a precise model of the interaction between lakes and en-vironments Recent studies pointed out that many naturals system and even the entire Earth are governed by nonlinear laws The profund consequences of this fact is that these system can display a cahotic behaviuor Nonliearity has been confirmed in the case of Baldeggersee where the answer of the lake to a nutrient input cycling was studied on the magnetic properties of the sediments (Fig 11)

50 100 time

Ncrit

min

Ncritmax

nutr

ient

s(b)

oxic

sub-

oxic

anoxic

AR

M r

atio

magnetization

oligotrophic

eutrophic

0

1

2

3

0 1 2 3

kA

RM

IR

M o

f BS

+B

H m

mA

minus1

IRMBS+BH IRMD+EX

1972

1982

1984

1999

1942

1910

1885

lt1700 (a)

Fig 11 (a) Cycling of Baldeggersee sediments through an eutrophication event The x-axis describes the total concentration of magnetosomes the y-axis shows a grain size parameter that becomes small if the particles are dissolved Before the industrializa-tion the lake was oligotrophic as shown by the low concentration of magnetosomes For a certain period the amount of magntosomes increased along with the primary production until reductive dissolution started in 1910 The concentration of magnto-somes was relatively constant until 1942 and decreases abruptly until a restoration program was started in 1982 Notice that recovering occurs along a different path as expected from a nonlinear system (b) A model for the magnetic properties of the sediments as a function of the nutrient imput simulated by a sinusoidal curve with some random spikes due to extreme local events (storms landslides) The model reproduces accurately the features of (a) [from the same source of Fig 10]

14

Anhysteretic remanent magnetization (ARM) and the King plot ARM is a particular kind of magntization acquired in an alternating field whose amplitude slowly decreases to zero If the field is perfectly symmetric the remanent magnetization of the sample after this treatement is zero (it is said that the sample has been AF demagnetized) However if a small constant bias field is added to the alternating field the sample acquires a magnetization called ARM This magnetization is proportional to the bias field and is typically much smaller than the saturation remanence An interesting property of the ARM is its extreme grain size sensitivity the highest ARMs are acquired by single-domain grains (which correspond to 30-100 nm grain size in magnetite) The efficiency of the ARM is evaluated by normalizing it with the saturation remanence or with the susceptibility Hence the parameters and (hereafter called ARM ratios) are sometimes used as rough grains size estimators High ARM ratios ( and ) are characteristic for fine-grained (single-domain) particles and magnetosome chains The so-called King plot combines the parameters χ and and a modified version is used to track the formation of authigenic magnetite in lake sediments (Fig 8)

DCH

DC rs( )ARM H M DC( )ARM H χ

3DC rs( ) 10 mAARM H M minus minusgt 1

DC( ) 5ARM H χ gt

DC( )ARM H χ

Fig 8 (a) The King plot is the susceptibility of the ferrimagnetic grains

is the so-called ARM susceptibility Dots are measurements of 01 microm magnetite particles at different concetrations Particles with a given grain size plot along lines whose slope is the ARM ratio Steeper lines correspond to smaller grain sizes (b) The ldquoOldfield plotrdquo is a modified version of the King plot where the ARM is normalized by the susceptibility (x-axis) and frequency dependence of suscep-tibility (y-axis) The shaded areas indicate typical values for detrital magnetite (A) and magnetosomes (B) [redrawn from F Oldfield Toward the discrimination of fine grained ferrimagnets by magnetic measurements in lake and near-shore marine sediments Journal of Geophysical Research 99 9045-9050 1994]

ferriχARM DCARM Hχ =

FDχ

10

Thermomagnetic curves Low temperature measurements can be used to identify mineral-specific transition temperatures as well as for the identification of superparamagnetic particles that become permanently magnetized below room temperature High temperature mea-surements of lake sediments rich in organic matter are difficult because the high surface-to-mass ratio of the authigenic minerals and the presence of organic matter accelerates the irreversible alteration of many magntic parameters above 200degC 4 Characterization of an eutrophication event

The goal of this case study is the characterization of the last eutrophication event of Baldeggersee which began in around 1900 For this purpose you will measure seven sediment samples labelled BAL-GR BA9-75 G101 G044 G010 AR23 and MV1 BAL-GR is a sample of sediment collected in a zone of the catchment area that was eroded by a small stream that ends in Baldeggersee This sample is expected to re-present the actual detrital input of the lake BA9-75 was collected near the bottom of the sediment core This sediment was deposited about 14000 years ago soon after the formation of the lake G101 G044 and G010 are sediment samples collected at depths of 101 44 and 10 cm respectively They represent three stages of the last eutrophication event G101 was deposited at the time of the first human settlements around the lake G044 marks the onset of eutrophication and G010 is totally anoxic Bot G044 and G010 display chemical varves (no bioturbation) AR23 is a sediment taken from the Aral sea (UzbekistanKazakstan) MV1 is a cultured sample of the magnetotactic bacterium Magetotactic Vibrium 1 and provides a reference for the magnetic properties of magnetosomes The results of low-field susceptibilty hysteresis and ARM measurememts are summarized in Tab 1 and Fig 9 Sample χ

3 1nm kgminus paraχ

3 1nm kgminus ferriχ

3 1nm kgminus ARMχ

3 1nm kgminus rs sM M cr cH H

BAL-GR BA9-75 G101 G044 G010 AR23 MV1

1195 510

1307 951 497

1580 158

326184594297207827

00

869326713654291753158

629 253 2140 3440 301 4250 3330

0208 0330 0237 0340 0249 0373 0482

3352 2158 1992 1857 2474 1743 1161

Tab 1 Summary of the measurements The low-field susceptibility χ has been mea-sured with a Kappabridge KLY-2 The paramagnetic susceptibility and the Day plot parameters and were obtained from hysteresis loops measured with a Micromag Vibrating Sample Magnetometer The ARM was imparted with a D-TECH D2000 AF system (maximum AF field 200 mT DC field 01 mT) and measured with a 2G cryogenic magnetometer The susceptibility of ARM was calculated by dividing the ARM magnetization by the DC field

paraχ

rs sM M cr cH H

ARMχ

11

00

02

04

06

BAL-GR

MV-1

AR23G044 BA9-75

G010G101

1 2 3 4 5 60

1

2

3

4

5

BAL-GR

AR23

MV-1 G044

G101

G010 BA9-75

0 20 40 60 80 100

(a) (b)

ferri3

AR

M3

nm kg

nm

kg

χ

χ

minus

minus

1

1

SD

MD

MM

H H

rss

cr c

Fig 9 (a) Representation of the measurements (open dots) on a Day plot The solid line is a model for the binary mixture of single-domain (SD) and multidomain (MD) magnetite (b) Representation of the measurements (open dots) on a King plot The solid line refers to the ARM ratio of 20 microm magnetite grains All lake sediments show a clear trend indicated by the dashed line to the right This line does not intersect the origin indicating a different composition of the samples Samples G010 BA9-75 and BAL-GR are also compatible with the line given by 20 microm magnetite The dashed line to the left is the calculated trend for magnetosomes [calculated after R Egli and W Lowrie Anhysteretic remanent magnetization of fine magnetic particles Journal of Geophysical Research 107 doi 1010292001JB000671 2002]

Figure 9 shows some interesting features that help us with the interpretation of the magnetic composition of the sediments The Day plot shows that the sediments are a mixture of relatively coarse magnetic grains probably of detrital origin and fine SD authigenic magnetite One endmember of this binary mixture are obviously the mag-netosomes in MV1 The other endmember is BAL-GR the material trasported into the lake from the catchment area The binary mixing trend is less steep than the mo-del curve in Fig 9a probably because BAL-GR does not contain only magnetite Mixtures of coarse magnetite maghemite or hematite will not necessary fall into the MD range predicted by the Day plot More detailed informations can be obtained from the King plot in Fig 9b Samples with the same magnetic composition but dif-ferent concentration of magnetic minerals would plot along lines that intercept the origin This is the case for G010 BA9-75 and BAL-GR BAL-GR and BA9-75 are expected to be similar in composition because during the early hystory of the lake the primary productivity was low and the production of authigenic minerals in the sediment is negligible The only difference between BAL-GR and BA9-75 is given by the concentration of ferrimagnetic minerals which is lower by a factor 3 in the lake sediment Why this difference Baldeggersee is a hard water lake and the sediments contain more than 50 calcium carbonate that precipitated from the water The lower concentration of magnetic minerals is simply produced by a dilution effect in

12

the nonmagnetic carbonate All lake sediment samples plot along a trend defined by a dashed line in Fig 9b This line does not intercept the origin demonstrating the variable composition of the lake sediments The second dashed line indicates the trend predicted for magnetosomes which is confirmed by the measurement of MV1 If we assume that lake sediments are binary mixtures of coarse detrital magnetite with a roughtly constant concentration and single-domain authigenic magnetite with a variable concentration the resulting mixing trend is a line whose slope is idetical to that of the single-domain grains The actual slope is slightly smaller than that pre-dicted by this model The reason of this discrepancy is that our simple model did not consider the production of extracellular magnetite which is in part superparamag-netic Superparamagntic grains are characterized by and lower the slope of the mixing trend predicted by the model

ARM 0χ =

One last feature to explain is the fact that despite G010 was sedimented during a period of very high primary productivity (around 1970 triggered by phosphor rich sewage waters) its properties are similar to those of BA9-75 (very low productivity) How is this possible To explain this apparent contradiction more detailed measure-ments are necessary The precise measurement of the distribution of switching field (Fig 7) allows the quantification of the different groups of magnetic grains that occur in the sediment With this method it was possible to dempnstrate that the autigenic magnetie was dissolved under anoxic conditions Fig 10 show the results of these measurements

Fig 10 Concentration of the two groups of magnetosomes BS and BH shown in Fig 7 as a function of depth for the uppermost 70 cm of sediments in Baldeggersee (Switzerland) The magnetic contributions have been normalized by the magnetization of the detrital plarticles Notice that the concentration of magnetosomes peaks at 44 cm and dorps to 13 of the original value at 10 cm This depth corresponds to the period of most severe eutrophication when the hypolimnion was completely anoxic The drop in the concentration of magneto-somes can be explained by ther dissolution The dissolution is more rapid for small particles therefore detrital magnetite was less affected The lake restoration program started in 1982 is clearly marked by an increase in the concentra-tion of magnetosomes The decreasing trend af-ter 1982 is due to the fact that the amout of oxygen pumped into the hypolimnium was de-creased [from R Egli Characterization of individual rock magnetic components by analy-sis of remanence curves 3 Bacterial magnetite and natural processes in lakes Physics and

Chemistry of the Earth 29 869-884 2004]

0 1

0

20

40

60

2

dept

h c

m

IRMBS IRMD+EX IRMBH IRMD+EX

1982

1942

13

5 Possible applications

We have seen how it is possible to characterize the iron cycle in lake sediments by quantifying the production of authigenic magnetite A lake is an ecosystem in equi-librion with the surrounding environment Its geochemical conditions depend on many factors that include the climate and local geological events in the catchment area These factors are more or less indirectly reflected in the properties of the sedi-ments Magnetic measurements of lake sediments have been used to roughly recon-struct the climate during the Holocene A fully quantitatie paleoclimatic reconstruc-tion needs to be based on a precise model of the interaction between lakes and en-vironments Recent studies pointed out that many naturals system and even the entire Earth are governed by nonlinear laws The profund consequences of this fact is that these system can display a cahotic behaviuor Nonliearity has been confirmed in the case of Baldeggersee where the answer of the lake to a nutrient input cycling was studied on the magnetic properties of the sediments (Fig 11)

50 100 time

Ncrit

min

Ncritmax

nutr

ient

s(b)

oxic

sub-

oxic

anoxic

AR

M r

atio

magnetization

oligotrophic

eutrophic

0

1

2

3

0 1 2 3

kA

RM

IR

M o

f BS

+B

H m

mA

minus1

IRMBS+BH IRMD+EX

1972

1982

1984

1999

1942

1910

1885

lt1700 (a)

Fig 11 (a) Cycling of Baldeggersee sediments through an eutrophication event The x-axis describes the total concentration of magnetosomes the y-axis shows a grain size parameter that becomes small if the particles are dissolved Before the industrializa-tion the lake was oligotrophic as shown by the low concentration of magnetosomes For a certain period the amount of magntosomes increased along with the primary production until reductive dissolution started in 1910 The concentration of magnto-somes was relatively constant until 1942 and decreases abruptly until a restoration program was started in 1982 Notice that recovering occurs along a different path as expected from a nonlinear system (b) A model for the magnetic properties of the sediments as a function of the nutrient imput simulated by a sinusoidal curve with some random spikes due to extreme local events (storms landslides) The model reproduces accurately the features of (a) [from the same source of Fig 10]

14

Thermomagnetic curves Low temperature measurements can be used to identify mineral-specific transition temperatures as well as for the identification of superparamagnetic particles that become permanently magnetized below room temperature High temperature mea-surements of lake sediments rich in organic matter are difficult because the high surface-to-mass ratio of the authigenic minerals and the presence of organic matter accelerates the irreversible alteration of many magntic parameters above 200degC 4 Characterization of an eutrophication event

The goal of this case study is the characterization of the last eutrophication event of Baldeggersee which began in around 1900 For this purpose you will measure seven sediment samples labelled BAL-GR BA9-75 G101 G044 G010 AR23 and MV1 BAL-GR is a sample of sediment collected in a zone of the catchment area that was eroded by a small stream that ends in Baldeggersee This sample is expected to re-present the actual detrital input of the lake BA9-75 was collected near the bottom of the sediment core This sediment was deposited about 14000 years ago soon after the formation of the lake G101 G044 and G010 are sediment samples collected at depths of 101 44 and 10 cm respectively They represent three stages of the last eutrophication event G101 was deposited at the time of the first human settlements around the lake G044 marks the onset of eutrophication and G010 is totally anoxic Bot G044 and G010 display chemical varves (no bioturbation) AR23 is a sediment taken from the Aral sea (UzbekistanKazakstan) MV1 is a cultured sample of the magnetotactic bacterium Magetotactic Vibrium 1 and provides a reference for the magnetic properties of magnetosomes The results of low-field susceptibilty hysteresis and ARM measurememts are summarized in Tab 1 and Fig 9 Sample χ

3 1nm kgminus paraχ

3 1nm kgminus ferriχ

3 1nm kgminus ARMχ

3 1nm kgminus rs sM M cr cH H

BAL-GR BA9-75 G101 G044 G010 AR23 MV1

1195 510

1307 951 497

1580 158

326184594297207827

00

869326713654291753158

629 253 2140 3440 301 4250 3330

0208 0330 0237 0340 0249 0373 0482

3352 2158 1992 1857 2474 1743 1161

Tab 1 Summary of the measurements The low-field susceptibility χ has been mea-sured with a Kappabridge KLY-2 The paramagnetic susceptibility and the Day plot parameters and were obtained from hysteresis loops measured with a Micromag Vibrating Sample Magnetometer The ARM was imparted with a D-TECH D2000 AF system (maximum AF field 200 mT DC field 01 mT) and measured with a 2G cryogenic magnetometer The susceptibility of ARM was calculated by dividing the ARM magnetization by the DC field

paraχ

rs sM M cr cH H

ARMχ

11

00

02

04

06

BAL-GR

MV-1

AR23G044 BA9-75

G010G101

1 2 3 4 5 60

1

2

3

4

5

BAL-GR

AR23

MV-1 G044

G101

G010 BA9-75

0 20 40 60 80 100

(a) (b)

ferri3

AR

M3

nm kg

nm

kg

χ

χ

minus

minus

1

1

SD

MD

MM

H H

rss

cr c

Fig 9 (a) Representation of the measurements (open dots) on a Day plot The solid line is a model for the binary mixture of single-domain (SD) and multidomain (MD) magnetite (b) Representation of the measurements (open dots) on a King plot The solid line refers to the ARM ratio of 20 microm magnetite grains All lake sediments show a clear trend indicated by the dashed line to the right This line does not intersect the origin indicating a different composition of the samples Samples G010 BA9-75 and BAL-GR are also compatible with the line given by 20 microm magnetite The dashed line to the left is the calculated trend for magnetosomes [calculated after R Egli and W Lowrie Anhysteretic remanent magnetization of fine magnetic particles Journal of Geophysical Research 107 doi 1010292001JB000671 2002]

Figure 9 shows some interesting features that help us with the interpretation of the magnetic composition of the sediments The Day plot shows that the sediments are a mixture of relatively coarse magnetic grains probably of detrital origin and fine SD authigenic magnetite One endmember of this binary mixture are obviously the mag-netosomes in MV1 The other endmember is BAL-GR the material trasported into the lake from the catchment area The binary mixing trend is less steep than the mo-del curve in Fig 9a probably because BAL-GR does not contain only magnetite Mixtures of coarse magnetite maghemite or hematite will not necessary fall into the MD range predicted by the Day plot More detailed informations can be obtained from the King plot in Fig 9b Samples with the same magnetic composition but dif-ferent concentration of magnetic minerals would plot along lines that intercept the origin This is the case for G010 BA9-75 and BAL-GR BAL-GR and BA9-75 are expected to be similar in composition because during the early hystory of the lake the primary productivity was low and the production of authigenic minerals in the sediment is negligible The only difference between BAL-GR and BA9-75 is given by the concentration of ferrimagnetic minerals which is lower by a factor 3 in the lake sediment Why this difference Baldeggersee is a hard water lake and the sediments contain more than 50 calcium carbonate that precipitated from the water The lower concentration of magnetic minerals is simply produced by a dilution effect in

12

the nonmagnetic carbonate All lake sediment samples plot along a trend defined by a dashed line in Fig 9b This line does not intercept the origin demonstrating the variable composition of the lake sediments The second dashed line indicates the trend predicted for magnetosomes which is confirmed by the measurement of MV1 If we assume that lake sediments are binary mixtures of coarse detrital magnetite with a roughtly constant concentration and single-domain authigenic magnetite with a variable concentration the resulting mixing trend is a line whose slope is idetical to that of the single-domain grains The actual slope is slightly smaller than that pre-dicted by this model The reason of this discrepancy is that our simple model did not consider the production of extracellular magnetite which is in part superparamag-netic Superparamagntic grains are characterized by and lower the slope of the mixing trend predicted by the model

ARM 0χ =

One last feature to explain is the fact that despite G010 was sedimented during a period of very high primary productivity (around 1970 triggered by phosphor rich sewage waters) its properties are similar to those of BA9-75 (very low productivity) How is this possible To explain this apparent contradiction more detailed measure-ments are necessary The precise measurement of the distribution of switching field (Fig 7) allows the quantification of the different groups of magnetic grains that occur in the sediment With this method it was possible to dempnstrate that the autigenic magnetie was dissolved under anoxic conditions Fig 10 show the results of these measurements

Fig 10 Concentration of the two groups of magnetosomes BS and BH shown in Fig 7 as a function of depth for the uppermost 70 cm of sediments in Baldeggersee (Switzerland) The magnetic contributions have been normalized by the magnetization of the detrital plarticles Notice that the concentration of magnetosomes peaks at 44 cm and dorps to 13 of the original value at 10 cm This depth corresponds to the period of most severe eutrophication when the hypolimnion was completely anoxic The drop in the concentration of magneto-somes can be explained by ther dissolution The dissolution is more rapid for small particles therefore detrital magnetite was less affected The lake restoration program started in 1982 is clearly marked by an increase in the concentra-tion of magnetosomes The decreasing trend af-ter 1982 is due to the fact that the amout of oxygen pumped into the hypolimnium was de-creased [from R Egli Characterization of individual rock magnetic components by analy-sis of remanence curves 3 Bacterial magnetite and natural processes in lakes Physics and

Chemistry of the Earth 29 869-884 2004]

0 1

0

20

40

60

2

dept

h c

m

IRMBS IRMD+EX IRMBH IRMD+EX

1982

1942

13

5 Possible applications

We have seen how it is possible to characterize the iron cycle in lake sediments by quantifying the production of authigenic magnetite A lake is an ecosystem in equi-librion with the surrounding environment Its geochemical conditions depend on many factors that include the climate and local geological events in the catchment area These factors are more or less indirectly reflected in the properties of the sedi-ments Magnetic measurements of lake sediments have been used to roughly recon-struct the climate during the Holocene A fully quantitatie paleoclimatic reconstruc-tion needs to be based on a precise model of the interaction between lakes and en-vironments Recent studies pointed out that many naturals system and even the entire Earth are governed by nonlinear laws The profund consequences of this fact is that these system can display a cahotic behaviuor Nonliearity has been confirmed in the case of Baldeggersee where the answer of the lake to a nutrient input cycling was studied on the magnetic properties of the sediments (Fig 11)

50 100 time

Ncrit

min

Ncritmax

nutr

ient

s(b)

oxic

sub-

oxic

anoxic

AR

M r

atio

magnetization

oligotrophic

eutrophic

0

1

2

3

0 1 2 3

kA

RM

IR

M o

f BS

+B

H m

mA

minus1

IRMBS+BH IRMD+EX

1972

1982

1984

1999

1942

1910

1885

lt1700 (a)

Fig 11 (a) Cycling of Baldeggersee sediments through an eutrophication event The x-axis describes the total concentration of magnetosomes the y-axis shows a grain size parameter that becomes small if the particles are dissolved Before the industrializa-tion the lake was oligotrophic as shown by the low concentration of magnetosomes For a certain period the amount of magntosomes increased along with the primary production until reductive dissolution started in 1910 The concentration of magnto-somes was relatively constant until 1942 and decreases abruptly until a restoration program was started in 1982 Notice that recovering occurs along a different path as expected from a nonlinear system (b) A model for the magnetic properties of the sediments as a function of the nutrient imput simulated by a sinusoidal curve with some random spikes due to extreme local events (storms landslides) The model reproduces accurately the features of (a) [from the same source of Fig 10]

14

00

02

04

06

BAL-GR

MV-1

AR23G044 BA9-75

G010G101

1 2 3 4 5 60

1

2

3

4

5

BAL-GR

AR23

MV-1 G044

G101

G010 BA9-75

0 20 40 60 80 100

(a) (b)

ferri3

AR

M3

nm kg

nm

kg

χ

χ

minus

minus

1

1

SD

MD

MM

H H

rss

cr c

Fig 9 (a) Representation of the measurements (open dots) on a Day plot The solid line is a model for the binary mixture of single-domain (SD) and multidomain (MD) magnetite (b) Representation of the measurements (open dots) on a King plot The solid line refers to the ARM ratio of 20 microm magnetite grains All lake sediments show a clear trend indicated by the dashed line to the right This line does not intersect the origin indicating a different composition of the samples Samples G010 BA9-75 and BAL-GR are also compatible with the line given by 20 microm magnetite The dashed line to the left is the calculated trend for magnetosomes [calculated after R Egli and W Lowrie Anhysteretic remanent magnetization of fine magnetic particles Journal of Geophysical Research 107 doi 1010292001JB000671 2002]

Figure 9 shows some interesting features that help us with the interpretation of the magnetic composition of the sediments The Day plot shows that the sediments are a mixture of relatively coarse magnetic grains probably of detrital origin and fine SD authigenic magnetite One endmember of this binary mixture are obviously the mag-netosomes in MV1 The other endmember is BAL-GR the material trasported into the lake from the catchment area The binary mixing trend is less steep than the mo-del curve in Fig 9a probably because BAL-GR does not contain only magnetite Mixtures of coarse magnetite maghemite or hematite will not necessary fall into the MD range predicted by the Day plot More detailed informations can be obtained from the King plot in Fig 9b Samples with the same magnetic composition but dif-ferent concentration of magnetic minerals would plot along lines that intercept the origin This is the case for G010 BA9-75 and BAL-GR BAL-GR and BA9-75 are expected to be similar in composition because during the early hystory of the lake the primary productivity was low and the production of authigenic minerals in the sediment is negligible The only difference between BAL-GR and BA9-75 is given by the concentration of ferrimagnetic minerals which is lower by a factor 3 in the lake sediment Why this difference Baldeggersee is a hard water lake and the sediments contain more than 50 calcium carbonate that precipitated from the water The lower concentration of magnetic minerals is simply produced by a dilution effect in

12

the nonmagnetic carbonate All lake sediment samples plot along a trend defined by a dashed line in Fig 9b This line does not intercept the origin demonstrating the variable composition of the lake sediments The second dashed line indicates the trend predicted for magnetosomes which is confirmed by the measurement of MV1 If we assume that lake sediments are binary mixtures of coarse detrital magnetite with a roughtly constant concentration and single-domain authigenic magnetite with a variable concentration the resulting mixing trend is a line whose slope is idetical to that of the single-domain grains The actual slope is slightly smaller than that pre-dicted by this model The reason of this discrepancy is that our simple model did not consider the production of extracellular magnetite which is in part superparamag-netic Superparamagntic grains are characterized by and lower the slope of the mixing trend predicted by the model

ARM 0χ =

One last feature to explain is the fact that despite G010 was sedimented during a period of very high primary productivity (around 1970 triggered by phosphor rich sewage waters) its properties are similar to those of BA9-75 (very low productivity) How is this possible To explain this apparent contradiction more detailed measure-ments are necessary The precise measurement of the distribution of switching field (Fig 7) allows the quantification of the different groups of magnetic grains that occur in the sediment With this method it was possible to dempnstrate that the autigenic magnetie was dissolved under anoxic conditions Fig 10 show the results of these measurements

Fig 10 Concentration of the two groups of magnetosomes BS and BH shown in Fig 7 as a function of depth for the uppermost 70 cm of sediments in Baldeggersee (Switzerland) The magnetic contributions have been normalized by the magnetization of the detrital plarticles Notice that the concentration of magnetosomes peaks at 44 cm and dorps to 13 of the original value at 10 cm This depth corresponds to the period of most severe eutrophication when the hypolimnion was completely anoxic The drop in the concentration of magneto-somes can be explained by ther dissolution The dissolution is more rapid for small particles therefore detrital magnetite was less affected The lake restoration program started in 1982 is clearly marked by an increase in the concentra-tion of magnetosomes The decreasing trend af-ter 1982 is due to the fact that the amout of oxygen pumped into the hypolimnium was de-creased [from R Egli Characterization of individual rock magnetic components by analy-sis of remanence curves 3 Bacterial magnetite and natural processes in lakes Physics and

Chemistry of the Earth 29 869-884 2004]

0 1

0

20

40

60

2

dept

h c

m

IRMBS IRMD+EX IRMBH IRMD+EX

1982

1942

13

5 Possible applications

We have seen how it is possible to characterize the iron cycle in lake sediments by quantifying the production of authigenic magnetite A lake is an ecosystem in equi-librion with the surrounding environment Its geochemical conditions depend on many factors that include the climate and local geological events in the catchment area These factors are more or less indirectly reflected in the properties of the sedi-ments Magnetic measurements of lake sediments have been used to roughly recon-struct the climate during the Holocene A fully quantitatie paleoclimatic reconstruc-tion needs to be based on a precise model of the interaction between lakes and en-vironments Recent studies pointed out that many naturals system and even the entire Earth are governed by nonlinear laws The profund consequences of this fact is that these system can display a cahotic behaviuor Nonliearity has been confirmed in the case of Baldeggersee where the answer of the lake to a nutrient input cycling was studied on the magnetic properties of the sediments (Fig 11)

50 100 time

Ncrit

min

Ncritmax

nutr

ient

s(b)

oxic

sub-

oxic

anoxic

AR

M r

atio

magnetization

oligotrophic

eutrophic

0

1

2

3

0 1 2 3

kA

RM

IR

M o

f BS

+B

H m

mA

minus1

IRMBS+BH IRMD+EX

1972

1982

1984

1999

1942

1910

1885

lt1700 (a)

Fig 11 (a) Cycling of Baldeggersee sediments through an eutrophication event The x-axis describes the total concentration of magnetosomes the y-axis shows a grain size parameter that becomes small if the particles are dissolved Before the industrializa-tion the lake was oligotrophic as shown by the low concentration of magnetosomes For a certain period the amount of magntosomes increased along with the primary production until reductive dissolution started in 1910 The concentration of magnto-somes was relatively constant until 1942 and decreases abruptly until a restoration program was started in 1982 Notice that recovering occurs along a different path as expected from a nonlinear system (b) A model for the magnetic properties of the sediments as a function of the nutrient imput simulated by a sinusoidal curve with some random spikes due to extreme local events (storms landslides) The model reproduces accurately the features of (a) [from the same source of Fig 10]

14

the nonmagnetic carbonate All lake sediment samples plot along a trend defined by a dashed line in Fig 9b This line does not intercept the origin demonstrating the variable composition of the lake sediments The second dashed line indicates the trend predicted for magnetosomes which is confirmed by the measurement of MV1 If we assume that lake sediments are binary mixtures of coarse detrital magnetite with a roughtly constant concentration and single-domain authigenic magnetite with a variable concentration the resulting mixing trend is a line whose slope is idetical to that of the single-domain grains The actual slope is slightly smaller than that pre-dicted by this model The reason of this discrepancy is that our simple model did not consider the production of extracellular magnetite which is in part superparamag-netic Superparamagntic grains are characterized by and lower the slope of the mixing trend predicted by the model

ARM 0χ =

One last feature to explain is the fact that despite G010 was sedimented during a period of very high primary productivity (around 1970 triggered by phosphor rich sewage waters) its properties are similar to those of BA9-75 (very low productivity) How is this possible To explain this apparent contradiction more detailed measure-ments are necessary The precise measurement of the distribution of switching field (Fig 7) allows the quantification of the different groups of magnetic grains that occur in the sediment With this method it was possible to dempnstrate that the autigenic magnetie was dissolved under anoxic conditions Fig 10 show the results of these measurements

Fig 10 Concentration of the two groups of magnetosomes BS and BH shown in Fig 7 as a function of depth for the uppermost 70 cm of sediments in Baldeggersee (Switzerland) The magnetic contributions have been normalized by the magnetization of the detrital plarticles Notice that the concentration of magnetosomes peaks at 44 cm and dorps to 13 of the original value at 10 cm This depth corresponds to the period of most severe eutrophication when the hypolimnion was completely anoxic The drop in the concentration of magneto-somes can be explained by ther dissolution The dissolution is more rapid for small particles therefore detrital magnetite was less affected The lake restoration program started in 1982 is clearly marked by an increase in the concentra-tion of magnetosomes The decreasing trend af-ter 1982 is due to the fact that the amout of oxygen pumped into the hypolimnium was de-creased [from R Egli Characterization of individual rock magnetic components by analy-sis of remanence curves 3 Bacterial magnetite and natural processes in lakes Physics and

Chemistry of the Earth 29 869-884 2004]

0 1

0

20

40

60

2

dept

h c

m

IRMBS IRMD+EX IRMBH IRMD+EX

1982

1942

13

5 Possible applications

We have seen how it is possible to characterize the iron cycle in lake sediments by quantifying the production of authigenic magnetite A lake is an ecosystem in equi-librion with the surrounding environment Its geochemical conditions depend on many factors that include the climate and local geological events in the catchment area These factors are more or less indirectly reflected in the properties of the sedi-ments Magnetic measurements of lake sediments have been used to roughly recon-struct the climate during the Holocene A fully quantitatie paleoclimatic reconstruc-tion needs to be based on a precise model of the interaction between lakes and en-vironments Recent studies pointed out that many naturals system and even the entire Earth are governed by nonlinear laws The profund consequences of this fact is that these system can display a cahotic behaviuor Nonliearity has been confirmed in the case of Baldeggersee where the answer of the lake to a nutrient input cycling was studied on the magnetic properties of the sediments (Fig 11)

50 100 time

Ncrit

min

Ncritmax

nutr

ient

s(b)

oxic

sub-

oxic

anoxic

AR

M r

atio

magnetization

oligotrophic

eutrophic

0

1

2

3

0 1 2 3

kA

RM

IR

M o

f BS

+B

H m

mA

minus1

IRMBS+BH IRMD+EX

1972

1982

1984

1999

1942

1910

1885

lt1700 (a)

Fig 11 (a) Cycling of Baldeggersee sediments through an eutrophication event The x-axis describes the total concentration of magnetosomes the y-axis shows a grain size parameter that becomes small if the particles are dissolved Before the industrializa-tion the lake was oligotrophic as shown by the low concentration of magnetosomes For a certain period the amount of magntosomes increased along with the primary production until reductive dissolution started in 1910 The concentration of magnto-somes was relatively constant until 1942 and decreases abruptly until a restoration program was started in 1982 Notice that recovering occurs along a different path as expected from a nonlinear system (b) A model for the magnetic properties of the sediments as a function of the nutrient imput simulated by a sinusoidal curve with some random spikes due to extreme local events (storms landslides) The model reproduces accurately the features of (a) [from the same source of Fig 10]

14

5 Possible applications

We have seen how it is possible to characterize the iron cycle in lake sediments by quantifying the production of authigenic magnetite A lake is an ecosystem in equi-librion with the surrounding environment Its geochemical conditions depend on many factors that include the climate and local geological events in the catchment area These factors are more or less indirectly reflected in the properties of the sedi-ments Magnetic measurements of lake sediments have been used to roughly recon-struct the climate during the Holocene A fully quantitatie paleoclimatic reconstruc-tion needs to be based on a precise model of the interaction between lakes and en-vironments Recent studies pointed out that many naturals system and even the entire Earth are governed by nonlinear laws The profund consequences of this fact is that these system can display a cahotic behaviuor Nonliearity has been confirmed in the case of Baldeggersee where the answer of the lake to a nutrient input cycling was studied on the magnetic properties of the sediments (Fig 11)

50 100 time

Ncrit

min

Ncritmax

nutr

ient

s(b)

oxic

sub-

oxic

anoxic

AR

M r

atio

magnetization

oligotrophic

eutrophic

0

1

2

3

0 1 2 3

kA

RM

IR

M o

f BS

+B

H m

mA

minus1

IRMBS+BH IRMD+EX

1972

1982

1984

1999

1942

1910

1885

lt1700 (a)

Fig 11 (a) Cycling of Baldeggersee sediments through an eutrophication event The x-axis describes the total concentration of magnetosomes the y-axis shows a grain size parameter that becomes small if the particles are dissolved Before the industrializa-tion the lake was oligotrophic as shown by the low concentration of magnetosomes For a certain period the amount of magntosomes increased along with the primary production until reductive dissolution started in 1910 The concentration of magnto-somes was relatively constant until 1942 and decreases abruptly until a restoration program was started in 1982 Notice that recovering occurs along a different path as expected from a nonlinear system (b) A model for the magnetic properties of the sediments as a function of the nutrient imput simulated by a sinusoidal curve with some random spikes due to extreme local events (storms landslides) The model reproduces accurately the features of (a) [from the same source of Fig 10]

14