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Mar$n et al. (1990, Nature) Iron in Antar*c Waters Jickells et al. (2005, Science) Global Iron Connec*ons Between Desert Dust, Ocean Biogeochemistry, and Climate Shi et al. (2012, Aeol. Res.) Impacts on iron solubility in the mineral dust by processes in the source region and the atmosphere: A review
Iron in Antar*c Waters
Mar*n et al. (1990, Nature)
Summary • Tes*ng the hypothesis that Antar*c phytoplankton suffer
from iron deficiency, preven*ng them from blooming and using up major nutrients in the southern ocean
• Highly produc*ve (shallow) Gerlache Strait waters with abundant Fe facilitate phytoplankton blooming and major nutrient removal
• Low produc*vity Drake Passage water, with very low levels of dissolved Fe, uses less than 10 % of the major nutrients available.
• Hypothesis: Iron s*mulated phytoplankton growth may have contributed to the drawning of atmospheric CO2 during glacial maxima
Gerlache Strait vs Drake Passage • Neri*c (shallow) Gerlache Strait shows Fe and Mn 50-‐60
*mes higher than open ocean waters of the Drake Passage (7.4 vs 0.1 nmol/kg)
• Insufficient Fe introduced by upwelling in the Drake passage and low atmospheric dust load
• In the shallow waters of the Gerlache Strait Fe input comes from Fe sediments and Fe released from mel*ng sea
• Assuming that Fe requirement of Phytoplankton is
5000N:1Fe -‐> 7.4 nmol/kg is enough to remove the ~ 24 umol NO3/kg occurring there.
• Clearly, no Fe limita*on -‐> which may explain the very high produc*vity reported (3 g C/(m2 day))
• This contrasts with the es*mate of 0.01 to 0.03 g C / (m2 day) in the Drake passage, which is less than 10 % of the carbon that could be fixed with the available nitrogen.
Conclusions • Very low dissolved Fe in the Drake passage supports the argument that present-‐day produc*vity is limited in offshore waters of the Antar*c because of Fe deficiency
• This may severely limit the power of the ‘biological pump’, contribu*ng to the raised CO2 typical of previous and present interglacial periods.
• -‐> Enhanced Fe from atmospheric dust may thus have contributed to the drawning down of CO2 during glacial maxima.
Global Iron Connec*ons Between Desert Dust, Ocean Biogeochemistry, and Climate
Jickells et al. (2005, Science)
Introduc*on • Iron (Fe): is an essen*al nutrient for all organisms, used in
a variety of enzyme systems, including those for photosynthesis, respira*on, and nitrogen fixa*on.
• Fe supply is a limi*ng factor on phytoplankton growth over vast areas of the modern ocean
• Iron supply: – Fluvial and glacial par*culate iron that is mostly
trapped near coastal areas (except where rivers discharge beyond the shelf)
– Hydrothermal inputs (rapidly precipitated at depth in the ocean). (This is disputed by Tagliabue et al., 2010 Nat. Geosc.; and other studies)
– Dominant external Fe input to the open ocean: aeolian dust (which can be par*cularly sensi*ve to global change pressures)
– Other atmospheric Fe inputs: volcanic, anthropogenic, extraterrestrial.. (lower Fe supply but more Fe soluble frac*on than dust)
Global iron and dust connec*ons
• Four components: land surface and dust availability, atmospheric loading, marine produc*vity, clima*c state.
• Solid arrowhead: posi*ve correla*on
• Open circle: nega*ve correla*on/forcing
• Open arrowhead: uncertain sign
• Mechanism in italics
• Water tap symbol: secondary mechanism modula*ng the primary mechanism
Climate effects on Dust/Iron Fluxes • Clima*c and geomorphological controls on source regions. • Controls on dust emission: rainfall, wind, surface
roughness, temperature, topography, vegeta*on cover. • Dust removal occurs by dry and wet deposi*on, depending
on par*cle size, rainfall paierns and transport al*tude
• Deposi*on es*mates: 1000 to 2000 Tg/year. Significant interannual variability. Highly uncertain. • Produc*on es*mated around 1700 Tg/year. Highly uncertain. 25 % reaching the oceans • Dust fluxes 2 to 20 *mes higher during last glacia*on (? Stronger dust winds, aridity, changes in
vegeta*on cover, lowered sea level, reduced precipita*on) • Land use prac*ces over recent decades have altered dust fluxes (up to 50%, very uncertain). Recent
work suggests 25 %. Global importance unclear, however regionally can be very important.
• Dust from the Sahel probably increased since the 1950’s (Climate and land use) • Variability of dust can be influenced by climate cycles such as el Niño-‐SO and NAO • Enhanced greenhouse warming could “green” the Sahel and southern Sahara. • Predic*ons for the next 100 years range from + 12 % increase to 60% decrease, depending on the
importance of land use changes and CO2 fer*liza*on (HIGHLY UNCERTAIN)
• In a biogeochemical context, the key flux to the oceans is not dust, but soluble or bioavailable iron.
• Fe content of soil dust is 3.5 % on average, variable globally depending on the mineralogical composi*on of the sources
• Fe solubility at the sources is small (<1 to 2%)
• Fe is processed during transport through: – photochemistry (photoreduc*on of Fe III to Fe II) – acidity, par*cularly during aerosol cloud processing (Emissions of acid precursors
have more than doubled from the preanthropogenic state.) – Organic complexa*on: natural (soil humic acids and plant terpenes) and
anthropogenic sources (biomass burning and industrial/urban emissions) may influence atmospheric iron cycling
• All these processes are affected by global change pressures.
Effects on Iron Fluxes
Dust/Iron Impacts on the Ocean • At seawater pH of 8: compe**on between adsorp*on to par*culates in the water column, biological
uptake and organic complexa*on (evolves over tens of days, the water residence *me of dust) • Oceanic profiles:
– Low surface dissolved Fe concentra*ons (0.03 to 1nmol/liter) – Deep water concentra*ons of 0.4 to 2 nmol/liter – Colloidal Fe present and poten*ally labile
• Dissolved Fe in oceans is predominantly organically complexed: this stabilizes it against rapid scavenging (residence *me of decades). Source, biological func*on and structure of organic iron-‐complexing ligands unknown.
• Studies suggest that these ligands have similar strong binding strength as siderophores. (Microbes release siderophores to scavenge Fe from mineral phases by forma*on of soluble complexes)
• Rela*ve importance of atmospheric and upwelling sources varies. Fe/N ra*os of deep water show that sustaining open ocean primary produc*on requires addi*onal input besides upwelling.
• Fe limita*on results in incomplete use of macronutrients N, P, Si, and low algal abundance in the Southern Ocean (HNLC)
• Fe availability influences algal community structure as well as overall produc*vity. • Open ocean phytoplankton generally need less iron than coastal species, which have evolved in a
more iron-‐rich environment, although iron-‐limited coastal systems are known • In addi*on to direct limita*on of primary produc*on in the HNLC regions, iron may limit (or co-‐limit
with P) nitrogen fixa*on by photosynthe*c diazotrophs in tropical oceans, where there are low nitrate concentra*ons in surface waters
• Overall effect will vary between ocean biogeochemical provinces
Dust/Iron Impacts on the Ocean and effects upon climate
• Models and ice core data yield very different results, predic*ng that glacial/interglacial changes in dust fluxes will change atmospheric CO2 by 5 to 45 parts per million (ppm) as a contribu*on to the total change of 80 to 100 ppm
• Radia*ve forcing (CO2)
• Twofold global rise in DMS -‐> global temp decrease of 1oC
• greenhouse gas forcing (nitrous oxide and methane)
• atmospheric oxidizing capacity (isoprene and carbon monoxide)
Research priori*es • Huge uncertain*es in our understanding of these interac*ons, requiring research
that integrates across the whole Earth system. Authors suggest the following research priori*es:
(i) dust deposi*on processes,
(ii) aerosol iron bioavailability,
(iii) the impact of iron on marine nitrogen fixa*on and trace gas emissions.
Impacts on iron solubility in the mineral dust by processes in the source region
and the atmosphere: A review
Shi et al. (2012, Aeolian Research)
Processes controlling specia*on of Fe and deposi*on
Introduc*on
• Fe bioavailability cannot be directly measured chemically -‐> it is assumed that dissolved Fe or highly reac*ve Fe in the dust is bioavailable
• FeT in dust: 1% to 5% depending on source region
• FFS observed to be from 0.1% to more than 80%
• FFS controlled by: – the mineralogy of the soils – atmospheric processes that can convert low-‐reac*vity Fe-‐bearing minerals
into highly soluble/bioavailable forms of Fe
• Biomass burning and anthropogenic pollu*on also provide Fe-‐bearing par*cles which can be a dominant source of bioavailable Fe in some areas of the ocean
Fe Solubility: defini*on and measurements
• Difference in the defini*on of ‘‘solubility’’ between the atmospheric and geochemical communi*es
• Geochemical: concentra*on (ac*vity) of a solute measured in equilibrium with a mineral phase and is therefore independent of mineral mass in the system considered.
• Atmospheric (FFS): ra*o (as a percentage) of the dissolved Fe concentra*on (typically auer filtra*on and passing through 0.2 or 0.45 lm pore size filter) in the filtrate rela*ve to the total Fe contained in the bulk sample
• FFS is heavily dependent on the proper*es of the sample, the extractant/solvent used, extrac*on *me and other experimental protocols. Big problem in the community and a nightmare for modelers (evalua*on)
Soil samples as surrogates for dust in aerosol studies
• Wet sieving has the effect of wevng the surface of the aerosol/dust and will remove the most soluble and labile Fe frac*on from the samples
• Beier to re-‐suspend soil and collect with size selec*ve PM samplers.
• Main reason of using soil: large amounts (g-‐kg), and not subject to changes in the atmosphere
Analy*cal techniques for Fe in dust • X-‐ray diffrac*on (XRD): semi-‐quan*ta*ve mineral composi*on. Problems to
iden*fy and differen*ate between Fe oxides • Sequen*al extrac*on: ascorbate and dithionite extrac*on of Fe oxides. • Ascorbate extrac*on: most reac*ve and poorly crystalline pool of Fe (FeA) • Dithionite extrac*on: quan*ta*vely solubilizes the remaining Fe (oxyhydr)oxides
phases including goethite and hema*te (FeD) • Direct reflectance spectroscopy (DRS) can be used to quan*fy the ra*o of
hema*te to goethite • FeT: X-‐ray fluorescence (XRF) spectrometer and Par*cle induced X-‐ray Emission
(PIXE), among others • Observa*on of individual dust par*cles:
– scanning electron microscopy (SEM) coupled with energy dispersive X-‐ray spectrometry (EDX) – transmission electron microscopy (TEM) coupled with EDX – selected area electron diffrac*on (SAED) and electron energy loss spectrometry (EELS) – synchrotron-‐based X-‐ray absorp*on spectroscopy (XAS) – size and high resolu*on morphology of individual par*cles down to nanometerscale,
mineralogy, Fe specia*on (Fe II / Fe III)
Processes in the dust source regions: Fe minerals in dust
• Ferrihydrite and other poorly crystalline Fe phases • Crystalline hema*te (Fe2O3), goethite (FeOOH) • Clay minerals, such as illite, mixed layer illite/smec*te, and smec*te
• Amorphous Fe minerals including nanopar*cles of ferrihydrite are highly reac*ve and poten*ally bioavailable (generally exhibit a grain size in the nanometer range)
• Ferrihydrite par*cles may be also formed during cloud processing
• Most abundant form of Fe oxides in dust or dust precursors are goethite and hema*te • Synthe*c and commercial Fe oxides are usually well crystallized with characteris*c
morphologies, larger size and well-‐defined surface area. • Fe in clays may also transform into chemically available forms (although they are considered
the most refractory Fe-‐containing minerals)
• Some of the poten*ally available Fe in ‘‘standard’’ clay minerals used in previous works is likely to be the adsorbed Fe oxides par*cles, and Fe located at the edge of clay par*cles while the remainder is refractory Fe held in the clay lavce structure
Processes in the dust source regions: Fe mineralogy in North-‐African dust
• (FeA + FeD)/FeT helps characterizing the Fe mineralogy
• Higher in Sahelian samples (~0.57) than in Saharan Samples (~0.36). Even lower in paleolake samples (~0.2)
• Explained by Fe weathering in soils (higher degree of transforma*on of original Fe-‐bearing minerals into Fe oxides)
• FeA/(FeA + FeD) is usually small (<<0.1), although it can be higher in ephemeral lakes
Forma*on of secondary Fe minerals during chemical weathering
primary Fe minerals Fe(III)
Fe oxyhydroxides Poorly cristalline e.g. Ferrydrite
Crystalline Fe oxides Mainly Hema*te and goethite
(Fe II)
Aluminosilicates Clay minerals
rapid oxida*on hydrolysis
FeSt FeD
FeA
Forma*on of secondary Fe minerals during chemical weathering
Atmospheric processing and impact on FFS
Gravita$onal seCling: • Inverse rela*onship between FFS in Atlan*c
aerosols and atmospheric dust concentra*ons
• First explained in terms of gravita*onal seiling: greater solubility at lower dust mass could be due to a larger surface area to volume ra*o of the finer dust par*cles
• Later results indicated that the size dependence can at most explain a small part of the measured variability in FFS
• Other studies also showed that the aerosol FFS was somewhat variable with size but in general, it did not increase with decreasing par*cle size
Acid processing of dust • Solubility of Fe oxides is highly dependent on the pH of
the aqueous medium it is in contact with, specially for pH < 3
• Interac*on of dust par*cles with cloud water, or cloud processing, provides the main mechanism for uptake of acid gases in the atmosphere, lowering the pH of the cloud water
• Correla*on of the FFS with concentra*ons of acid species in aerosols ambiguous due to complexity
• E.g mineral dust ouen contains a high percentage of carbonate can neutralize the acid in contact with the dust
Atmospheric processing and impact on FFS
Acid processing of dust • First studies found that dissolved Fe appeared in solu*on when a
sample of Saharan dust was exposed to acidic solu*ons at pH 2 which is the pH relevant to wet aerosols.
• When the pH of the solu*on was increased to 5–6, a range
commonly measured in cloud water, the dissolved Fe concentra*on was considerably reduced
• Mineralogy is an important factor in controlling the amount of dissolved Fe
• Commercial Fe oxide (goethite, hema*te, and magne*te) samples have a much lower FFS (pH 2 for 2 h) compared to clay minerals
• There is currently discussion/disagreement whether clays or amorphous Fe nanopar*cles on the surface of clays are responsible for the observed solubility
Atmospheric processing and impact on FFS
Acid processing of dust • Shi’s 3 pool model:
– The Fe dissolu*on kine*cs of samples from Asia and Sahara) could be accurately described using a simple cumula*ve model assuming first-‐order dissolu*on kine*cs of 3 acid-‐extractable pools of Fe.
– They hypothesize: • “fast” Fe pool is low reac*vity dry ferrihydrite and/or
poorly crystalline Fe(III) oxyhydroxides • “slow” Fe pool represents both crystalline Fe oxide
phases (goethite and/or hema*te) and Fe-‐containing clay minerals
• “intermediate” Fe pool: nano-‐sized Fe oxides
• Importance of dust/liquid ra*o as a control for Fe solubiliza*on in dust (problem: dust/liquid ra*os in the lab are approximately 3 orders of magnitude smaller than what is expected in dust aerosol par*cles)
• Because of this, the high Fe concentra*on from the fast pool may suppress the dissolu*on of other Fe phases from the dust
Atmospheric processing and impact on FFS
Photo-‐reduc$on and organic complexa$on • It is possible to photo-‐reduce solid Fe oxides in
solu*on to form dissolved Fe2+ under the condi*ons of UV light at rela*vely low pH
• Photo-‐reduc*on when ac*ng alone has limited impact on FFS
• Organic ligands such as formate, acetate and oxalate are found in atmospheric par*cles and clouds
• These are able to form complexes with dissolved Fe and thus may increase the FFS
Atmospheric processing and impact on FFS