Paleoclimatology of Dust

8/13/2019 Paleoclimatology of Dust

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Reconstruction of LGM climate from the dust in ice cores and marine sediments

Abstract

Recent studies have shown that mineral dust can affect the earth radiation budget,

biogeochemical cycle, and precipitation. The Inter-governmental Panel on Climate Change

(IPCC) has identified dust as a radiative forcing agent in its climate change reports and considered

it as a climate change variable (Solomon et al., 2007). By examining the level of dust

concentration in ice cores and marine sediment cores, we can infer important information about

the climate of the past. Though dust in ice cores and marine sediments is a better indicator of

dust deposition rather than emission, study of dust activity from various spatially distributed

proxies can give a tentative picture of dust sources, transport and deposition in the past. In

addition, analysis of dust particle size distribution and chemical composition can reveal

information about the wind pattern and climate coupling among different regions of the earth (Xiao

et al., 1995; Stuut et al., 2002). A number of ice cores and marine sediment samples have been

studied to infer paleo dust activity in the earth (Basile et al., 1997; Ruddiman, 1997; Delmonte et

al., 2004). In an effort to reconcile dust data from different sources, The Dust Indicators and

Records of Terrestrial and Marine Paleoenvironments (DIRTMAP) data base has been

established (Kohfeld and Harrison, 2001). This database provides a comprehensive information

of dust at different time-scale and also provides basis for comparison with dust simulation from

current earth system models.

In this work, I review the studies of ice cores and marine sediments including DIRTMAP

focusing on the dust preserved in these proxies in order to understand the last glacial maximum

(LGM) climate. I also examine the discrepancy on dust information from different proxies and their

possible causes. This study will provide a tentative picture of climate of the past along with the

distribution of paleo dust sources during the last glacial maximum.


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1. The dust cycle

Atmospheric dust has implications for earth radiation budget, biogeochemical cycle,

precipitation, human health and visibility. Dust cycle is very complex as the dust has a diverse

origin and a number of atmospheric variables act on it between source and sink. Availability of

fine sediment is a pre-condition for dust emission which has both organic and inorganic origin. In

the paleo-perspective, dust was mainly of inorganic origin associated with rock weathering,

volcanic activity, sea salt emission, and grinding and flouring under large mass such as ice sheets.

In the present day, dust is further complicated by the addition of particles from biomass and fossil

fuel burning. Figure 1 shows the schematic diagram of the dust cycle involving different stages

such as origin, transport and deposition.

Figure 1. The dust cycle showing different stages of the dust cycle and their evolution, adapted

from Shao et al., 2011.


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2. Dust proxies

2.1 Ice cores

Several ice cores have been drilled in Antartica and Greenland for paleoclimate

reconstruction. Though these ice cores were not explicitly drilled for deriving dust records, their

dust content have been extremely useful for climate reconstruction. In addition to temperature

signal and samples of atmospheric gases, ice cores also contain deposited dust which can be

used as a proxy for several climate variables including wind and precipitation. Because dust

deposition in the ice cores results mainly from wet deposition, it also carries precipitation signal.

Further, as the precipitation events involves isotope fractionation, stable oxygen isotope (18)

can also indicate wet deposition. However, because the fractionation is affected by a number of

factors such as source water, seasonality, altitude, latitude etc., precaution is necessary in

interpreting 18record in regards to wet deposition.

The mineralogy, chemical composition and particle size distribution of the dust in the ice

cores give indications about the various aspects of the dust cycle. For example, finer mode in the

particle size distribution (PSD) would mean distant dust sources and coarser mode can imply

proximity to the active dust sources. Ice cores may also contain signals of volcanic eruptions

which can be identified by chemical analysis as the volcanic dust mainly consist of sulfate

particles. In the present climate, dust PSD distribution is lognormally distributed in the range 0.1

to 10 as observed in different field studies. However, there are no such observations of

atmospheric dust concentration in the Antarctic and Greenland where ice cores have been drilled.

Dust concentration data in the ice cores are obtained mostly by direct measurements of particle

mass/number concentrations made with either a Coulter Particle Counter or a laser sensor (e.g.

Lambert et al., 2008).

Deposited dust in the ice cores is most suitable for understanding the sink processes but

it can also provide source and transport pathways as the particle size of deposited dust show


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variable distribution depending upon the source and transport pathways. Dust deposition also

depends upon the snow accumulation rate, vertical profile of the long-range transport, and the

dry/wet deposition of the atmosphere associated with the hydrological cycle. Dust deposited in

the ice core can be the result of both wet and dry deposition, dry deposition being more effective

near source regions because coarser particles tend to settle first.

Ice core records used for dust study has been compiled by DIRTMAP team (Kohfeld and

Harrison, 2001) which have provided a broader understanding of the dust cycle. In addition to

these studies, there are a number of recent studies based on additional ice cores, which are

discussed later.

2.2 Marine sediments

Inorganic material in marine sediments also provide useful information of paleoclimate.

Weathering and erosion process creates a range of fine sediments which are carried to the ocean

through various Aeolian and fluvial processes. Because the sedimentation rate is very low, ocean

basin sediment cores are not very useful proxies. So, sediment cores are good from ocean-land

margins or open ocean lands where there is high sedimentation rates. One main problem for

using marine sediments to infer dust is that the organic or biogenic fraction must be removed to

get useful information about the potential source and chemical composition of the deposited

sediment. Interpretation of dust from marine sediment is more complicated than from ice cores

because sediment in marine sediment is derived from various sources such as Aeolian, fluvial,

weathering and deposition and melting of ice or glaciers. They can also be affected by re-

suspension, gyres, western boundary currents and upwelling. Biogenic fraction can be easily

removed by separating the carbonate fraction but the problem is that some carbonate is also

present in Aeolian dust, specially originating from Sahara, which can underestimate aeolin dust

(Ruddiman, 1997).


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3. Paleoclimate reconstruction

3.1 From ice cores

Petit et al., 1999 analyzed dust concentration in vostok ice core dating back to 420,000

BP and showed that the dust flux is strongly affected by orbital forcing (Figure 2). They also

compared that dust flux in Vostok ice core with ice core results from Greenland and found that

dust concentration in Anatartica correlates well with that in Greenland ice cores dust

concentration. This and many other studies have consistently shown that the dust flux was about

15-25 times higher during the last glacial maximum than the Holocene (Figure 2 (e)). This could

be because of the strengthening of wind speed which is also supported by increased sodium

concentration (sea salt proxy) during the glacial maximum (Figure 2(d)). Such a large increase in

dust flux during the LGM must have affected the regional radiation budget considerably, given our

present understanding of net cooling effect of dust as presented in the IPCC report (Bernstein et

al., 2007). So the increased dust flux during the LGM could have positive feedback on temperature

driven by the stronger orbital cycle. As a direct effect, dust layer scatters the shortwave radiation

reaching the earth and it can enhance the formation of ice in Polar Regions, increasing the albedo.

In the other hand, dust deposition on the snow enhances melting of ice which reduces the albedo

and contributes to the ice volume. Given such complex feedbacks, our understanding of the effect

of dust on radiation budget is still limited though climate system models have improved greatly

both computationally and physically. One of the key problem behind is the complex dust-cloud

interaction mechanism.


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Figure 2. Time series of different proxies at Vostock Ice core (a) deuterium profile (temperature

proxy) (b) 18Oatm (c) seawater 18O (ice volume proxy) (d) sodium profile (proxy for marine

aerosol) (e) dust profile (volume of particles measured using a Coulter counter). Adapted from

Petit et al., 1999.

Lambert et al., 2008 analyzed the ice core dust concentration in EPICA dome C and

found that dust depositional flux and temperature are well correlated during glacial times.They

observed an interesting relationship between temperature (indicated by D) and dust depositional

flux (Figure 3). The crescent shape of the dust flux vs. Drelationship suggests that the dust

fluxes have a higher temperature sensitivity as the climate becomes colder. As most of the present

dust sources are located in the tropics, the above relationship means that the coupling of high

and low latitude became stronger as the temperature becomes colder during the glacial times.

The relation between dust and temperature is weak during the interglacial times (D values

greater than - 405 per mil) which can be due to a number of factors such as reduced wind speed,

precipitation, increased transport towards high latitude etc. The particle size distribution of the


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dust in EPICA ice core shows that the dust particle follows a lognormal distribution with a modal

diameter of about 2 (Lambert et al., 2008), which is surprisingly similar to the PSD of present

day atmospheric dust. As there are no dust sourses close to Antartica, the dust must have been

advected from low lattitudes in the mid to lower troposphere. For such fine particles, dry deposition

is inefficient so the dust deposited in Antartica could either be a result of wet deposition or because

of increased transport.

Figure 3. Dust-temperature relationship at an Antartic ice core (EPICA dome C) adapted from

Lambert et al., 2008. Green and blue dots represent data from 0-430 kyr BP and 430-800 BP,

respectively.

Delmonte et al., 2004 analyzed dust concentration and particle size distribution of dust in

three Antartic ice cores. By analyzing samples (each of 5-6 cm thickness representing about 3


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years of accumulation), they observed about 850 ppb dust concentration maxima during LGM

and a minima of 10 ppb (about 12 kyr BP) during the pre-holocene. The dust isotopic signature

(Sr-87/Sr-86) in three ice cores appear almost identical indicating that they all share a common

dust source. Further, the isotopic signature was compared to the fine soil fraction from South

America which also matched nicely, confirming the dust source to be of Patagonian origin.

Strontium (Sr) and Neodimium (Nd) are used to trace the dust source locations (Biscaye, 1974).

The ratio Sr-87 (radiogenic)/Sr-86(non-radiogenic) changes with time in rocks and soils and it is

preserved wherever they are carried by fluvial/Aeolian action. Simmilarly, Nd isotope ratios are

used to provide information on the source of igneous melts as well as to provide age data. The

various reservoirs within the solid earth will have different values of initial 143Nd/144Nd ratios,

especially with reference to the mantle (Biscaye, 1974).

The Greenland ice cores have also been very useful for studying dust changes in

millennial scale which is not seen in the Antartic ice cores. Fuhrer, Wolff, & Johnsen, 1999 studied

the calcium concentration (mineral dust proxy) in the Greenland ice cores and found high levels

of dust in colder periods than interstadials. The reason behind this change in dust concentration

during stadials/interstadials is yet to be understood. D-O events are the result of calving of North

American ice sheets which have also been related to the strengthening/weakening of Atlantic

Meriodional Overturning Circulation (AMOC).

Moreno et al., 2002 conducted another study based on lithogenic fraction of sediment in

the Mediterranean region and showed that the aeolian dust transport varied with the millennial

scale DansgaardOescher (DO) events, with higher dust transport from the Sahara during cold

stadial periods of the DO cycles. Because the DO events occur in short time scale (about 1500

years), changes in dust concentrations in this time scale reflect changes in wind speed rather

than the sediment availability in the dust source regions.


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3.2 From marine sediments

Ruddiman, 1997 examined terrigenous sediment flux in 12 tropical atlantic marine

sediment cores over the last 25,000 years. They assumed terrigenous sediment flux to be of

Aeolian origin and subtracted CaCO3 and opal fraction (both organic) from the total sediment to

get the Aeolian sediment flux. They obtained most of the cores from topographic highs which

reduces the effect of turbidities and hemiplegic sediment. However, not all stations are in

topographic highs and some are near the coast in west-Africa which can receive sediment from

fluvial processes and some gravity deposits as well. They found that the terrigenous flux (Aeolian)

was greater during the glacial times (Figure 4) in all cores, consistent with the ice core records.

They also found that the dust input didnt change following the early Holocene moisture maximum

which made them believe that the change in dust flux is less related to aridity than to the wind.

Figure 4. Mean dust flux for glacial and interglacial (now) period.


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Another study by Sirocko, et al., 1991 investigated marine sediment cores in the Arabian

gulf. Their method is similar to that of Ruddiman, 1997 but here they also separated biogenic opal

by dissolving in potassium polytungstate solution. With this the net lithogenic sedimentation rate

was calculated. Further, they separated biogenic and Aeolian carbonates using carbonate oxygen

isotope and found that about 15% of carbonate in marine cores was of Aeolian origin. Results

show that the average bulk lithogenic sedimentation rate in Arabian Sea didnt change over the

last 27,000 years. The sedimentation rate gradually decreases northwest to southeast (which is

also the direction of shamal wind, a dominant wind pattern in the region at the present) but again

increases in the southern border of Indian subcontinent and east of Oman probably because of

river borne sediment. The effect of Shamal wind on dust is also seen in the PSD, the fraction of

coarse grains (greater than 6) which decreases northwest to southeast. The coarser grain size

during the LGM could be because of the strengthening of wind.

Magnetic susceptibility of marine sediment has also been studied for identifying breaks in

paleo climate patterns and to verify regional climate trends from other proxies. Such studies are

mainly focused in the Atlantic and Mediterranean region to track the input of Aeolian sediment

especially from the Sahara. In general, marine sediment deposited during wet climate show lower

magnetic susceptibility as a result of lower dust production. However, magnetic susceptibility is

affected by the presence of magnetic (e.g. hematite) and paramagnetic (e.g. clay) substances

and also by the digenetic dissolution of magnetic materials. Because of these issues, they are

often interpreted together with chemical analysis of the sediment. Petit et al., 1990 compared the

magnetic susceptibility of the sediment core from Indian Ocean to the dust flux from Vostok ice

core and found that both of these proxies compare well in representing higher dust flux during the

LGM.


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4. Implications to biogeochemical cycle

A large amount of dust is deposited on the ocean which can modify the biogechemical

cycle as the dust contains nutrition (e.g. Iron and Phosphorous) for phytoplankton. As the

productivity is essentially nutrition-limited, mineral dust input is one of the most important factor

that governs the dust cycle, which ultimately affects the carbon cycle. Generally speaking, one

may believe that the photosynthesis was suppressed during the glacial times which would

increase the CO2 in the atmosphere. However, atmospheric CO2 results from the net effect of a

number of carbon sources and sinks. Solubility of CO2 in the ocean depend upon a number of

factors including salinity, pH and temperature. For example, atmospheric CO2 can be decreased

by the increased solubility of CO2 in water at lower temperature. The so called Iron Hypotheses

(Martin & Fitzwater, 1988) suggested that iron rich atmosphere during glacial times deposited a

lot of iron in the subantartic region increasing the productivity and hence lowering the CO2. It

should also be noted that the degassing of CO2 in the CO2 rich upwelling region would also

increase atmospheric CO2. Given this relation of the dust cycle to the carbon cycle, the lower

level of CO2 during the LGM cant be looked in isolation with the dust feedback.

5. Comparison with model results

Mahowald et al., 1999 compared model simulated dust deposition with ice core and

marine sediment data. Their results indicate that the model simulated dust depositional flux over

tropical and mid-latitude regions is in good agreement with global reconstructions, but it failed to

simulate the 15-25 fold increase observed in glacial dust input over Antarctica. This shortcoming

is currently attributed to an incomplete representation of circulations in the current models. In the

study, the concentration of dust in the ice core was converted to the deposition rate by multiplying

with ice accumulation rate. However, precipitation field is one of the most problematic field in

current GCM and the model doesnt simulate the precipitation/snowfall well in general. Their

results suggest that vegetation played key role in expanding the dust sources during the LGM.


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The precipitation was low during the LGM which reduced the vegetation consequently increasing

the extent of dust sources in the tropics. They also found that lowered CO2 during LGM also

reduced vegetation because it increased evapotranspiration, and reduced photosynthesis by the

C3 plants. It should be noted that their model dont consider the radiative feedback of dust

aerosols and its effect on albedo. Their model also doesnt consider effect of dust on increasing

productivity on the ocean which has feedback through CO2.

Albani et al. 2012 simulated LGM dust conditions using a state of the art coupled climate

model (CESM). By comparing model simulated dust deposition with ice core dust concentration

and particle size distribution, they showed that in addition to the Patagonian dust sources,

Australian source also contributed to the increased dust flux in the Antarctica during the LGM.

Though the land and atmosphere model used in their study is very advanced, the ocean model

used is a slab model which dont really represent the existing ocean circulations including the

atlantic meriodional overturning circulation (AMOC). For accurate dust simulation, instantaneous

wind fields are extremely important as the dust emission is initiated when the wind speed exceeds

a critical threshold known as threshold friction speed. In this regard, ability of the model can be

questioned as the wind simulated by these models is 3-6 hourly average. The current model

outputs show that the 6 hourly average wind doesnt exceed ~ 12 /so the dust emission due

to large scale atmospheric phenomena such as thunderstorm and cyclone cant be simulated by

these models. Also the current models have main problem in reproducing precipitation (which

ultimately affect soil moisture that modifies threshold wind speed for dust emission).

Another issue in modeling is that the simulation of the model is greatly sensitive to the

temporal and spatial resolution of the model. Further, current dust model has the main problem

that they cant accurately represent the spatial distribution of dust sources when compared to

satellite measurements. So, they rely on tuning with the help of satellite datasets (which also have

problems because the dust observed is a mixture of mineral dust, biomass burning and

transported dust). In addition to the problems within the model configuration, inability of the


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models to represent LGM climate conditions is also because of the unavailability of high resolution

input data (soil types, soil moisture, vegetation data, etc) and boundary conditions.

Lunt & Valdes, 2001 conducted a back trajectory analysis of the air mass at Dome C to

understand the transport and sources of dust. They used Ice sheets and land-ocean mask from

a previous study and used CO2 concentration of 200 ppm for forcing. Present day boundary

conditions used were observed SST, ice sheet and orography and CO2 of 345 ppm. Results show

that most of the back trajectories (no. of trajectories passing through each source in a 30 days

period) belong to Patagonian dust source. However, present day back trajectory tracks the dust

source back not only to South America, but also to Australia and South Africa driven mainly by

the westerlies (Figure 5). Another interesting result they observed is that the seasonality is less

pronounced for Patagonian dust sources compared to Australian and South African sources. It is

also seen that present day transport is much stronger than that during the LGM. Given similar

climatic conditions between the LGM and the present, Antarctic dust cant be attributed only to

Patagonia during the LGM. It is interesting to note that present day major dust sources are located

in the Middle East and North Africa and the Patagonian source doesnt exist at all. In this regard,

model didnt show the dust sources that originate from the main dust sources of present day, so

the model used in their study also has weaknesses. Further this work only simulate dry deposition

and not the wet depositions which can be very high as the snow-scavenging is very important in

the Antarctica. One important point to be noted is that since transport is more efficient in the

present than LGM, higher dust during glacial cant be explained by transport, there must be some

changes in the origin/sources or sinks of dust during the glacial time.


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Figure. 5. Back trajectory of air mass for a destination at EPICA dome C, Antartica. It can be seen

that dust to Antartica is directed from diverse source/direction. (Obtained by running HYSPLIT

back trajectory model at http://ready.arl.noaa.gov/HYSPLIT.php)

6. Conclusion

Reconstructing climate of the past from paleo proxies is reliable only when sufficient

observations across the globe are available since climate has large regional differences. Most of

the studies based on ice cores and marine sediment proxies until now generally agree on the

increased dust flux during the glacial times. However, there is an ongoing debate on the origin or

the sources of dust during the LGM. Even with the increasing number of proxies in the future, we

can expect that this debate will go a long way as the deposited dust is controlled by the aridity in

the dust source regions as well as the strength of transporting wind. Chemical/mineral

composition can constrain the distance of the dust sources but they cant completely describe the

sources and direction of the dust especially when the dust originates from multiple sources. In


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this context, climate system modeling has provided promising opportunity to understand the dust

cycle as a component of the whole climate system. Using past boundary conditions, fully coupled

global circulation model can be used to understand the dust transport and deposition processes.

However, the boundary conditions such as topography and sea surface temperature cant be

specified accurately for the past in coarse-resolution climate models. In addition, even the

complex fully coupled models existing today have difficulty in simulating dust sources and

distribution in longer time scale because of our poor understanding of basic physical processes

such as dust-radiation interaction, dust-albedo feedback, biogeochemical cycle and dust-cloud

interaction. With the improvements in modeling and increase in the no. and distribution of proxy

records, we can expect that the various unsolved questions including increased dust flux up to

15-25 times of that of present in the high-lattitude will be resolved in the near future.


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Paleoclimatology of Dust