Fis2007c

Fis2007c
Earth and Planetary Science Letters 260 (2007) 340 – 354
www.elsevier.com/locate/epsl
Reconstruction of millennial changes in dust emission, transport and
regional sea ice coverage using the deep EPICA ice cores from
the Atlantic and Indian Ocean sector of Antarctica
Hubertus Fischer a,⁎, Felix Fundel a , Urs Ruth a , Birthe Twarloh a , Anna Wegner a ,
Roberto Udisti b , Silvia Becagli b , Emiliano Castellano b , Andrea Morganti b ,
Mirko Severi b , Eric Wolff c , Genevieve Littot c , Regine Röthlisberger c ,
Rob Mulvaney c , Manuel A. Hutterli d,c , Patrik Kaufmann d ,
Urs Federer d , Fabrice Lambert d , Matthias Bigler d,g ,
Margareta Hansson e , Ulf Jonsell e , Martine de Angelis f ,
Claude Boutron f , Marie-Louise Siggaard-Andersen g ,
Jorgen Peder Steffensen g , Carlo Barbante h,i ,
Vania Gaspari h , Paolo Gabrielli i , Dietmar Wagenbach j
a
Alfred-Wegener-Institute for Polar and Marine Research, Columbusstrasse, D-27568 Bremerhaven, Germany
Department of Chemistry, University of Florence, Via della Lastruccia 3, 50019 Sesto Fiorentino (Florence), Italy
c
British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 0ET, UK
d
Climate and Environmental Physics, Physics Institute, University of Bern, Sidlerstr.5, 3012 Bern, Switzerland
e
Department of Physical Geography and Quaternary Geology Stockholm University, 106 91 Stockholm, Sweden
Laboratoire de Glaciologie et Géophysique de l'Environnement (LGGE), CNRS-UJF, BP96 38402 Saint-Martin-d'Hères Cedex, France
g
Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen OE, Denmark
h
Department of Environmental Sciences, University Ca' Foscari of Venice, Dorsoduro 2137, 30123 Venice, Italy
i
Institute for the Dynamics of Environmental Processes-CNR, Dorsoduro 2137, 30123 Venice, Italy
j
Institute for Environmental Physics, University of Heidelberg, INF229, 69120 Heidelberg, Germany
b
f
Received 12 March 2007; received in revised form 31 May 2007; accepted 1 June 2007
Available online 12 June 2007
Editor: M.L. Delaney
Abstract
Continuous sea salt and mineral dust aerosol records have been studied on the two EPICA (European Project for Ice Coring in
Antarctica) deep ice cores. The joint use of these records from opposite sides of the East Antarctic plateau allows for an estimate of
changes in dust transport and emission intensity as well as for the identification of regional differences in the sea salt aerosol
source. The mineral dust flux records at both sites show a strong coherency over the last 150 kyr related to dust emission changes in
the glacial Patagonian dust source with three times higher dust fluxes in the Atlantic compared to the Indian Ocean sector of the
Southern Ocean (SO). Using a simple conceptual transport model this indicates that transport can explain only 40% of the
atmospheric dust concentration changes in Antarctica, while factor 5–10 changes occurred. Accordingly, the main cause for the
⁎ Corresponding author.
E-mail address: [email protected] (H. Fischer).
0012-821X/$ - see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.epsl.2007.06.014
H. Fischer et al. / Earth and Planetary Science Letters 260 (2007) 340–354
341
strong glacial dust flux changes in Antarctica must lie in environmental changes in Patagonia. Dust emissions, hence environmental
conditions in Patagonia, were very similar during the last two glacials and interglacials, respectively, despite 2–4 °C warmer
temperatures recorded in Antarctica during the penultimate interglacial than today. 2–3 times higher sea salt fluxes found in both
ice cores in the glacial compared to the Holocene are difficult to reconcile with a largely unchanged transport intensity and the
distant open ocean source. The substantial glacial enhancements in sea salt aerosol fluxes can be readily explained assuming sea ice
formation as the main sea salt aerosol source with a significantly larger expansion of (summer) sea ice in the Weddell Sea than in
the Indian Ocean sector. During the penultimate interglacial, our sea salt records point to a 50% reduction of winter sea ice
coverage compared to the Holocene both in the Indian and Atlantic Ocean sector of the SO. However, from 20 to 80 ka before
present sea salt fluxes show only very subdued millennial changes despite pronounced temperature fluctuations, likely due to the
large distance of the sea ice salt source to our drill sites.
© 2007 Elsevier B.V. All rights reserved.
Keywords: paleoclimate; ice core; Antarctica; sea salt; mineral dust
1. Introduction
Antarctic glacial/interglacial temperature changes
have been reconstructed over up to eight glacial cycles
using deep ice cores drilled on the Antarctic plateau (Petit
et al., 1999; Watanabe et al., 2003; EPICA community
members, 2004; Brook et al., 2005; EPICA community
members, 2006; Jouzel et al., in press) and on coastal ice
domes (Steig et al., 1998; Morgan et al., 2002). Such
climate records are generally interpreted as being
representative for the whole SO region, which plays a
key role in glacial/interglacial climate changes (Blunier
et al., 1997; Knorr and Lohmann, 2003; Stocker and
Johnsen, 2003) and the global carbon cycle (Archer et al.,
2001; Köhler et al., 2005; Toggweiler et al., 2006).
However, while the large-scale glacial/interglacial
Fig. 1. Map of the Antarctic continent indicating the EPICA drill sites in
Dronning Maud Land (EDML) facing the Atlantic sector of the SO and
at Dome C (EDC) facing the Indian Ocean sector of the SO together
with previously drilled deep ice cores on the Antarctic plateau.
changes are imprinted in all those records (Watanabe
et al., 2003) the influence of different air mass origin
allows us to reconstruct also regional differences in
climate evolution (Morgan et al., 2002). Moreover, while
temperature changes have been documented in many
Antarctic ice cores, regional information on accompanying environmental changes is still limited. Especially,
temporally resolved information on a dust induced iron
fertilization of marine phytoplankton productivity (Martin, 1990) and on reduced gas exchange and decreased
mixing of the ocean due to an increase in sea ice cover
(Toggweiler, 1999; Stephens and Keeling, 2000; Köhler
et al., 2005) is needed to constrain the carbon cycle/
climate feedback. Independent information on mineral
dust deposition and sea ice coverage in different regions
of Antarctica can be deduced from aerosol records in deep
Antarctic ice cores (Petit et al., 1990; Delmonte et al.,
2002; Röthlisberger et al., 2002; Wolff et al., 2003; Udisti
et al., 2004; Wolff et al., 2006; Fischer et al., 2007).
Within the European Project for Ice Coring in
Antarctica (EPICA) two deep ice cores have recently
been drilled (Fig. 1). The core at Dome C (EPICA
community members, 2004) (EDC: 75°06′S, 123°21′E,
3233 m above sea level) in the Indian Ocean sector of
Antarctica comprises undisturbed ice core records over
the last approximately 800 kyr (Jouzel et al., in press). The
second ice core was drilled at Kohnen station in Dronning
Maud Land (EPICA community members, 2006)
(EDML: 75°00′S, 00°04′E, 2882 m above sea level).
With a snow accumulation rate 2–3 times higher than at
EDC it provides higher resolution records down to Marine
Isotope Stage (MIS) 4. Moreover, due to its location it
represents the first ice core closely linked to climate
changes in the Atlantic sector of the SO.
In the following, we report on mineral dust (represented by non-sea salt calcium (nssCa2+)) and sea salt
aerosol (represented by sea salt sodium (ssNa+)) records
derived from both ice cores in decadal to centennial
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H. Fischer et al. / Earth and Planetary Science Letters 260 (2007) 340–354
resolution. The change of these parameters in the EDC ice
core has been previously discussed for the last 45 kyr in
high resolution (Röthlisberger et al., 2002) and in
millennial resolution over the last 740 kyr (Wolff et al.,
2006). Here we extend the discussion of the centennial
sea salt and mineral dust record from EDC to the last
150 kyr. More importantly, we contrast the EDC data to
the first continuous aerosol records from the Atlantic
sector of the Antarctic ice sheet as archived in the deep
EDML ice core. This way we can make use of the coherencies and differences between the two records
located on opposite sides of the East Antarctic plateau
allowing for an estimate of changes in transport intensity and for the identification of regional differences
in aerosol source strengths.
maximum synchronization uncertainty at 130 ka BP is
450 yr and even larger for older ages. However, during
most of the record the maximum synchronization
uncertainty is smaller and on average about 40 yr (Ruth
et al., 2007). Note, that this maximum synchronization
uncertainty is a conservative estimate and the typical
synchronization uncertainty is even smaller (on average
6 yr over the last 130 kyr). Thus, the interpolation error is
always small compared to the absolute dating uncertainty
and allows for unprecedented synchronization of these two
independent climate archives. Among others, this becomes
crucial when directly comparing aerosol fluxes at both sites
at a given point in time.
2. Methods
Samples for chemical analyses have been taken using
a continuous flow melting device, which precludes contact of the melt water with contaminated ice core surfaces
(Röthlisberger et al., 2000). EDML Concentrations of
Ca2+ and Na+ presented here have been determined
using ion chromatography (IC), while the top 580 m of
Ca2+ data at Dome C have been determined using continuous flow analysis (CFA) (Röthlisberger et al., 2002;
Wolff et al., 2006). Analytical details for the EDC
chemistry data set are summarized in Wolff et al. (2006).
The uncertainty in IC analyses is mainly determined by
the lab dependent blank contribution, while the instrument detection limit is one order of magnitude lower. The
average blank contribution for EDML lies between 3
and 50% of the Ca2+ concentration for average glacial and
interglacial conditions, respectively, and between 2
and 5% in the case of Na+. The reproducibility of the
Ca2+ and Na+ data is typically better than 10% and even
better than 5% during glacial conditions. During
interglacial periods the reproducibility of Ca2+ concentrations lower than 2 ppb deteriorates to around 50%. The
CFA based EDC Holocene record is not affected by this
blank contribution. For the EDML ice core the top 113 m
were not sampled. The depth interval 113–450 m in the
EDML ice core covering the time period from 1200 to
6700 a BP has been sampled in 5 cm resolution and then
averaged to 1 m intervals for this study. Analyses of these
6400 high-resolution samples were done in 4 continuous
batches in 4 different laboratories. Due to different lab
procedures blank values and reproducibility of the
different batches vary from lab to lab. To account for
the interlaboratory differences in mean blank contribution
and to gain a homogeneous record throughout the
Holocene, the concentration values have been corrected
by subtracting the mean concentration difference in
neighboring 5 m depth intervals in each of the adjacent
2.1. Age scales
The age scale (called EDC3) for the EDC core was
derived using an accumulation and ice flow model
constrained by independent age markers (Parrenin et al.,
2007). For internal coherence, a dependent EDML age
scale (called EDML1) has been derived for the last 150 kyr
by synchronizing the EDC and EDML ice core (Ruth et al.,
2007; Severi et al., 2007) using volcanic horizons in
sulfate, in the dielectric profile as well as electrolytic
conductivity down to a depth of 2366 m (equivalent to
128 ka before present (BP) where present is defined as
1950). Below this depth no unambiguous volcanic
synchronization could be established and we relied on a
few pronounced dust match points to synchronize both
cores. Due to the hemispheric and often global impact of
volcanic eruptions on sulfuric acid deposition and due to
the common glacial dust source for EDML and EDC in
Patagonia (Grousset et al., 1992; Basile et al., 1997;
Delmonte et al., 2004a,b) this procedure provides
unambiguous isochrones in both ice cores and allows for
direct comparison of regional differences in the two cores
independent of absolute dating uncertainties. Below
150 ka BP no unambiguous matching has been accomplished yet (Ruth et al., 2007). Accordingly, we will restrict
the discussion of the EDML chemistry data to the last
150 kyr.
For this time period the absolute dating uncertainty of
the EDC3 age scale is always better than 6000 yr (Parrenin
et al., 2007). During MIS 3 the absolute uncertainty of the
age scale is less than 1500 yr. More important for our
comparison of the EDC and EDML ice cores is the internal
consistency of the EDC3 and the EDML1 age scale, which
is defined by the synchronization uncertainty. The
2.2. Chemical analyses
H. Fischer et al. / Earth and Planetary Science Letters 260 (2007) 340–354
batches with the depth interval 273–450 m taken as
reference interval. To avoid a bias by high concentration
peaks we used the median to estimate the mean
concentration level. In doing so, all data are corrected to
the same average blank contribution, assuming that no
change in average atmospheric concentrations occurred in
the adjacent 5 m intervals used for the correction,
representing a time interval of approximately 150 yr.
With this procedure we are able to avoid interlaboratory
offsets between the batches while retaining as much longterm variability in the individual batches as possible. For
the sodium record the average correction was −0.8 ppb
(the maximum correction was −2.4 ppb applied to one
batch) which is less than 5% (maximum 14%) of the
average concentration. For the calcium record the
correction was on average 0.2 ppb (maximum correction
−3.0 ppb applied to one batch), i.e. about 9% (maximum
130%) of the average concentration. Note, that these
interlaboratory corrections affect only the Holocene
record and are rather small for Na+. For Holocene
samples with low Ca2+ concentrations these corrections
are significant and we have to state that we cannot
determine the lower envelope of Holocene Ca2+ concentrations with certainty using IC. Note however, that these
corrections do not affect any of our conclusions on glacial
343
aerosol variability, where Ca2+ concentrations are more
than one order of magnitude higher. The depth interval
450–2774 m has been analyzed entirely in one lab in 1 m
resolution and no interlaboratory offsets had to be
corrected.
The contributions of sea salt and mineral dust to the
total Ca2+ and Na+ content have been corrected using the
sea salt Ca2+/Na+ and the average crustal Ca2+/Na+ ratio
of Rm = 0.038 and Rt = 1.78 (Bowen, 1979) (in bulk
weight units), respectively (Röthlisberger et al., 2002).
While this correction is of minor importance for interglacial sea salt aerosol (on average 10%) it becomes
important for mineral dust in the Holocene where approximately 24% of the total Ca2+ content in the EDML core
is derived from sea salt aerosol. In the peak glacial (20–
30 ka BP) this effect is reversed. Here nssCa2+ is on
average only 5% lower than total Ca2+ while about 28%
of the total sodium is of crustal origin. The composition of
crustal tracers as recorded in ionic ice core data may differ
from Rt which is based on the bulk elemental composition
of insoluble crustal material. Note, that Rt may vary
dependent on the composition of crustal material as well
as on size dependent fractionation processes of different
minerals during the formation of mineral dust aerosol. Ice
core observations at EDC point to an Rt which may be
Fig. 2. Na+ (grey line) and Ca2+ (black line) concentration records (logarithmic scale) as measured on the EDML (top) and EDC ice cores (bottom) on
their individual depth scale in 1 m resolution. The dashed-dotted lines indicate pronounced isochronous dust changes during the last two glacial/
interglacial transitions in both records.
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H. Fischer et al. / Earth and Planetary Science Letters 260 (2007) 340–354
Fig. 3. Sea salt Na+ (ssNa+) and non-sea salt Ca2+ (nssCa2+) flux records (logarithmic scale) over the last 150 kyr at EDML and EDC on the common
EDML1/EDC3 age scale (Parrenin et al., 2007; Ruth et al., 2007). For comparison past accumulation rates at the site of deposition are given as
derived from isotope temperature changes (δ18O, δD) at EDML and EDC (EPICA community members, 2004, 2006) which have been used for flux
calculations. Data are shown in 1 m resolution (thin grey lines) and 500 yr averages (thick black line for EDML and thin black line for EDC), the latter
accounting for the loss in temporal resolution down core. Also depicted are reconstructed changes in benthic δ18O representative of long-term sea
level changes on the new EDML1/EDC age scale (Bintanja et al., 2005).
closer to 1 (Bigler et al., 2006). While the effect of such a
variability in Rt changes the nssCa2+ estimate only by a
few percent, its effect on ssNa+ is considerable. For
instance reducing Rt within reasonable bounds to 1
(Bigler et al., 2006) or increasing it to 3.5 changes the
ssNa+ estimate by 10–30% for glacial conditions in
Antarctica (Fischer et al., 2007). Note, that Rt may also
change from glacials to interglacials due to potentially
shifting sources and weathering conditions, however,
within reasonable limits of Rt this effect is too small to
significantly affect our conclusions.
2.3. Accumulation rates and deposition fluxes
The total average deposition flux Jtot at an ice core
site (Fischer et al., 2007) can be approximated by
Jtot ¼ Cice A ¼ vdry Cair þ W =qair Cair A:
The first term on the right hand side of this equation
parameterizes the dry deposition and the second term the
wet deposition flux with vdry the total mean dry
deposition velocity, W the scavenging ratio, ρair the
density of air, Cair the atmospheric aerosol concentration
and A the average accumulation rate. In low accumulation areas dry deposition dominates and the total flux
is essentially independent of changes in accumulation
and almost proportional to the atmospheric aerosol
concentrations. Accordingly, we will discuss aerosol
flux records in this study, which are more representative
of atmospheric aerosol concentrations. Knowledge of
the latter is a prerequisite for quantitative estimates of
source strength or transport changes in the past. The
total deposition flux was calculated by multiplying the
measured ice concentration Cice with reconstructed
accumulation rates (Figs. 2 and 3). Both at EDC and
EDML the accumulation rate is estimated from the
H. Fischer et al. / Earth and Planetary Science Letters 260 (2007) 340–354
stable water isotope (δ18O, δD) profiles (after correction
for changes in the isotopic composition of sea water due
to continental ice mass changes) essentially assuming
thermodynamic control of the water vapor saturation
pressure (EPICA community members, 2006; Parrenin
et al., 2007). Second order corrections of non-thermodynamic effects on the accumulation rate at EDML have
been made using the spatial variability in accumulation
rates upstream of the drill site (EPICA community
members, 2006) derived from firn cores and an extended
surface radar survey (Oerter et al., 2000; Steinhage et al.,
2001; Graf et al., 2002; Rybak et al., 2005) and at EDC
by a one-dimensional flow model (Parrenin et al., 2007).
The error of the reconstructed accumulation rates is
estimated to be 30% for glacial times and significantly
less for warm periods, where the precipitation regime is
similar to today. This is also supported by comparing
these continuous thermodynamically derived accumulation rates at EDML with discrete accumulation rates
derived from the unstrained layer thickness between
volcanic match points (Severi et al., 2007). Both
accumulation estimates agree typically within ± 20%.
Because the accumulation information based on δ18O
can be calculated continuously in high resolution over
the entire length of the core, we will use the thermodynamically derived accumulation rate in the following
to derive aerosol fluxes. Based on the errors in
accumulation rates and ion concentrations, the aerosol
fluxes are constrained to better than 30% for the large
majority of the samples and to better than 60% for very
low Holocene Ca2+ concentrations. Note, that although
these errors are considerable they are small compared
to the order of magnitude change in glacial/interglacial
Ca2+ concentrations and cannot explain systematic
glacial variations on millennial time scales in concentration and flux records.
2.4. Upstream correction
In contrast to the EDC ice core, which is located on a
dome position, the EDML ice core lies at a saddle
position of the ice divide with small (about 1 m/yr)
horizontal flow velocities. Accordingly, while the ice at
Dome C originates at the current drill site, deeper ice at
EDML originates from upstream positions at higher
altitudes, where temperatures and accumulation rates are
lower and the atmospheric aerosol concentrations may
be slightly different. E.g. ice with an age of 150 ka BP
has been deposited 160 km upstream (EPICA community members, 2006; Huybrechts et al., 2007). Note, that
geographic effects in aerosol concentrations are small
compared to the glacial/interglacial changes (Sommer
345
et al., 2000; Göktas et al., 2002). As outlined above the
true total aerosol flux at the site of deposition should be
almost independent from the local snow accumulation
rate in low accumulation areas where dry deposition
prevails. Accordingly, we used the local accumulation
rate at the site of initial deposition to calculate total
aerosol fluxes.
3. Results
The Ca2+ and Na+ concentration records for both ice
cores are shown on a depth scale in Fig. 2. The EDC
record provides unprecedented data over 8 full glacial
cycles, while the EDML record shows only 2 unambiguous glacial maxima. In return, the EDML record
exhibits higher annual layer thicknesses down to a depth
of 1900 m, allowing for higher resolution records down
to an age of approximately 80 ka BP. The concentration
records show comparable glacial/interglacial variations
both at EDML and EDC with 1–2 orders of magnitude
higher sea salt and mineral dust concentrations during
glacial periods and significant millennial dust variability
during marine isotope stage 2–4. Similar glacial/
interglacial variability has been also reported from
Vostok (de Angelis et al., 1997) and Taylor Dome
(Mayewski et al., 1996) as far as temporal resolution
allowed.
One major achievement for the EPICA ice cores is the
unambiguous stratigraphic link between the two EPICA
ice cores (Ruth et al., 2007; Severi et al., 2007). This
common EDML1/EDC3 age scale allows for direct
comparison of aerosol flux records and their interpretation
in terms of atmospheric aerosol concentrations. In Fig. 3
the ssNa+ and nssCa2+ flux records are plotted. The
accompanying changes in the snow accumulation rate are
given for comparison. Clearly, both records show a very
high covariance between both sites in each of the aerosol
species. However, nssCa2+ fluxes at EDML are about 3
times higher than at EDC showing that atmospheric
nssCa2+ concentrations are substantially higher at EDML
than at EDC. Isotopic fingerprinting for Dome C samples
showed that Patagonia is the dominant source of mineral
dust deposited at that site in the glacial (Basile et al., 1997;
Delmonte et al., 2007) as also supported by dust modeling
(Lunt and Valdes, 2001; Mahowald et al., 2006). In the
interglacial an additional Australian dust source may be
possible at Dome C (Revel-Rolland et al., 2006;
Delmonte et al., 2007). Isotopic fingerprinting of mineral
dust has not been accomplished yet at EDML, but a strong
influence of Australian dust sources to EDML appears to
be very unlikely, given its geographic location which is
much closer to South America and downwind of the
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H. Fischer et al. / Earth and Planetary Science Letters 260 (2007) 340–354
cyclonically curved atmospheric pathway from Patagonia
(Reijmer et al., 2002). Accordingly, we attribute the
higher atmospheric nssCa2+ concentrations at EDML to
the greater proximity of EDML to the Patagonian dust
source.
Also sea salt fluxes are about a factor of 2 higher at
EDML than at EDC indicating higher atmospheric sea
salt aerosol concentrations in Dronning Maud Land.
Note, that in the case of sea salt wet deposition may have
a stronger influence than is the case for dust. Transport of
sea salt aerosol is closely linked to cyclonic activity and a
higher contribution by wet deposition of sea salt aerosol
may be expected. This holds especially at EDML, where
large parts of the recent annual accumulation is derived
from a few single snow fall events per year (Reijmer and
van den Broeke, 2001), while at EDC only non-frontal
diamond dust is observed. Model studies show that
circum-Antarctic cyclone activity in the glacial was most
likely reduced (Krinner and Genthon, 1998) due to the
extended sea ice cover and an accompanying northward
shift of the atmospheric polar front, reducing the effect of
single precipitation events on sea salt transport to EDML
at that time. Given the high covariance between both
records and the extremely low glacial accumulation of
1–3 cm WE/yr at both sites a dominant influence of wet
deposition during cold climate periods appears to be
unlikely. This implies that glacial sea salt fluxes are
largely representative of the true variations in atmospheric concentrations.
Most striking are the substantial changes in dust and
sea salt fluxes over the last glacial cycle similarly found
at both sites. The nssCa2+ flux decreased by a factor of
about 10 during the last two glacial/interglacial transitions (Fig. 3) at both sites. At the same time the ssNa+
flux decreased at EDC by a factor of 2 and at EDML by a
factor of 3 from the Last Glacial Maximum (LGM) to the
Holocene and by a factor of 4–8 during the penultimate
transition, essentially due to much lower ssNa+ fluxes
during the interglacial MIS 5.5. Apart from the glacial/
interglacial changes in dust emissions, strong millennial
variations in the nssCa2+ flux are also observed, while
such variations are very subdued in the sea salt record.
plateau (r2 = 0.82 for 100 yr averages), we are now able to
constrain the influence of transport changes on the
changes in mineral dust flux in Antarctica. Assuming
the atmospheric aerosol concentration close to the source
is given by Cair(0) and an air parcel is transported away
from the source, the atmospheric aerosol concentration
Cair at any time t along the transport trajectory can to first
order be approximated by
Cair ðtÞ ¼ Cair ð0Þexpðt=sÞ
or
log½Cair ðtÞ=Cair ð0Þ ¼ t=s
where t is the transport time and the average atmospheric
residence time τ along the transport route is controlled by
wet and dry deposition (Hansson, 1994). τ is typically on
the order of a few days to weeks (Mahowald et al., 1999;
Gong et al., 2002; Reijmer et al., 2002; Werner et al.,
2002). If two sites with transport times t1 and t2 (e.g.
EDML and EDC) are considered, the logarithmic air
concentration at site 2 can be expressed by the concentration at site 1 according to
log½Cair ðt2 Þ ¼ log½Cair ðt1 Þ Dt=s
ð1Þ
where Δt =t2 −t1 is the transport time difference along the
two trajectories and where we assume that the atmospheric
residence time along both transport routes is comparable.
4. Discussion
4.1. Mineral dust
A key question is, how much of the glacial/interglacial
and millennial variations in the glacial is due to changes in
the aerosol transport or changes in aerosol source strength.
Using the coherent change in glacial dust fluxes at both
EPICA sites from opposite sides of the East Antarctic
Fig. 4. Scatter plot of logarithmic nssCa2+ (light grey dots) and ssNa+
(dark grey dots) fluxes in the EDC vs. EDML ice cores in 100 yr
resolution according to Eq. (1). If transport remains unchanged the
difference of the data points from the first bisecting line should stay
constant. For visual guidance a linear regression through all data points
has been added which shows a slope close to 1 for nssCa2+ (dark grey
line) and around 0.6 for ssNa+ (light grey line).
H. Fischer et al. / Earth and Planetary Science Letters 260 (2007) 340–354
Accordingly, if the aerosol is derived from the same source
at both sites (as is the case for glacial Patagonian dust in
Antarctica) the difference in the logarithmic atmospheric
aerosol concentration (thus, the deposition flux in
Antarctic low accumulation areas), should be Δt/τ. The
offset Δt/τ may change for different climate conditions.
For example if transport velocities intensify as hypothesized for colder climate conditions (Petit et al., 1999), Δt
and, thus, the logarithmic offset should become smaller. If
the atmospheric residence time τ is considerably lengthened as hypothesized for cold periods (Yung et al., 1996;
Petit et al., 1999) the offset should also be reduced.
As shown in Fig. 4 the difference of logarithmic
nssCa2+ fluxes in the ice (representative of atmospheric
nssCa2+ concentrations) at EDC and EDML stayed rather
constant, providing observational evidence that transport
parameters for dust transport from Patagonia to Antarctica
have not changed strongly over the last glacial cycles.
Alternatively, both Δt and τ may have changed in the
same way compensating each other's transport effect.
However, it seems unlikely that this was the case during
all time periods over the last 150 kyr. In any case the
change in atmospheric concentration at the drill site is
only dependent on the joint effect of transport time and
residence time which, based on our results, was limited. In
Table 1 the average differences (representative of Δt/τ) of
logarithmic fluxes between EDML and EDC are
summarized for individual time periods. Based on our
conceptual model it becomes clear that Δt/τ is between 1
and 2 for all time periods over the last glacial cycle,
implying that the difference in transport time is similar to
the atmospheric residence time and is on the order of a
week approximately in line with back trajectory studies
(Reijmer et al., 2002). For the glacial intervals, where
Patagonian dust is dominant (Grousset et al., 1992; Basile
et al., 1997; Delmonte et al., 2004a), the offset lies
between 1 and 1.5 with the lower values for peak glacial
conditions (LGM, MIS6). This points to a somewhat
faster transport or a larger residence time during the
coldest periods but the decrease in Δt/τ from MIS3 to the
LGM is still within the uncertainty in Table 1. Taking the
values in Table 1 at face value a LGM decrease in Δt/τ
347
compared to MIS3 could explain increases in the
atmospheric nssCa2+ concentration of approximately
40% assuming that this change were entirely due to
changes in atmospheric residence time and if we assume
both an average atmospheric residence time and a transport time to EDML of approximately a week during MIS
3 (Gong et al., 2002). Only if we use an unrealistically
long transport time, which is 6 times larger than the
atmospheric residence time, can we explain the observed
factor of 5 changes in the glacial nssCa2+ flux records with
our simple conceptual transport model. The largest
difference in Table 1 is found between the Holocene
and the LGM. Taken at face value this would point to a
slower transport or a reduced atmospheric residence time
during the Holocene, which could explain a glacial
increase by 70%. However, the Holocene Ca2+ flux at
EDML has to be regarded as an upper limit due to the
relatively high blank contribution in this time interval and
accordingly the real difference in Δt/τ in Table 1 is
significantly smaller. E.g. reducing the average Holocene
concentration at EDML for a blank contribution of only
1 ppb would reduce Δt/τ to 1.5, i.e. a value similar to
MIS3 and 4.
Interestingly, the scatter of the nssCa2+ flux data in
Fig. 4 decreases with increasing flux values, i.e. for colder
climate conditions. This can only partly be explained by
the larger measurement uncertainty for low Ca2+
concentrations during warm periods. Accordingly, this
suggests a higher transport variability or a variable input of
Australian dust export to Antarctica for warm climate
conditions. In summary, we conclude that both transport
intensity as well as variability changed over time but
played only a secondary role for the observed dust changes
in line with atmospheric circulation models (Krinner and
Genthon, 1998; Reader and McFarlane, 2003; Mahowald
et al., 2006). Thus, the observed changes in nssCa2+ fluxes
predominantly reflect changes in source strength.
For glacial dust this implies strongly increased dust
emissions in Patagonia, which are most pronounced
during the LGM. The penultimate glacial but also the
cold interval between 60–70 ka BP show extraordinarily
high nssCa2+ fluxes comparable to the environmental
Table 1
Average ± standard deviation of the difference in logarithmic nssCa2+ fluxes in 100 yr resolution at EDML and EDC representing the average ratio of
the transport time difference between the two sites and the atmospheric residence time as controlled by wet and dry deposition en route
Time
period
Holocene
LGM
MIS3
MIS4-5.4
MIS5.5
MIS6
1–10 ka BP
20–30 ka BP
30–60 ka BP
60–120 ka BP
120–130 ka BP
130–150 ka BP
Δt/τ
(2.01 ± 0.47)
0.96 ± 0.17
1.41 ± 0.40
1.56 ± 0.84
(1.45 ± 0.51)
1.16 ± 0.39
The Holocene value represents an upper limit due to the larger blank contribution to ion chromatographically determined Ca2+ concentrations at
EDML compared to the CFA data used at EDC. During MIS5.5 both EDML and EDC are affected by the blank level.
348
H. Fischer et al. / Earth and Planetary Science Letters 260 (2007) 340–354
conditions during the LGM. Parallel to the nssCa2+ flux
maxima in Antarctica, terrigenous records in marine
sediments in the northern Scotia Sea also point to higher
mineral dust input from southern Patagonian sources
(Diekmann et al., 2000), however this study did not
distinguish between aeolian and lateral transport of
suspended particles to the sediment. The rest of the last
glacial is characterized by intermediate nssCa2+ fluxes,
hence mineral dust emissions in Patagonia, which are
strongly but not linearly related to temperature changes
recorded in Antarctica (Fig. 5) (Wolff et al., 2006).
Especially, each of the Antarctic Isotope Maxima (AIM)
(EPICA community members, 2006) indicated in Fig. 3
is accompanied by a significant decline in nssCa2+ fluxes
at EDML and EDC. This points to synchronous changes
(reduced aridity and/or lower wind speeds) in the Patagonian source regions during AIMs. Of special importance in Fig. 3 are also the apparently comparable
environmental conditions during the last two interglacials. This points to similar precipitation rates and wind
speeds in Patagonia while temperatures in Antarctica
were 2–4 °C higher in MIS5.5 than in the Holocene
(EPICA community members, 2006).
Little is known about the long-term change in local
Patagonian climate beyond the LGM. For the LGM,
terrestrial records ((Markgraf et al., 1992) and references
therein) and climate models (Wainer et al., 2005) suggest
increased aridity leeside of the Andes and a northward
expansion and potential strengthening of the westerlies
(Moreno et al., 1999). The latter is connected to the
northward expansion of sea ice found in marine sediment
records (Gersonde et al., 2005) and in line with our sea
salt reconstructions discussed in Section 4.2. Model
simulations on the change in the southern hemisphere
westerlies are inconsistent (Shin et al., 2003; Butzin
et al., 2005; Otto-Bliesner et al., 2006). While an
idealized atmospheric model without continents shows a
clear glacial northward shift of peak westerly winds
(Williams and Bryan, 2006), one model study with a
complex coupled ocean/atmosphere climate model (Shin
et al., 2003) shows even a slight southward shift of the
peak wind stress in the SO. Interestingly, a northward
expansion of the westerlies is also observed for recent
winter conditions compared to summer in NCEP/NCAR
reanalysis data (Kalnay et al., 1996), while the core of the
westerlies in the SO does not shift in latitude.
Another possible factor acting on dust emission may
be the reduction of sea level exposing substantial shelf
areas especially on the Atlantic side off Patagonia.
Comparison of the EDC nssCa2+ record with coast line
reconstructions for the Argentinian shelf (Wolff et al.,
2006) showed that the major glacial/interglacial nssCa2+
decrease occurred prior to any significant sea level rise.
In addition the millennial climate variability during MIS
3 is also accompanied by significant nssCa2+ flux
variations (Fig. 3), but has not been accompanied by sea
level changes at similar high frequencies. In contrast to
the delayed global sea level rise relative to the mineral
dust decrease, an early glacier retreat before 17 ka BP
has been reported from Tierra del Fuego and other mid-
Fig. 5. Temperature dependence of nssCa2+ (left) and ssNa+ (right) fluxes in 500 yr resolution at EDML (dark grey dots) and EDC (light grey dots).
Surface temperatures have been derived from δ18O (EDML) and δD (EDC) using the spatial isotope/temperature gradient in the respective region
(EPICA community members, 2004, 2006).
H. Fischer et al. / Earth and Planetary Science Letters 260 (2007) 340–354
latitude glaciers (Sugden et al., 2005; Schaefer et al.,
2006), supporting a close connection of glaciation and
continental dust sources in southern South America.
Outwash of mineral dust material has been strongly
enhanced during glacial expansion of southern Andean
ice masses, providing a potential active source for
mineral dust mobilization in the glacial. In summary we
suggest that a combination of changes in glacial
outwash, aridity and wind speed are mainly responsible
for the glacial/interglacial mineral dust changes
recorded in Antarctica (Mahowald et al., 2006) while
the influence of sea level changes is small for the rapid
decrease in dust fluxes at the beginning of the termination and during MIS 3.
4.2. Sea salt
A quantification of transport changes for sea salt
aerosol is even more difficult as in the case of dust. Sea
salt concentration increases more strongly at EDML
than at EDC for colder climate conditions leading to an
increase in the difference of logarithmic sea salt
concentration in Fig. 4. However, this cannot be used
to quantify transport changes as in the case of glacial
dust because the sea salt source regions for the Atlantic
and Indian Ocean sector of Antarctica are not the same
and sea salt source strength as well as transport may
have changed independently in both regions. In view of
the largely expanded sea ice coverage around Antarctica
a longer transport time from the open ocean to the
Antarctic plateau is expected in glacial periods and any
intensification of transport from the open ocean would
have to overcompensate that effect. An increased inflow
of marine air masses from the open ocean leading to an
increased sea salt aerosol flux seems also in contradiction with the strongly depleted stable water isotope
(δ18O, δD) levels (EPICA community members, 2004,
2006) and reduced snow accumulation rates at both
sites, pointing to a stronger depletion of marine air masses in water vapor and most likely also sea salt aerosol
during transport.
Also an explanation of the site dependent glacial
increase in sea salt fluxes in terms of sea salt source
strength is not straightforward. The canonical interpretation of changes in sea salt aerosol is an increased sea/air
particle flux at the open ocean surface due to increased
wind speeds. This, however, is in contradiction to the
greatly increased sea ice cover (Gersonde et al., 2005)
during glacial periods. Aerosol formation from brine
water, as for example experienced during the formation
of frost flowers in sea ice leads and polynyas, has
recently been identified as an additional effective source
349
for sea salt aerosol (further on referred to as “sea ice
salt”). Precipitation of mirabilite (Na2SO4·10H2O) from
sea water during brine formation leads to a characteristic
sulfate depletion compared to the average sea water
composition. This fractionation has been regularly
observed in atmospheric sea salt aerosol concentrations
at coastal Antarctic sites in winter (Wagenbach et al.,
1998; Rankin et al., 2000) showing the importance of the
brine water source for the local sea salt aerosol budget.
Brine and frost flower formation was also suggested as a
possible mechanism for the glacial/interglacial increase
in sea salt concentrations in ice cores on the high
Antarctic plateau (Wolff et al., 2003, 2006). Only recently, year-round aerosol sampling on the East Antarctic
plateau revealed evidence of sulfate depleted sea salt
aerosol during austral winter (Hara et al., 2004), which
we interpret as significant influence of sea ice formation
on the sea salt aerosol budget also for the interior of
Antarctica (Wolff et al., 2006; Fischer et al., 2007). In the
following we will explore whether we can explain the
observed changes in sea salt fluxes, if we take sea ice salt
formation into account, while recognizing that there
remains some controversy about this interpretation as
long as we have no quantitative source estimate for sea
ice salt production.
Due to wind driven opening of leads this sea ice salt
aerosol formation process occurs throughout the sea ice
cover and continuously in coastal polynyas. Based on
diatom evidence in SO sediment records it was shown
that the circum-Antarctic winter sea ice cover during the
LGM was enhanced by approximately a factor of two
from 19 to 39 · 106 km2 (Gersonde et al., 2005). Although the reconstruction of the summer sea ice edge is
hampered by the lack of siliceous sediments in regions
of second year sea ice, some evidence for substantial
summer sea ice exists for the LGM in the Atlantic and
Indian Ocean sector of the SO (Gersonde et al., 2005).
This indicates that the glacial summer sea ice was as
large as recent winter sea ice coverage in the Atlantic
sector while it was not extended in the Indian Ocean
sector of the SO. Sea surface temperatures in summer
sea ice regions (e.g. in coastal polynyas in the Weddell
Sea) have been well below the freezing point during the
LGM (Gersonde et al., 2005), allowing for considerable
sea ice formation also during summer. This summer sea
ice salt formation would add to the sea ice salt aerosol
emission in winter and further enhance the annually
averaged aerosol source strength of this process. If we
take at first order the sea ice salt contribution to the
Antarctic sea salt budget to be proportional to the total
sea ice cover in winter and summer we expect a factor of
2 increase in the Indian ocean and a factor up to 3
350
H. Fischer et al. / Earth and Planetary Science Letters 260 (2007) 340–354
increase in the Atlantic sector of the SO, favorably in
line with our observations in the EDC and EDML ice
cores.
Concentrating on the variability in sea salt flux in
parallel to millennial climate variations during MIS 3
(Fig. 4) it becomes clear that a warming during AIMs
does not lead to a clear response in sea salt fluxes. This
casts doubt on a quantitative use of sea salt in ice cores as
sea ice proxy. During peak glacial times the potential sea
ice salt formation zone extends as far north as 50°S. Sea
ice salt produced in areas at the outer edge of the sea ice
cover has to travel a very long distance before it reaches
an ice core site on the Antarctic plateau. Due to this long
transport time it experiences substantial depletion by wet
and dry deposition en route. Accordingly, we suggest
that the farther north the sea ice salt formation zone
extends during cold periods the less efficient is this edge
area to add to the sea salt aerosol budget at Antarctic ice
core drill sites. Looking at the sea salt variations in MIS 3
in more detail (Fig. 6) we see that at EDC no imprint on
sea salt fluxes can be found during AIMs whatsoever
while the record at EDML suggests a tendency to rapidly
reduced sea salt fluxes at the onset of most of the
warming events and a gradual recovery thereafter. Although this effect is rather weak it may point to a decline
in the summer sea ice extent in the Weddell Sea sector
which may react more sensitively to the warming events
during MIS 3.
A major difference in ssNa+ fluxes exists for the
penultimate warm period. During MIS 5.5 the sea salt
flux in the EDML ice core was reduced by a factor of 4
while it was lowered by a factor of 2–3 at EDC compared to Holocene conditions in line with significantly
warmer temperatures at EDC and EDML (EPICA
Fig. 6. High-resolution (100 yr averages) plots of stable water isotopes, ssNa+ fluxes and nssCa2+ fluxes in the EDML (black line) and EDC (grey
line) ice cores during the time interval 22–42 ka BP on the common EDML1/EDC3 age scale (Parrenin et al., 2007; Ruth et al., 2007). Isotope
temperatures show Antarctic Isotope Maxima (AIM) during this period (EPICA community members, 2006) which are also reflected in lowered
nssCa2+ fluxes at both sites (indicated by arrows in the bottom panel, note the logarithmic y-scale). Sea salt aerosol at EDML tends to a rapid decline
at the onset of most of the AIMs (indicated by arrows in the middle panel) while no significant changes in sea salt aerosol can be observed at EDC.
H. Fischer et al. / Earth and Planetary Science Letters 260 (2007) 340–354
community members, 2006). Because summer sea ice is
nearly absent in the Indian Ocean sector for Holocene
conditions, this implies that only a reduction in the
winter sea ice salt source could explain the sea salt
decrease in this region during the warmer MIS 5.5. In
the Atlantic sector of the SO an even stronger decline in
sea ice coverage during MIS 5.5 is indicated by our
ssNa+ data, possibly related to a reduction of both winter
and summer sea ice in the Weddell Sea. This is also
supported by changes in marine sediments, indicating a
southward shift of the opal deposition belt by 3–5°
latitude (Bianchi and Gersonde, 2002) at that time.
5. Conclusions
Sea salt and mineral dust aerosol records are
representative for basin-wide atmospheric catchment
areas of aerosol transported onto the East Antarctic ice
sheet. The records allow us to draw a clearer picture on the
timing of environmental changes in Patagonia and the SO
and their influence on aerosol formation in parallel to
climate variations in Antarctica over the last 150 kyr.
Sea salt records at both sites allow for the first time a
basin-wide estimate of sea salt aerosol emissions, which
support an accompanying change in sea ice cover over the
complete time interval from the penultimate glacial to the
present. The coherence of the two sea salt records from
opposite sides of the Antarctic ice sheet indicates a
waxing and waning of sea ice, which was generally synchronous in the Atlantic and the Indian Ocean sector of
the SO on glacial/interglacial time scales. The sensitivity
of sea salt fluxes to temperature changes (Fig. 5),
however, is higher in the Weddell Sea sector, a fact
which we attribute to a more vigorous change in sea ice
cover and the additional role of summer sea ice extent on
sea ice salt formation in this area. We note, that for very
cold climate conditions sea salt changes become increasingly insensitive to temperature changes probably related
to the long transport distance of aerosol formed at the
northern margin of the sea ice cover. Accordingly, the use
of sea salt as quantitative proxy for sea ice coverage is
hampered and requires a rigorous calibration against
marine sediment data for different time slices, experimental studies of the modern sea ice salt source as well as
refined model studies about its change in the past.
Using our dust records from sites facing different
sectors of the SO with greatly different distances to
Patagonia and using a simple conceptual transport model
we can rule out a substantial variation in transport as
being the main factor being responsible for the observed
nssCa2+ changes during glacial times as also supported
by atmospheric circulation models (Krinner and Gen-
351
thon, 1998; Reader and McFarlane, 2003). Instead our
results point to temporal changes in the strength of the
Patagonian mineral dust source in parallel to temperature
variations in Antarctica. The three times higher nssCa2+
fluxes at EDML also point to a threefold higher aeolian
dust input into the Weddell Sea sector of the SO, potentially related to a stronger dust fertilization in this
region. This effect may be amplified by a potential dust
storage effect of the greatly extended winter and summer
sea ice coverage as indicated in our ssNa+ record. The
extension of summer and winter sea ice in this region is
also expected to have a strong effect on bottom water
formation, gas exchange and wind shear of the surface
ocean likely related to a reduced ventilation of the deep
SO with strong impacts on the global carbon cycle
(Köhler et al., 2005). Accordingly, our data will allow for
a better time resolved, basin-wide quantification of
potential dust fertilization on marine export productivity
and investigation of the effect of the sea ice extent on gas
exchange and water mass formation.
Acknowledgments
This work is a contribution to the European Project for
Ice Coring in Antarctica (EPICA), a joint European
Science Foundation/European Commission scientific
programme, funded by the EU (EPICA-MIS) and by the
national contributions from Belgium, Denmark, France,
Germany, Italy, the Netherlands, Norway, Sweden,
Switzerland and the United Kingdom. The main logistic
support was provided by IPEV and PNRA (at Dome C)
and AWI (at Dronning Maud Land). We thank the logistics
and drilling teams and all helpers in the field for making
the science possible. This is EPICA publication no. 170.
Appendix A. Supplementary data
Supplementary data associated with this article can
be found, in the online version, at doi:10.1016/j.
epsl.2007.06.014.
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