Rth2008a

Rth2008a
Clim. Past, 4, 345–356, 2008
www.clim-past.net/4/345/2008/
© Author(s) 2008. This work is distributed under
the Creative Commons Attribution 3.0 License.
Climate
of the Past
The Southern Hemisphere at glacial terminations:
insights from the Dome C ice core
R. Röthlisberger1 , M. Mudelsee1,2 , M. Bigler3 , M. de Angelis4 , H. Fischer5,6 , M. Hansson7 , F. Lambert6 ,
V. Masson-Delmotte8 , L. Sime1 , R. Udisti9 , and E. W. Wolff1
1 British
Antarctic Survey, Natural Environment Research Council, Cambridge, UK
Risk Analysis, Hannover, Germany
3 Niels Bohr Institute, University of Copenhagen, Denmark
4 Laboratoire de Glaciologie et Géophysique de l’Environnement, Grenoble, France
5 Alfred Wegener Institut, Bremerhaven, Germany
6 Climate and Environmental Physics, University of Bern, Switzerland
7 Department of Physical Geography and Quaternary Geology, Stockholm University, Sweden
8 Laboratoire des Sciences du Climat et de l’Environnement, Gif-sur-Yvette, France
9 Department of Chemistry, University of Florence, Italy
2 Climate
Received: 21 May 2008 – Published in Clim. Past Discuss.: 19 June 2008
Revised: 12 September 2008 – Accepted: 22 October 2008 – Published: 9 December 2008
Abstract. The many different proxy records from the European Project for Ice Coring in Antarctica (EPICA) Dome C
ice core allow for the first time a comparison of nine glacial
terminations in great detail. Despite the fact that all terminations cover the transition from a glacial maximum into an
interglacial, there are large differences between single terminations. For some terminations, Antarctic temperature increased only moderately, while for others, the amplitude of
change at the termination was much larger. For the different
terminations, the rate of change in temperature is more similar than the magnitude or duration of change. These temperature changes were accompanied by vast changes in dust and
sea salt deposition all over Antarctica.
Here we investigate the phasing between a South American dust proxy (non-sea-salt calcium flux, nssCa2+ ), a sea ice
proxy (sea salt sodium flux, ssNa+ ) and a proxy for Antarctic temperature (deuterium, δD). In particular, we look into
whether a similar sequence of events applies to all terminations, despite their different characteristics. All proxies are
derived from the EPICA Dome C ice core, resulting in a
relative dating uncertainty between the proxies of less than
20 years.
At the start of the terminations, the temperature (δD) increase and dust (nssCa2+ flux) decrease start synchronously.
The sea ice proxy (ssNa+ flux), however, only changes once
Correspondence to: R. Röthlisberger
([email protected])
the temperature has reached a particular threshold, approximately 5◦ C below present day temperatures (corresponding
to a δD value of −420‰). This reflects to a large extent the
limited sensitivity of the sea ice proxy during very cold periods with large sea ice extent. At terminations where this
threshold is not reached (TVI, TVIII), ssNa+ flux shows no
changes. Above this threshold, the sea ice proxy is closely
coupled to the Antarctic temperature, and interglacial levels
are reached at the same time for both ssNa+ and δD.
On the other hand, once another threshold at approximately 2◦ C below present day temperature is passed (corresponding to a δD value of −402‰), nssCa2+ flux has
reached interglacial levels and does not change any more, despite further warming. This threshold behaviour most likely
results from a combination of changes to the threshold friction velocity for dust entrainment and to the distribution of
surface wind speeds in the dust source region.
1
Introduction
The climate of the late Quaternary has been marked by repeated changes between glacial and interglacial periods. The
reasons for such changes are still not entirely understood, although it seems clear that orbital forcing and internal feedback mechanisms involving greenhouse gases play a vital
role (Huybers, 2006; Köhler and Fischer, 2006 and references therein). There are various processes that are influenced by and that exert an influence on the evolution of
Published by Copernicus Publications on behalf of the European Geosciences Union.
346
R. Röthlisberger et al.: Glacial terminations in the EPICA Dome C ice core record
-360
TI
δD (‰)
-380
TII
TIII
TIV
TV
TVI
TVII
TVIII
TIX
-400
-420
1
1.5
log ss-Na+ flux
(μg/m2/a)
2
2.2
2.5
2.4
3
log nss-Ca2+ flux
(μg/m2/a)
-440
2.6
2.8
3
3.2
0
200,000
400,000
Age (a B.P.)
600,000
800,000
2+ and ssNa+
Fig. 1.Figure
Overview1.ofOverview
entire data set.
0.55 m
averages
of δD
year averages
of the
logarithm
nssCayear
ofShown
entirearedata
set.
Shown
are(green)
0.55and
m 100
averages
of δD
(green)
andof100
2+
+
flux (grey) overlaid by a 11-point running average (blue, red). Y-axes of nssCa flux and ssNa flux have been reversed in order to facilitate
averages
of the logarithm of nssCa2+ and ssNa+ flux (grey) overlaid by a 11-point running
comparison
with δD.
average (blue, red). Y-axes of nssCa2+ flux and ssNa+ flux have been reversed in order to
temperature and atmospheric CO2 . In high southern latcomparison
with δD.
itudes,facilitate
the impact
of sea ice extent
and its connection to
Southern Hemisphere winds and other factors that may contribute to ocean upwelling are of particular interest (Le Quere
et al., 2007; Toggweiler et al., 2006).
The ice core from Dome C, Antarctica, that has been
drilled in the framework of the European Project for Ice
Coring in Antarctica (EPICA), provides a record of the last
nine glacial – interglacial terminations in terms of changes
in high latitude temperature (Jouzel et al., 2007), changes in
greenhouse gases (Siegenthaler et al., 2005; Spahni et al.,
2005) and various aerosols (Lambert et al., 2008; Wolff et
al., 2006). Here we look into the pattern and phasing at terminations in different parameters, namely δD representing
Antarctic temperature, nssCa2+ flux, a proxy for aspects of
South American climate (Röthlisberger et al., 2002; Wolff
et al., 2006), and ssNa+ flux, which is related to the sea ice
extent around Antarctica (Wagenbach et al., 1998; Wolff et
al., 2003). The aim is to identify robust pattern and phaserelationships at glacial terminations between Antarctic temperature, South American conditions and the sea ice based
on the ice core record from Dome C.
2
2.1
Methods
Data
The ice core was drilled from 1996 to 2004 and has been
analysed for stable water isotopes (δD, (Jouzel et al., 2007))
at 55 cm resolution. The analysis of the soluble impurities e.g. sodium (Na+ ) and calcium (Ca2+ ) has been done
by seven European laboratories with different methods, and
low-resolution data along most of the core, using a previous age-scale, have already been published (Wolff et al.,
2006). In this study, we used the data obtained by continuous flow analysis (CFA), (Röthlisberger et al., 2000), which
resulted in a high-resolution record (of the order of 1 cm,
corresponding to less than a year in the Holocene, approximately 3 years at 410 ka BP during marine isotope stage
(MIS) 11, and 20 years at 800 ka BP during MIS 20). In
the top part (0 to 450 ka BP), these data were downsampled
to 20 years resolution by using the median of the data in
each 20-a interval in order to reduce the computing time.
Below that, computing time was within reasonable limits
for the high-resolution data, so that 1 cm data were used
for further analysis. Fluxes, being representative of atmospheric concentrations at sites where dry deposition is assumed to dominate, were calculated using the accumulation
rates derived from the EDC3 timescale (Parrenin et al., 2007)
(Fig. 1). Compared to nssCa2+ and ssNa+ flux, the accumulation rate changed independently during terminations,
20
Clim. Past, 4, 345–356, 2008
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R. Röthlisberger et al.: Glacial terminations in the EPICA Dome C ice core record
347
# simulations
# simulations
log nss-Ca2+ flux
μg/m2/a
δD (‰)
thus the uncertainty in reconstructing accumulation rate will
-360
T II
translate into the ssNa+ and nssCa2+ fluxes. However, in
x1
-380
view of flux changes of the order of a factor of 2 to 4 in
200
t1
ssNa+ fluxes and 7 to 30 in nssCa2+ fluxes over the nine
-400
100
glacial-interglacial terminations, the 30% uncertainty in ac200
cumulation rate is relatively small.
0
-420
100
Both Ca2+ and Na+ in the ice core originate from sea salt
aerosol and terrestrial dust. However, Ca2+ is predominantly
x2
0
-440
of terrestrial origin, while Na+ derives mainly from sea salt.
t2
We calculated the non-sea-salt fraction of Ca2+ of terrestrial
-460
origin (nssCa2+ ) and the sea-salt fraction of Na+ (ssNa+ )
124,000
128,000
132,000
136,000
140,000
Age (a B.P.)
as in Röthlisberger et al. (2002), using a Ca2+ /Na+ weight
ratio of 1.78 for terrestrial material (Rt), and 0.038 for sea
Figure 2. Example of RAMPFIT results for δD at Termination II. The black line represents
water (Rm). The contribution from crustal material to total
Fig. 2. Example of RAMPFIT results for δD at Termination II.
the rampThe
that black
best fits
therepresents
data based the
on weighted
least-squares
regression.
Arrows
line
ramp that
best fits the
data based
on indicate
Na+ depends on the composition of the dust source material,
the
levels
x1
and
x2
and
the
change
points
t1
and
t2.
The
histograms
show
the
change
points
weighted
least-squares
regression.
Arrows
indicate
the
levels
x1
and higher ratios Rt of the terrestrial source material could
+
and
x2
and
the
change
points
t1
and
t2.
The
histograms
show
the
for 400 bootstrap simulations. The distribution of these simulated change points is used to
be used (Bigler et al., 2006), resulting in lower ssNa conchange points for 400 bootstrap simulations. The distribution of
centrations during glacial periods, but hardly any changes for
derive an estimate of the uncertainty of the change points.
these simulated change points is used to derive an estimate of the
interglacial periods. The effect of choosing different values
0 of the change points.
uncertainty
2+
for Rt on nssCa is negligible. For the aim of this study,
T IV
the exact amplitude of glacial-interglacial changes in ssNa+
does not affect the timing of the changes, and the results ret2* and1 x2*. This procedure is repeated 400 times, and the
main within error bars regardless of which values are used to
standard deviation of these 400 t1* values is used as boot+
2+
calculate ssNa and nssCa .
strap standard error for t1 (see histograms
in Fig.
t2: 341,632
± 2972). The
All parameters were measured along the same core, i.e.
t2*: 341,130 ± 116
bootstrap
2 standard errors for x1, t2 and x2 were calculated
the records are all on the same timescale (EDC3, Parrenin
t1: 336,250 ± 294
analogously.
et al., 2007). The uncertainty in matching the three paramet1*: 336,390 ± 358
While the algorithm of RAMPFIT provides an objective
ters is always much smaller than the resolution of δD. There3 for the change points in a given data set, there are
estimate
fore, the data offers excellent control of the relative timing
334,000 some
336,000
338,000 that
340,000
344,000
nevertheless
parameters
need to342,000
be chosen
subbetween the proxies. However, the uncertainty of the abAge (a B.P.)
jectively
in
the
fitting
procedure
(e.g.
the
selection
of
the
fit
solute age is estimated 3 ka at 100 ka BP and approximately
2+
interval)
that
influence
the
result.
For
this
study,
we
norFigure
3.
Example
of
RAMPFIT
results
for
nssCa
flux
at
Termination
IV.
The
y-axis has
6 ka for older sections of the ice core. In terms of event duramally
chose
2
ka
beyond
either
end
of
the
termination.
In
been reversed. The orange line corresponds to a ramp fitted over the entire section; the black
tions, the accuracy of the chronology is estimated to be 20%
some instances,
weramps.
chose
in order
to for
ex-the single
back to 410 ka BP and possibly 40% for older sections (Parlines correspond
to two separate
Theshorter
estimatedintervals
change points
are given
clude
sections
of
the
data
that
had
large
deviations
from
a
renin et al., 2007).
ramp (t1, t2) and for the corresponding two-step ramp (t1*, t2*).
constant level and would thus distort the ramp. The parameters used for each termination are listed in the supple2.2 RAMPFIT
21
mentary material (Table S1: http://www.clim-past.net/4/345/
2008/cp-4-345-2008-supplement.pdf).
In order to estimate the exact timing of a glacial termination in an objective way, we used a regression approach
RAMPFIT minimizes the systematic deviations from con(RAMPFIT, (Mudelsee, 2000)). RAMPFIT is a weighted
stant glacial and interglacial levels and from the assumed linleast-squares method that fits a ramp to the data. It estimates
ear change from glacial to interglacial conditions. However,
the level of a parameter for glacial (x2) and interglacial (x1)
a glacial termination does not necessarily conform to this
conditions and a linear change between the change points t1
simplified shape. For example, the interglacial levels need
and t2 (Fig. 2). A measure of the uncertainty of these estinot be constant, or the termination may not progress linearly,
mated change points is based on a set of 400 bootstrap simbut may change slope over time. This is illustrated in Fig. 3,
ulations for each parameter and each termination (Mudelsee,
where the Termination IV in nssCa2+ flux is shown. As2000; Politis and Romano, 1994).
suming a linear change over the entire termination results in
change points that deviate from the change points that result
A simulated time series is generated by adding a sequence
from the assumed two-step termination. Alternatively, the
of successive residuals to the fitted ramp. The length of the
glacial or interglacial levels are not constant, as for example
sequence of residuals is defined by the persistence of the data
the interglacial levels in δD in Termination III and IV, which
(four times the persistence time), thus preserving the autorereach high values early in the interglacial, but drop after a
gressive properties of the original data in the simulated time
few thousand years (Fig. 4). This impacts the resulting levels
series. The ramp fitting is then repeated on the simulated
and thus change points that RAMPFIT estimates. In other
data, giving a set of simulated ramp parameters, t1*, x1*,
www.clim-past.net/4/345/2008/
Clim. Past, 4, 345–356, 2008
Figure 2. Example of RAMPFIT results for δD at Termination II. The black line represents
the ramp that best fits the data based on weighted least-squares regression. Arrows indicate
the levels x1 and x2 and the change points t1 and t2. The histograms show the change points
for 400 bootstrap
simulations. The distribution of these simulated
change points isetused
348
R. Röthlisberger
al.:toGlacial
terminations in the EPICA Dome C ice core record
derive an estimate of the uncertainty of the change points.
In order to study the sensitivity of the westerly winds to
changes in sea surface temperature, a series of experiments
using different prescribed SST patterns were made (Fig. 7).
The SST patterns were designed as follows: One set of ex1
periments, labelled GLO in Fig. 7, was run with SST being
reduced globally by 1◦ C to 4◦ C. A second set of experiments
t2: 341,632 ± 297
(EXT) was run with only extratropical (30◦ to pole) SST bet2*: 341,130 ± 116
2
ing reduced by 1◦ C to 4◦ C, with a linear reduction of the
t1: 336,250 ± 294
t1*: 336,390 ± 358
cooling towards 0◦ C at 20◦ and no cooling over the tropics. In another set of experiments (GRA), SST cooling in3
creases linearly from the equator to the polar regions. And
334,000 336,000 338,000 340,000 342,000 344,000
experiments with increasing extensions of sea ice (ICE) withAge (a B.P.)
out changes to SST were made to separate the effect of SST
Figure 3. Example of RAMPFIT results for nssCa2+ flux at Termination
IV. The y-axis has
from sea ice. Further details of the model experiment setup
Fig. 3. Example of RAMPFIT results for nssCa2+ flux at Terminabeen reversed.
TheThe
orange
linehas
corresponds
to a ramp
fitted
over the
section; the black
are given in Sime et al. (2008).
tion IV.
y-axis
been reversed.
The
orange
lineentire
corresponds
0
log nss-Ca2+ flux
μg/m2/a
T IV
lines correspond
to two
separate
ramps.
The
estimated
are given fortothe single
to a ramp
fitted
over the
entire
section;
thechange
black points
lines correspond
separate
The estimated
change
ramp (t1,two
t2) and
for the ramps.
corresponding
two-step ramp
(t1*,points
t2*). are given for the
single ramp (t1, t2) and for the corresponding two-step ramp (t1*,
t2*).
words, the choice of the fit interval used for RAMPFIT had
some influence on the resulting change point estimates.
While we are aware of the limitations of the ramp model,
we still think it provides valuable insights into the phasing at
glacial terminations. A model is supposed to bring out the
major properties of a system rather than reflecting the full
complexity of the data. Additional parameters in a model that
would lead to a better fit between model and data do not necessarily improve the understanding of the system. In the case
of quantifying phasing at glacial terminations as presented
in this paper, we decided that a ramp, and in some cases a
two-step ramp, provides a sufficiently complex description of
the data, especially for the main purpose of defining change
points.
2.3
Model experiments
Experiments using the HadAM3 model (Pope et al., 2000)
were made in order to investigate the relationship between
climate parameters in South America and Antarctica. In
particular, we looked into the response of westerly winds
to changes in sea surface temperatures (SST), and how the
changes in winds relate to changes in Dome C temperature.
The atmospheric model HadAM3 has a regular latitude longitude grid with a lower horizontal resolution of
2.5◦ ×3.75◦ , and 19 hybrid coordinate levels in the vertical.
The model climatology is similar to that observed. Additionally, where run in its coupled state, the model produces realistic simulations of coupled wind-dependent low-frequency
variability e.g. El Nino Southern Oscillation and North
Atlantic Oscillation events (Gordon et al., 2000; Collins et
al., 2001), suggesting its suitability for investigating the behaviour of the westerlies.
Clim. Past, 4, 345–356, 2008
3
Results and discussion
21
The final ramps as calculated using RAMPFIT are shown in
Fig. 4 and the estimated change points and error estimates
are given in Table 1. A schematic of a termination is shown
in Fig. 5. Across all terminations, δD and nssCa2+ flux start
to change synchronously (within error estimate, indicated by
arrow (a) in Fig. 5), while ssNa+ flux shows a delayed onset of the glacial termination by a few thousand years (arrow
(b) in Fig. 5). On the other hand, the end of the termination
is normally synchronous between ssNa+ flux and δD (arrow
(d) in Fig. 5). Interglacial levels of nssCa2+ flux are either
reached at the same time as δD and ssNa+ flux (Terminations
I, V, VI, VII, VIII) or several thousand years earlier (Terminations II, III, IV, IX; arrow (c) in Fig. 5). In Terminations
VI and VIII, the amplitude of the temperature change was
relatively small. In these terminations, ssNa+ flux did not
change significantly.
The δD in the ice core record is used primarily as a proxy
for Antarctic temperature. However, especially at glacial terminations, other factors that have an influence on δD such
as isotopic composition of seawater, moisture source region
and local topography changes potentially changed quite significantly too. Therefore, the timing of the change in temperature compared to the changes in δD may be different.
However, the change point estimates with RAMPFIT were
also done on the reconstructed Dome C temperature Tsite (not
shown), which takes changes in moisture source region into
account (Stenni et al., 2003). Estimates for the beginning of
interglacial periods (t1) tend to be a few hundred years later
in Tsite than in δD, however, considering the uncertainty in
the estimates, the change points were not significantly different from the change points estimated based on δD.
While the time difference between the start of the deglaciation in δD and the change point in ssNa+ flux tends to vary
between 1 and 5 ka, the level of δD (−420±5‰) at the beginning of the termination in ssNa+ flux (t2) seems to be
nearly constant over all terminations (Table 2). Similarly,
www.clim-past.net/4/345/2008/
628,000
Age (a B.P.)
636,000
738,000
Age (a B.P.)
746,000
T IX
Fig. 4. Glacial terminations and ramps (black) estimated by RAMPFIT. δD in ‰(green), nssCa2+ (blue) and ssNa+ fluxes (red) in µg/m2 /a.
Y-axes for ssNa+ and nssCa2+ have been reversed.
Figure 4. (continued)
780,000
3.2
2.8
2.4
788,000
Age (a B.P.)
796,000
1.5
δD
528,000
Age (a B.P.)
536,000
-440
24
520,000
3.2
2.8
2.4
1.5
240,000
3.2
2.8
2.4
-440
-420
-400
-380
-360
120,000
3.2
2.8
2.4
-440
-420
-400
-380
-360
8,000
3.2
2.8
2.4
-440
-420
-400
-380
-360
248,000
Age (a B.P.)
128,000
Age (a B.P.)
256,000
136,000
12,000
16,000
Age (a B.P.)
3
2.5
2
1.5
1
T III
T II
3
2.5
2
1.5
1
3
2.5
2
1.5
1
20,000
TI
reversed.
23
22
nssCa2+ (blue) and ssNa+ fluxes (red) in μg/m2/a. Y-axes for ssNa+ and nssCa2+ have been
Figure 4. Glacial terminations and ramps (black) estimated by RAMPFIT. δD in ‰ (green),
3
2.5
2
1
3
2.5
-420
-400
-380
416,000
3.2
2.8
2.4
2
1.5
T VI
TV
-440
432,000
346,000
3
2.5
2
1.5
1
1
424,000
Age (a B.P.)
338,000
Age (a B.P.)
T IV
-420
-400
-380
330,000
3.2
2.8
2.4
-440
-420
-400
-380
-360
Figure 4. (continued)
3
2.5
2
1
-440
3
2.5
-420
-400
-380
730,000
3.2
2.8
2.4
1.5
2
1
-440
3
2.5
-420
-400
-380
620,000
3.2
2.8
T VIII
-440
2
1.5
2.4
1
-420
-400
T VII
δD
log ss-Na+ flux
δD
log ss-Na+ flux
δD
log ss-Na+ flux
δD
log ss-Na+ flux
log nss-Ca2+ flux
log nss-Ca2+ flux
log nss-Ca2+ flux
log ss-Na+ flux
δD
log ss-Na+ flux
δD
log ss-Na+ flux
log nss-Ca2+ flux
log nss-Ca2+ flux
log nss-Ca2+ flux
δD
log ss-Na+ flux
δD
log ss-Na+ flux
log nss-Ca2+ flux
log nss-Ca2+ flux
www.clim-past.net/4/345/2008/
log nss-Ca2+ flux
-380
R. Röthlisberger et al.: Glacial terminations in the EPICA Dome C ice core record
349
Clim. Past, 4, 345–356, 2008
350
R. Röthlisberger et al.: Glacial terminations in the EPICA Dome C ice core record
Table 1. The timing of glacial terminations in δD, log(nssCa2+ flux) and log(ssNa+ flux). t1 corresponds to the time when interglacial
levels are reached, t2 to the time when the first deviation from glacial levels is observed. In some instances the analysis with RAMPFIT
was done over two subsections in order to take account of a two-step shape of the termination. See methods for details regarding RAMPFIT.
The uncertainty in t1 and t2 for δD is likely underestimated by RAMPFIT for Terminations VI, VII, VIII, IX due to the coarse temporal
resolution of the data. For these terminations, the values in brackets are derived by RAMPFIT. The values in italic correspond to the average
spacing of the data at that age, which is used as an estimate of the uncertainty.
nssCa2+
δD
ssNa+
t1
±
t2
±
t1
±
t2
±
t1
±
t2
±
I
11 686
14 796
235
147
12 350
17 630
252
150
10 990
14 150
620
132
13 970
17 630
548
86
11 330
186
13 370
239
II
129 459
82
135 402
110
132 091
190
135 371
110
129 731
192
133 731
103
III
242 867
248 293
111
641
246 528
251 915
148
718
245 491
248 111
354
347
246 871
252 091
339
303
242 891
455
246 451
202
IV
334 103
301
341 121
403
336 392
339 872
358
114
338 752
341 132
353
116
334 792
313
338 732
198
V
425 813
471
430 693
617
425 053
201
430 833
151
425 973
426
429 013
405
VI
529 157
(246) 340
531 284
(233) 540
529 055
234
531 405
203
VII
626 073
(83) 720
629 486
(58) 1320
625 922
218
629 886
203
625715
158
627 438
129
VIII
737 354
(225) 580
740 867
(214) 590
737 784
184
741 171
192
IX
787 298
(221) 640
796 449
(106) 1040
789 788
325
796 937
346
787 083
789 590
96
128
788 270
792 007
70
115
ssNaflux
nssCaflux
1.6
δD
2
δD (‰) t2ssNaflux
Average
Standard deviation
Standard error
d)
-400
2.4
2.8
-420
c)
−412.6
−416.2
−426.8
−419
−426.4
–
−419
–
−420.2
−392
−401.1
−413.2
−400.4
−392.8
(−416.6)
−405.3
(−417.4)
−407.2
−420
5.1
1.9
−402
7.6
2.9
-440
3.2
b)
a)
0
2
4
6
Age (ka)
8
10
Figure 5. Schematic of a glacial termination. Arrows indicate change points in nssCa2+ flux
Fig. 5. Schematic
of a glacial termination. Arrows indicate change
and ssNa+ flux (Y-axis for log fluxes reversed). Dashed lines indicate the threshold levels in
points in nssCa2+ flux and ssNa+ flux (Y-axis for log fluxes reδD.
versed).
Dashed lines indicate the threshold levels in δD.
3.6
4
(a)
the timing
of the end of the termination in
nssCa2+ flux
3.2
3
(t1) and
δD varies considerably (0 to 2.5 ka), but the level
2.8
2
of δD2.4(−402±8‰) is more or less constant
(Table 2). In
other words, there are threshold levels in δD1 that correspond
2
to the onset of changes in ssNa+ flux and0 to the end of
log nssCa2+ flux (μg/m2/a)
I
II
III
IV
V
VI
VII
VIII
IX
δD (‰) t1nssCaflux
-380
log ssNa+ flux (μg/m2/a)
Termination
-360
δD
Table 2. Levels of δD at the change points of glacial sea salt
(t2ssNaflux ) and interglacial dust (t1nssCaflux ). Missing values in
t2ssNaflux and values in parentheses in t1nssCaflux represent terminations where the thresholds for ssNa+ flux and nssCa2+ flux were
not reached.
TI
T II
T III
T IV
TV
T VI
1.6
T VII
1.2
-460
T II
T III
T IV
TV
T VI
T VII
T VIII
-440
(b)
TI
T VIII
T IX
Clim. Past, 4, 345–356, 2008
log fluxes
Term.
T IX
-420
-400
δD (‰)
-380
-360
-1
-460
-440
-420
-400
-380
www.clim-past.net/4/345/2008/
δD (‰)
-360
Figure 6. Sea salt (a) and dust flux (b) versus δD over glacial terminations. The thresholds are
and ssNa flux (Y-axis for log fluxes reversed). Dashed lines indicate the threshold levels in
δD.
R. Röthlisberger et al.: Glacial terminations in the EPICA Dome C ice core record
3.6
351
4
(a)
(b)
log nssCa2+ flux (μg/m2/a)
log ssNa+ flux (μg/m2/a)
3.2
2.8
2.4
TI
T II
T III
2
T IV
TV
T VI
1.6
T VII
3
2
TI
T II
1
T VI
0
T VIII
-440
T IV
TV
T VII
T VIII
T IX
1.2
-460
T III
T IX
-420
-400
δD (‰)
-380
-360
-1
-460
-440
-420
-400
δD (‰)
-380
-360
Fig. 6. Sea salt (a) and dust flux (b) versus δD over glacial terminations. The thresholds are indicated by a vertical dashed line. They
are reflected in the different slopes in the relationship between sea salt (dust) and δD above and below the thresholds. The data from each
termination are plotted in a different colour.
Figure 6. Sea salt (a) and dust flux (b) versus δD over glacial terminations. The thresholds are
indicated
byflux.
a vertical
dashedgive
line.
They
reflected
in the
different
the
on various
transport
models,slopes
changes in
in long-range
changes
in nssCa2+
These thresholds
rise to
the are Based
transport (e.g. shorter transport times due to stronger winds)
apparent phase lags at the start or end of terminations. At
relationship between sea salt (dust) and δD above
and below the thresholds. The data from
are unlikely to account for a large proportion of the observed
glacial inceptions, the same threshold value seems to hold
+
2+
changes (Krinner and Genthon, 2003; Lunt and Valdes,
for ssNa
(not shown).are
However,
flux tendscolour.
to
eachflux
termination
plottednssCa
in a different
build up more gradually at glacial inceptions. Nevertheless,
2001), in line with evidence based on the size distribution of
the dust particles (Lambert et al., 2008). Based on the comthe threshold found for glacial terminations defines the level
parison of the dust records from the two EPICA ice cores,
below which the millennial-scale variability in nssCa2+ flux
Fischer et al. (2007) conclude that transport and lifetime efcorrelates with δD.
fects have changed dust fluxes in Antarctica by less than
These thresholds are illustrated in Fig. 6. While there
a factor of 2, while Lambert et al. (2008) suggest that apseems to be a fairly close relationship between nssCa2+ flux
and δD during glacial periods up to δD of approximately
proximately a factor 5 of the glacial-interglacial change in
dust flux might be explained by changes to the atmospheric
−402‰, the relationship vanishes beyond this point and
lifetime while another factor of 5 is due to changes
nssCa2+ flux seems to be unrelated to δD. For ssNa+ flux,
25at the
source. However, the change in atmospheric lifetime due to
there is a good correlation with δD for values higher than apa reduced hydrological cycle is difficult to quantify. Results
prox. −420‰, however, below that, the ssNa+ flux seems to
from a dust tracer model forced by a GCM showed only a
stay more or less constant.
marginal increase in lifetime, although with considerable un3.1 Coupling of South America and Antarctica
certainty due to a poorly constrained scavenging ratio (Lunt
and Valdes, 2002). The model produced a greatly increased
The generally close coupling between South American dust
Patagonian dust source during the LGM, mostly due to a deflux and Antarctic climate during cold glacial conditions has
crease in soil moisture, with some contribution of decreased
been discussed recently based on the dust particle numbers in
vegetation and increased land area during periods of low sea
the Dome C ice core compared to δD (Lambert et al., 2008),
level.
and similar threshold levels below which the coupling manBased on results from an atmospheric general circulation
ifests itself are derived. The factors that could influence the
model (HadAM3), we looked into the relationship between
dust deposition in Antarctica are various parameters at the
changes in temperature at Dome C and various parameters of
source (size of source area; conditions at the source, i.e. soil
South American climate. The model provides a good repremoisture, surface wind speed, vegetation, snow cover), atsentation of the present-day westerlies (Sime et al., 2008) and
mospheric long-range transport (i.e. wind systems and wind
modern Antarctic climatology (Connolley and Bracegirdle,
speed) as well as the atmospheric lifetime of the dust parti2007). In order to study the sensitivity of the South Amercles. The implication of the threshold is that there is a point in
ican climate to changing conditions, several model experithe evolution of the climate system (represented by Antarcments were run with the present-day setup but with imposed
tic temperature) beyond which one or more of these factors
sea surface temperature (SST) anomalies (Sime et al., 2008).
either ceases to change or ceases to influence dust.
A robust result from the Sime et al. study is the weakening
of the westerly winds over southern high latitudes associated
www.clim-past.net/4/345/2008/
Clim. Past, 4, 345–356, 2008
352
R. Röthlisberger et al.: Glacial terminations in the EPICA Dome C ice core record
most relevant latitudes could lead to a strong change in dust
entrainment. This could therefore provide a link between
the dust entrainment and the temperature at Dome C. The
effect of changes in SST on dust entrainment may also be
influenced by associated changes in precipitation. Increasing Southern Hemisphere SST leads to increased precipitation over Patagonia, therefore reducing the potential dust uplift due to raised soil moisture. Increased precipitation also
promotes vegetation cover, as the major limiting factor for
vegetation growth in Patagonia is precipitation (Markgraf et
al., 2002), which further reduces dust uplift. Additionally,
changes in South American topography due to ice cap disintegration at glacial terminations may have had an influence
on wind pattern over the South American dust source.
From this analysis it is difficult to distinguish between
the possible impacts of wind changes over dust source regions and of non-linear processes involved in dust entrainFig. 7. Relationship between South American wind speed and temFigure
7. Relationship
South
American wind
speed
temperature
C forIta is likely that the threshold velocity for dust enment.
perature
at Dome between
C for a set
of experiments
with
theand
Hadley
centre at Dome
atmospheric
model
HadAM3.general
Each circulation
data pointmodeltrainment
set of
experimentsgeneral
with thecirculation
Hadley centre
atmospheric
HadAM3. was reduced during glacial periods due to less
represents
annual
average
wind
speeds
over
South
American
landprecipitation
and therefore less vegetation cover. AdditionEach data point represents
annual average wind speeds over South American landmasses
masses south of 40◦ S against modelled temperature at Dome C for
ally,
the
average
wind speed was increased during glacial
south
of 40°S model
against experiment.
modelled temperature
at Dome C for
different
model
GLO
different
GLO experiments
have
uniform
SSTexperiment.
periods,
and
potentially
also the gustiness (i.e. the likeliexperiments
SST cooling
fromwhich
present-day
form 1 to
cooling have
from uniform
present-day
conditions
rangeconditions
form 1 towhich
4◦ C rangehood
of substantially higher than average wind speeds over
by the
thesubscript.
subscript.EXT
EXT
experiments
uniform
SST from present4°Cas
as indicated
indicated by
experiments
havehave
uniform
SST cooling
short periods of time). Due to the non-linear relationship
cooling from present-day conditions over latitudes poleward of 30◦ ,
day conditions over latitudes poleward of
30°, with a linear
reduction towards 0°Cbetween
cooling at dust entrainment and wind speed, especially near
◦
◦
with a linear reduction towards 0 C cooling at 20 and no cooling
an
entrainment threshold, even small increases in average
20° over
and no
cooling
over
the
tropics.
GRA
experiments
have
a
zonally
uniform
SST
cooling
the tropics. GRA experiments have a zonally uniform SST
wind
speeds and gustiness could have a significant impact on
cooling
which
linearly
increases
from to
thethe
equator
the Polar
Re-experiments have
which
linearly
increases
from
the equator
Polar toRegions.
ICE
dust
gions.
ICE
experiments
have
increasing
extensions
of
sea-ice.
Fur2 entrainment. The combination of the changes in wind
increasing extensions of sea-ice. Further experiment details are available in Sime et al.
speed distribution and threshold velocity could qualitatively
ther experiment details are available in Sime et al.
explain the observed threshold behaviour in the Dome C
nssCa2+ flux record. The low dust flux levels during mild
with a warming at Dome C. This weakening appears to destages could thus reflect average wind speeds falling below
pend on a reduction in the meridional surface temperature
the entrainment threshold, possibly in combination with regradient across the Southern Hemisphere; the gradient itduced gustiness and higher threshold velocities due to inself is generally dominated by changes in Antarctic tempercreased soil moisture and vegetation cover. However, in orature. The weakening simulated is liable to cause changes in
der to make a quantitative assessment of the causes for the
dust transport. However, Lunt and Valdes (2001) note that
threshold found in the nssCa2+ flux record at Dome C, a
changes of this type cannot be responsible for more than a
model with a higher resolution that includes tracer uplift and
small fraction of the dust changes seen in the Dome C record.
transport
26 is required.
Over the South American dust entrainment region, the
wind speed also decreases in response to Dome C warming.
Figure 7 shows that for simple zonally uniform SST anomaly
experiments, the mean annual wind speed over South America depends almost linearly on Dome C temperature. This
is partly dependent on latitudinal shifts in the belt of the
westerlies, but changes in the intensity and width of the wind
band also play a significant role (Sime et al., 2008). Further
experiments in Sime et al. (2008) indicate that the magnitude of the decrease is dependent on zonal variations in SST
anomalies (not shown here). However, the general wind
speed decrease is robust over the range of realistic experiments performed.
Dust entrainment is non-linearly related to surface wind
speed, so that a modest decrease in surface wind speed at the
Clim. Past, 4, 345–356, 2008
3.2
Sea salt – sea ice relationship
In Antarctica, a large proportion of sea salt aerosol originates
from sea ice surfaces rather than open water (Rankin et al.,
2000; Wagenbach et al., 1998; Jourdain et al., 2008). Therefore, sea salt fluxes at Dome C have been used to infer past
changes in sea ice extent in the Southern Ocean sector close
to Dome C (Wolff et al., 2006; Wolff et al., 2003). As a
first order approximation, one would expect to see a coupling between Antarctic temperature and sea ice extent. This
should manifest in a general agreement in the δD and ssNa+
flux records from Dome C. While there indeed seems to be a
strong correlation between δD and ssNa+ during mild stages
(r=−0.80, significant at the 95% confidence level (Mudelsee,
2003), using 55 cm averages for ssNa+ flux and δD and
www.clim-past.net/4/345/2008/
R. Röthlisberger et al.: Glacial terminations in the EPICA Dome C ice core record
353
Table 3. Rate of change in δD over terminations.
Termination
Duration
(years)
Change rate
(‰/year)
±
Rate of change (‰/year)
0.03
Ia
664
−0.025 0.0170
Ib
2834
−0.011 0.0011
0.02
II
5943
−0.011 0.0003
IIIa
3661
−0.013 0.0009
IIIb
3622
−0.003 0.0011
0.01
IV
7018
−0.010 0.0009
V
4880
−0.008 0.0016
VI
2127
−0.007 0.0015
0
VII
3413
−0.010 0.0029
I
II
III
IV
V
VI
VII VIII
IX
VIII
3513
−0.007 0.0011
Termination
IX
9151
−0.005 0.0004
Figure
8. Rate
of change
calculated
based
on termination
ramps
in δD. For T I and T III, two
Fig.
8. Rate
of change
calculated
based
on termination
ramps
in δD.
For
T I and
T III,
twotoramps
were
fitted
data
(see 1),
Fig.and
4 and
ramps
were
fitted
the data
(see
Fig.to 4the
and
Table
the rate of change has been
Table 1), and the rate of change has been calculated for each ramp
calculatedError
for each
separately.
Error
bars correspond
one standard error. The black
separately.
barsramp
correspond
to one
standard
error. The to
black
In view of this, the delayed onset of changes in ssNa+ flux
diamondatatTTI Icorresponds
correspondsto
to the
the short
short warming
warming around
diamond
around 12
12ka
ka BP.
BP. It seems as if this warming
with respect to the start of the warming at glacial terminaIt was
seems
as if this warming
was the
exceptionally
however,
the short duration compared to
exceptionally
fast, however,
uncertaintyfast,
is large
due to the
tions can be seen as the time when the sea ice proxy starts to
uncertainty is large due to the short duration compared to the unrespond
changes
the
uncertainty
in
the
determination
of
t1
and
t2.
The
early
part
of
Termination
III to
seems
to be in sea ice again, i.e. when sea ice has recertainty in the determination of t1 and t2. The early part of Tertreated
far
enough
mination
III seems
be exceptionally
slow (blacktodiamond)
exceptionally
slowto (black
diamond) compared
the othercomterminations (see also Fig. 4).so that further changes leave an imprint in
the sea salt aerosol flux at Dome C. The end of the terminapared to the other terminations (see also Fig. 4). Also, TerminaAlso, Termination IX seems to progress slower than average. However, the uncertainty of the
tion is synchronous in δD and ssNa+ , reflecting the expected
tion IX seems to progress slower than average. However, the unrate of change
is most
likely is
considerably
The horizontal
lines represent
the Antarctic temperature and sea ice. A
certainty
of the rate
of change
most likelyunderestimated.
considerably underesrelationship
between
timated.
represent the average
of change
of two
the reduced sensitivity of sea salt flux as a sea
averageThe
rate horizontal
of change lines
of 0.0091±0.0025
‰/year ,rate
calculated
from allconsequence
data except the
of 0.0091±0.0025‰/year , calculated from all data except the two
ice proxy during full glacial conditions is that we cannot infer
blackdata
datapoints.
points.
black
the timing of sea ice changes in relation to changes in CO at
2
all data with δD>−420‰), the relationship seems to break
down during cold glacial conditions (r=−0.19, still significant at the 95% confidence level, for data with δD<−420‰).
Obviously, during this time, one expects large sea ice extent around Antarctica, extending to over a thousand kilometres from the coast into the Southern Ocean (Gersonde
et al., 2005). Sea salt aerosol produced at the distant margin of the sea ice cover will need to be transported over
such long distances before reaching Dome C. However, it
has been shown that the atmospheric sea salt aerosol concentration rapidly decreases with increasing transport distance,
with only a small percentage of the original amount remaining after 500 km of transport (Minikin et al., 1994). Increasingly colder conditions will likely be accompanied by additional sea ice at the outer edge, at a distance of several
hundred kilometres. But despite adding a considerable sea
salt source area, the transport distance is so large that only
a small fraction of this extra sea salt aerosol makes it to the
East Antarctic plateau (Fischer et al., 2007). Although this
requires confirmation, it appears likely that eventually, the
effect of additional sea ice cannot be discriminated any more
in the sea salt records. In other words, the sensitivity of sea
salt flux at Dome C as a proxy for sea ice was decreased substantially during times of very large sea ice extent.
www.clim-past.net/4/345/2008/
the start of a glacial termination based on the ice core record
alone. Conclusions made in earlier papers (Röthlisberger et
al., 2004; Wolff et al., 2006) require confirmation based on
independent sea ice reconstructions.
3.3
Rate of change
The results from RAMPFIT can also be used to calculate the
rate of change over each glacial termination. The glacial
– interglacial amplitude is estimated as the difference between x1 and x2, while the duration of a termination and
its uncertainty were calculated directly by RAMPFIT. The
uncertainty in duration, as well as in t1 and t2 for δD (see
Table 1), is likely underestimated by RAMPFIT for Termina27
tions VI, VII, VIII, IX. This is caused by the coarse temporal
resolution of the δD data. For these four terminations, the
average spacing between data points is used as an estimate
of the uncertainty of the change points and the duration, but
this may still underestimate the true uncertainty.
As seen in Fig. 8, the rate of change in δD was rather
similar for all terminations, of the order of 0.01‰/year,
which is equivalent to approximately 2◦ C/ka. Only for
the early part of Termination III and for Termination IX
the temperature seemed to rise at a slower rate (see Table 3). This was also observed in the rate of change in
nssCa2+ flux (Fig. S1: http://www.clim-past.net/4/345/2008/
Clim. Past, 4, 345–356, 2008
354
R. Röthlisberger et al.: Glacial terminations in the EPICA Dome C ice core record
cp-4-345-2008-supplement.pdf). The second warming step
in Termination I, on the other hand, may have been exceptionally fast, as previously identified based on an independent analysis of the same data set (Masson-Delmotte et al.,
2006). However, the uncertainty of this large rate of change
is substantial, and the average rate of change observed during
the other terminations lies well within the error bar. Generally, rates of change for ssNa+ and nssCa2+ flux over
the corresponding intervals were also rather similar for all
terminations. The first step in nssCa2+ flux change at Termination IV was faster than average, however, it was followed
by a period of rather constant nssCa2+ flux (which was not
seen in δD), before resuming the change into full interglacial
conditions (see Fig. 4). Averaged over the entire Termination IV, the rate of change in nssCa2+ flux was very similar
to the one observed at other terminations.
This implies that regardless of the final amplitude of the
glacial – interglacial temperature change, the climate system
keeps changing at a steady pace. The duration of the termination is therefore shorter in the case where the interglacial
temperatures were cool compared to the cases where rather
warm interglacial temperatures were reached. This could be
viewed as an external trigger (orbital forcing) timing the start
of a glacial termination, but internal amplifiers and feedbacks
(e.g. sea ice – albedo – temperature feedbacks or temperature
– CO2 feedbacks) governing the rate of change; the factors
that determine at what point (in time or climate) the termination ends remain uncertain.
4
Conclusions
The analysis of the nine glacial terminations recorded in the
Dome C ice core has provided insights into the phasing at
glacial termination. Over all terminations, a consistent pattern emerged, involving threshold values beyond which a
coupling between Antarctic temperature and Patagonian dust
proxy (nssCa2+ flux) on the one hand and the response of the
sea ice proxy (ssNa+ flux) on the other hand manifested itself.
Changes in South American dust emissions and Antarctic temperature are synchronous during cold glacial conditions but the dust response fades for conditions warmer
than approximately 2◦ C below the present-day temperature
at Dome C. The close link between dust and Antarctic temperature may be caused by the changes in wind pattern and
precipitation over Patagonia that co-evolve with changes in
temperature at Dome C.
Sea salt aerosol is closely linked to Antarctic temperature
for interglacial conditions and conditions down to approximately 5◦ C cooler than present day. For these conditions,
sea salt aerosol at Dome C can be used as a first-order proxy
of sea ice in the Indian Ocean sector around Antarctica. For
colder climate, the proxy is reaching some sort of saturation
and fails to respond to potential further increases in sea ice
Clim. Past, 4, 345–356, 2008
extent. One result of this analysis is that we are no longer
safe in suggesting that sea ice responded late in terminations
and using this to apportion causes of CO2 change; this conclusion was probably an artefact of the apparent threshold in
response to sea ice change (Röthlisberger et al., 2004; Wolff
et al., 2006).
The rate of change over glacial terminations as determined
from the duration and the amplitude of the changes in δD
seems to be rather similar over all glacial terminations. This
suggests that once a glacial termination is triggered, the climate system progresses at its own pace. An exception with
regard to this is a 3000 a period early on in Termination III
where the rate of change seemed to be reduced significantly
compared to the other terminations. It remains to be seen
what caused this period to progress more slowly.
Acknowledgements. This work is contribution No. 211 to the
European Project for Ice Coring in Antarctica (EPICA), a joint
European Science Foundation/European Commission scientific
programme, funded by the EU and by national contributions from
Belgium, Denmark, France, Germany, Italy, The Netherlands,
Norway, Sweden, Switzerland and the UK. The main logistic
support at Dome C was provided by IPEV and PNRA.
Edited by: G. Lohmann
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