cp 7 397 2011

cp 7 397 2011
Clim. Past, 7, 397–423, 2011
www.clim-past.net/7/397/2011/
doi:10.5194/cp-7-397-2011
© Author(s) 2011. CC Attribution 3.0 License.
Climate
of the Past
A comparison of the present and last interglacial periods
in six Antarctic ice cores
V. Masson-Delmotte1 , D. Buiron2 , A. Ekaykin3 , M. Frezzotti4 , H. Gallée2 , J. Jouzel1 , G. Krinner2 , A. Landais1 ,
H. Motoyama5 , H. Oerter6 , K. Pol1 , D. Pollard7 , C. Ritz2 , E. Schlosser8 , L. C. Sime9 , H. Sodemann10 , B. Stenni11 ,
R. Uemura1,12 , and F. Vimeux1,13
1 Laboratoire
des Sciences du Climat et de l’Environnemen, IPSL-CEA-CNRS-UVSQ, UMR 8212, Gif-sur-Yvette, France
and UJF, Laboratoire de Glaciologie et Géophysique de l’Environnement (LGGE, UMR 5183), Grenoble, France
3 Arctic and Antarctic Research Institute, 38 Beringa St., 199397 St. Petersburg, Russia
4 ENEA, Rome, Italy
5 Research Organization of Information and Systems, National Institute of Polar Research, 10-3, Midoricho, Tachikawa,
Tokyo, 190-8518, Japan
6 Alfred Wegener Institute for Polar and Marine Research, Helmholtz Association, Bremerhaven, Germany
7 Earth and Environmental System Institute, Pennsylvania State University, University Park, USA
8 Institute of Meteorology and Geophysics, University of Innsbruck, Innsbruck, Austria
9 British Antarctic Survey, Cambridge, UK
10 Norwegian Institute for Air Research, NILU, Kjeller, Norway
11 Department of Geosciences, University of Trieste, Trieste, Italy
12 Department of Chemistry, Biology and Marine Science, University of the Ryukyus, Nishihara, Okinawa, Japan
13 Institut de Recherche pour le Développement, IRD, Laboratoire HydroSciences Montpellier, HSM, UMR 5569,
CNRS-IRD-UM1-UM2, Montpellier, France
2 CNRS
Received: 27 September 2010 – Published in Clim. Past Discuss.: 26 October 2010
Revised: 2 March 2011 – Accepted: 10 March 2011 – Published: 28 April 2011
Abstract. We compare the present and last interglacial periods as recorded in Antarctic water stable isotope records
now available at various temporal resolutions from six East
Antarctic ice cores: Vostok, Taylor Dome, EPICA Dome C
(EDC), EPICA Dronning Maud Land (EDML), Dome Fuji
and the recent TALDICE ice core from Talos Dome. We first
review the different modern site characteristics in terms of ice
flow, meteorological conditions, precipitation intermittency
and moisture origin, as depicted by meteorological data, atmospheric reanalyses and Lagrangian moisture source diagnostics. These different factors can indeed alter the relationships between temperature and water stable isotopes. Using
five records with sufficient resolution on the EDC3 age scale,
common features are quantified through principal component
analyses. Consistent with instrumental records and atmospheric model results, the ice core data depict rather coherent
and homogenous patterns in East Antarctica during the last
two interglacials. Across the East Antarctic plateau, regional
Correspondence to:
V. Masson-Delmotte
([email protected])
differences, with respect to the common East Antarctic signal, appear to have similar patterns during the current and
last interglacials. We identify two abrupt shifts in isotopic
records during the glacial inception at TALDICE and EDML,
likely caused by regional sea ice expansion. These regional
differences are discussed in terms of moisture origin and in
terms of past changes in local elevation histories, which are
compared to ice sheet model results. Our results suggest that
elevation changes may contribute significantly to inter-site
differences. These elevation changes may be underestimated
by current ice sheet models.
1
Introduction
In the context of global warming, documenting past natural climatic variability in polar regions offers a benchmark
against which to test Earth system models (Masson-Delmotte
et al., 2006b). The current and last interglacial periods provide useful case studies to explore climate feedbacks in response to orbital forcing (NorthGRIP-community-members,
2004; Jouzel et al., 2007; Otto-Bliesner et al., 2006). The
Published by Copernicus Publications on behalf of the European Geosciences Union.
398
V. Masson-Delmotte et al.: A comparison of the present and last interglacial periods in six Antarctic ice cores
Figures EDML EDML (a)
(b) Figure 1a. 1 Location of deep drilling sitees going baack to MIS5
5.5: Vostok,, Dome Fujji, EPICA Figure 1b. Surface elevation (upper line), ice core depths (vertical rectangles) and position of Dome C (a)
(here labeelled “EPICA
and drilling
Dro
onning sites
Mau
ud going
Land, Taaylor and TALDICE. The Fig.
1.C Location
of A”) deep
backDome
toe MIS5.5:
Vostok,
Dome Fuji, EPICA Dome C (here labelled “EPICA”) and Dronning
the Holocene (yellow rectangles) and Last Interglacial (red rectangles) periods (as defined in grey shLand,
ading displ
lays presen
nt day and
Antaarctic elevattion. The b grey
shadin
g displays monthly Maud
Taylor
Dome
TALDICE.
Theblue shading
displays
present
elevation.
blueexception shading
displays
monthly
Table 2, using day
EDC3 Antarctic
age scale for all records The
with the of TD). Note that the mean
sea
ice concentration
in (left) and A
Februaryugust (right
(left) and
August
(right).
Maps
were
generated
using
the
NSIDC
software
(http://www.nsidc.org).
mean se
ea ice conce
entration in
n February (
t). Maps we
ere generat
ed using horizontal axis is not proportional to distance. The
distance
from
EPICA
Dome
Vostok
isEPICA 560 Do
km,
Vostok to Dome F ∼1500 km. (b) Surface elevation (upper line), ice
the NSI
DC softwar
re (http://w
www.nsidc.o
org). C
The todistance from
m ome and
C to Vostok Vfrom is core
depths
(vertical
rectangles)
and position of the Holocene (yellow rectangles) and last interglacial (red rectangles) periods (as defined in
560 km,
, and from V
Vostok to D
Dome F ~150
00 km. Table
2, using EDC3 age scale for all records with the exception of TD). Note that the horizontal axis is not proportional to distance.
last interglacial period appears exceptionally warm in East
Antarctica, in the context of the past 800 ka (thousand of
years) (Watanabe et al., 2003; Jouzel et al., 2007).
We focus here on the description of Antarctic climate variability, which can be documented in high resolution by ice
core records of water isotopic composition (Masson et al.,
2000). Ice core isotopic composition is affected by climate
and water cycle variability through changes in evaporation
conditions, air mass distillation history, and local condensation conditions including snowfall
50 intermittency (Noone and
Simmonds, 1998; Jouzel et al., 2003; Masson-Delmotte et
al., 2006a; Sodemann and Stohl, 2009). Glaciological features can also affect ice core records through changes in local
elevation (Vinther et al., 2009) and in ice origin (Huybrechts
et al., 2007). The motivation of this study is to compare the
temporal trends and the spatial variability of Antarctic ice
core records of water isotopic composition for the present
and past interglacial periods, which are now available from
six Antarctic ice core sites (Table 1, Fig. 1), and to begin to
identify the processes accounting for inter-site differences.
A previous comparison of eleven Antarctic ice core
records spanning the Holocene period (Masson et al., 2000)
has revealed robust features, such as the early Holocene optimum and millennial variability, but also different local or
regional characteristics, especially in the Ross Sea sector.
Since this synthesis effort, many new records have become
available, such as the Siple Dome record in West Antarctica (Brook et al., 2005), two EPICA ice cores at Dome C
(Jouzel et al., 2007) and Dronning Maud Land (EPICAcommunity-members, 2006), the Dome Fuji ice core (Watanabe et al., 2003), and the Taylor Dome (Grootes et al., 2001)
and TALDICE ice cores in the Ross Sea sector (Stenni et
Clim. Past, 7, 397–423, 2011
al., 2011). The last five records also span the last interglacial, only covered by the Vostok (VK) core fifteen years
ago (Jouzel et al., 1993).
Hereafter, we compare the present and last interglacial ice
core water stable isotope records (0 to 145 ka) from the six
available records, by historical order, Vostok, Dome F (hereafter DF), EPICA Dome C (EDC), EPICA Dronning Maud
Land (EDML), Talos Dome ice core (TALDICE) and Taylor Dome ice core (TD) (Fig.
1, Table 1). All are located
51
on the East Antarctic Plateau, at elevations between 2315 to
3810 m, they face different ocean basins, with EDML and
Dome F being situated in front of the Atlantic Sector, Vostok, EDC in front of the Indian Ocean sector and TD and
TALDICE in the Ross Sea sector. Seasonal changes in sea ice
cover are particularly large in the Atlantic and Ross sea sector
(Fig. 1), potentially modifying the seasonal moisture origin
for the nearby sites (Sodemann and Stohl, 2009). Present-day
annual mean temperature ranges between −40 to −57 ◦ C at
these sites (Table 1). Most of our study focuses on the five ice
core sites offering high resolution records of both the current
and last interglacial periods, which is not the case for TD
due to the strong compression of the last interglacial ice at
this site (Fig. 1b, Table 2).
In Sect. 2, we introduce the orbital and deglacial contexts
of the present and last interglacial, and the patterns and sequences of events previously identified by comparison of the
EPICA Dome C records with climate reconstructions from
other latitudes (Masson-Delmotte et al., 2010a). Section 3
describes the meteorological and glaciological contexts at the
different sites. Section 4 presents the available stable isotope
records from the ice cores, and the dating uncertainties. Section 5 analyses the similarities and differences between the
www.clim-past.net/7/397/2011/
V. Masson-Delmotte et al.: A comparison of the present and last interglacial periods in six Antarctic ice cores
399
Table 1. Site characteristics. Accumulation (Acc.) is given in mm water equivalent per year (equivalent to 1 kg m−2 yr−1 ).
Site
Short
name
VK
Drilling
period
1982–1989
(Hole 3)
Latitude
Longitude
Vostok
78◦ 280 S
106◦ 480 E
Elevation
m a.s.l.
3488
Mean annual temperature
Acc.
10 m wind speed
Comments
−55.3 ◦ C (1958 to 2008 with
8 missing years)
Seasonal amplitude: 36.1 ◦ C
(www.nerc-bas.ac.uk/icd/gjma/)
21.5
4.2 m s−1
www.aari.aq/stations/vostok/vostok en.html
Records for the Holocene have
been compiled from several shallow
cores (BH7, BH8). Records for the
LIG are provided by core 3G.
EPICA Dome C
EDC
1993–2004
75◦ 060 S
123◦ 210 E
3233
−54.5 ◦ C
Seasonal amplitude: 34.9 ◦ C
(www.meteoclimat.net)
∼25
2.9 m s−1
(1984–2003)
Records for the Holocene are from
core EDC96 and for the LIG from
EDC99.
EPICA
Dronning
Maud Land
(Kohnen
Station)
EDML
2001–2006
75◦ 000 S
00◦ 040 E
2882
−44.6 ◦ C
Seasonal amplitude: 26.9 ◦ C
64
4.9 m s−1
M. van den Broeke (2010)
Main core data
Talos Dome
TALDICE
2004–2008
72◦ 490 S
159◦ 110 E
2315
−41 ◦ C
Seasonal amplitude: 24.5 ◦ C
(AWS Priestley station)
80
∼4.5–5.1 m s−1
(M. Frezzotti, personal communication, 2010)
Main core data
Dome Fuji
DF
1995–1996
and
2003–2007
77◦ 190 S
39◦ 400 E
3810
−57 ◦ C
Seasonal amplitude: ∼40 ◦ C
Fujita and Abe (2006)
23
5.4 m s−1
(http://polaris.nipr.ac.jp/∼domef/home/eng/page4.html)
Records are from DF1 ice core.
Taylor Dome
TD
1993–1994
77◦ 470 S
158◦ 430 E
2365
−43 ◦ C
?
Main core data
50–70
Table 2. Depth range where the present interglacial (in Antarctica, with EDC3 age scale, 0–12.1 kyr) and last interglacial (with EDC3 age
scale, 116–132.6 kyr) ice is found in each deep ice core (in m), mean number of years per m of ice for each ice core and each time period,
and thinning ratio between the two interglacial periods (estimated here as the ratio between the number of last interglacial years per m of ice
to the number of Holocene years per m of ice). The beginning of last interglacial is found in the middle of the ice depth at DF (59%), Vostok
(52%) and EDC (53%) but closer to bedrock at EDML (86%) and TALDICE (88%). Thinning is minimum at Vostok, due to the impact of
subglacial Lake Vostok on ice flow, and is maximum for TALDICE and Taylor Dome above a dry bedrock.
Holocene
depth
range
(0–12.2 ka
EDC3)
Number
of
Holocene
years per
m of ice
Holocene
sampling
Holocene
resolution
Last
interglacial
depth range
(116–132.6
ka EDC3)
Total ice
depth
Last
interglacial
depth span
Number of
Last
Interglacial
years per m
of ice
Sampling
available
Last
interglacial
resolution
DF
0–372 m
VOSTOK
0–274 m
DC
EDML
33 yr
∼0.25 m
∼8 yr
1637–1800 m
3035.2 m
163 m
102 yr
0.50 m
∼50 yr
3.1
44 yr
between 0
0.50 m
∼220 yr
and 138 m
(BH8); 5 m
down to
1413 m.
∼22 yr
1615–1903 m
3623 m
288 m
58 yr
2m
∼115 yr
1.3
0–376 m
32 yr
0.55 m
∼18 yr
1524–1744 m
3259.7 m
220 m
75 yr
0.55 m
∼40 yr
2.3
0–709 m
17 yr
0.50 m
∼8 yr
2284–2381 m
2774 m
97 m
171 yr
0.50 m
∼85 yr
10.1
TALDICE
0–691 m
18 yr
1.0 m
∼18 yr
1384–1419 m
1620 m
(but
bedrock
not
reached)
35 m
490 yr
1.0 m
0.05 m
∼490 yr
∼25 yr
27.2
TD
0–359 m
34 y
∼0.15 m
∼5 yr
525–530 m
554 m
5m
∼750 yr
∼0.22 cm
∼740 yr
22.1
present and last interglacials and among the ice core records,
using different methods. A strong homogeneity is depicted,
as well as site-specific anomalies which have similar patterns
during the present and last interglacial. We examine the potential sources of biases linked with changes in moisture origin, using the available deuterium excess – based temperature reconstructions. We finally compare the stable isotope
anomaly specific to each deep drilling site with past elevation
www.clim-past.net/7/397/2011/
Thinning
ratio
between
Holocene
and last
interglacial
reconstructions derived from ice flow models, before a summary of our results and their implications (Sect. 6) is given.
Clim. Past, 7, 397–423, 2011
400
2
V. Masson-Delmotte et al.: A comparison of the present and last interglacial periods in six Antarctic ice cores
Orbital and deglacial contexts for the present and
last interglacials
The present (Holocene or Marine Isotopic Stage 1) and last
interglacial period (Marine Isotopic Stage 5.5, or Eemian)
(Shackleton et al., 2003) has occured under different orbital hereafter also noted LIG for last interglacial configurations and has exhibited different mean climatic levels, amplitudes and trends at different latitudes (Masson-Delmotte
et al., 2010a) (Fig. 6b and c). Eccentricity has been much
stronger during the last interglacial than during the Holocene,
enhancing, the impact of precession and seasonal contrasts.
The phase between precession and obliquity has also been
different. During this last interglacial, obliquity reached its
maximum at 131 kyr (thousand of years before present) followed by a minimum in the precession parameter at 127 kyr,
while the Holocene precession parameter minimum occurred
at 12 kyr, followed by an obliquity maximum at 10–9 kyr.
While the orbital configuration is well known, the exact
mechanisms relating changes in Antarctic climate and orbital parameters remain controversial, with ice core studies
pointing to a link with Northern Hemisphere summer insolation (Kawamura et al., 2007), albeit with large lags with
respect to precession and obliquity (Jouzel et al., 2007), and
modelling studies pointing to the importance of local seasonal insolation (Huybers and Denton, 2008; Timmermann
et al., 2009) and possible biases due to changes in accumulation seasonality (Huybers, 2009). The present and last interglacial periods offer the possibility to explore the response
of climate to orbital forcing with roughly comparable contexts in terms of ice volume (Bintanja et al., 2005) and greenhouse gas concentrations (Siegenthaler et al., 2005; Loulergue et al., 2008), two of the major feedbacks at play during
glacial-interglacial transitions (Hansen et al., 2008; MassonDelmotte et al., 2010a).
The onset of the current interglacial in Greenland has been
precisely dated thanks to the GICC05 annual layer counting on Greenland ice cores (Rasmussen et al., 2006). The
abrupt warming ending the Younger Dryas cold period is
recorded at 11 703 years before year 2000 A (Vinther et al.,
2006). In Antarctic ice cores, several parameters can be
used to detect the onset and end of warm intervals: records
of local climate in ice core δ 18 O or δD, records of sea salt
or terrestrial aerosol deposition reflecting regional climate
conditions (in relationship with the sources of sea salt or
dust and transportation) in ice core chemistry, or records of
global atmospheric composition. In the EPICA Dome C
ice core, a δD threshold of −403‰ (Holocene average)
(EPICA-community-members, 2004) to −405‰ (threshold
marking the end of the glacial correlation between dust flux
and EDC δD, 10‰ below the late Holocene δD average)
(Röthlisberger et al., 2008; Petit and Delmonte, 2009) was
defined as the lower limit of an interglacial. Using a threshold of −405‰ on EDC δD and the EDC3 age scale (Parrenin
et al., 2007a) leads to an onset of the Antarctic present day
Clim. Past, 7, 397–423, 2011
interglacial at 12.2 kyr (1950 AD, or Before Present), therefore about 500–600 years before the onset of the Holocene
recorded in Greenland ice cores, and the parallel atmospheric
CH4 concentration rise (Severinghaus and Brook, 1999).
During the last interglacial, the EDC final abrupt methane
increase is dated at ∼128.6 kyr, while δD crosses the “interglacial” threshold at ∼132.4 kyr (3.8 ka earlier) and at
∼116 kyr, when the glacial inception appears in phase between northern and southern high latitudes (Landais et al.,
2005). There are therefore differences between the timing of
the onset of warm Antarctic intervals and the timing of interglacial periods as seen from the Northern Hemisphere, and
the early part of “Antarctic interglacial periods” are known
to be still affected by the final decay of glacial ice sheets and
associated changes in freshwater flux (Debret et al., 2009;
Renssen et al., 2010).
Atmospheric general circulation models equipped with
water stable isotopes have been extensively used to explore
the climatic controls on present day and glacial Antarctic
snowfall isotopic composition, and, so far, have simulated
a rather constant isotope-temperature relationship in Central East Antarctica between glacial and present-day conditions (Jouzel et al., 2007). Due to the difficulty of simulating past climates warmer than today in central Antarctica
in response to changes in orbital forcing (Overpeck et al.,
2006; Masson-Delmotte et al., 2010b), only few modelling
studies have been dedicated to the stability of the isotopetemperature relationship under warmer conditions (Schmidt
et al., 2007; Sime et al., 2008). The Dome C, Vostok and
Dome Fuji ice core records spanning the last 340 ka were
compared and model simulations were used to propose explanations for the differences amongst the records (Sime et
al., 2009b). Their climate projections showed relatively homogeneous temperature change induced by the A1B projection scenario across the three long East Antarctic ice-cores
sites. This, alongside with the comparison of ice core records
and isotopic modelling, led them to interpret the differences
in stable isotope ratios as reflecting changes in the isotopetemperature relationships, especially between Dome Fuji and
Dome C. This work points to non-linearities in some of the
isotope-temperature relationships, and to some uncertainty
in the individual core isotope-temperature conversions. They
concluded that peak Antarctic temperatures during the last
interglacial could have been more than 6 ◦ C above presentday. However, we note that an increased CO2 warming scenario is an imperfect analogue for the boundary conditions of
past interglacials.
It has been argued (Masson et al., 2000; Masson-Delmotte
et al., 2010a) that the early Holocene and last interglacial
optima recorded in EPICA Dome C isotopic records are
caused by a bipolar see-saw pattern occurring under interglacial contexts and caused by the Northern Hemisphere ice
sheet deglacial history, similarly to glacial Antarctic Isotopic
Maxima (Capron et al., 2010). As the deglacial freshwater
feedback is not part of standard climate simulations, this may
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V. Masson-Delmotte et al.: A comparison of the present and last interglacial periods in six Antarctic ice cores
be the reason why climate models do not simulate any significant annual mean Antarctic warming during the last interglacial unless they take into account Greenland ice sheet
meltwater (Masson-Delmotte et al., 2010b). Recent simulations show Antarctic warming reaching 1–2 ◦ C and larger at
Dome F than at Dome C, in response to freshwater forcing
during the last interglacial. The simulated Antarctic warming
can reach up to 5 ◦ C warming in response to both freshwater
forcing and to the removal of the West Antarctic Ice Sheet
(Holden et al., 2010), suggesting that changes in Antarctic
topography may also be significant. Northern Hemisphere
deglacial feedbacks are also expected to be at play during the
early Holocene optimum (or Antarctic Isotopic Maximum
number 0) (Masson-Delmotte et al., 2010a). The question of
changes in topography is one motivation for exploring past
differences between ice core records.
3
Deep drilling sites: climatological and glaciological
characteristics
The six deep drilling locations are all situated in the central
East Antarctic Plateau, at elevations varying from 2315 m to
3810 m a.s.l. (above sea level). They have different modern
climatological backgrounds, with an annual mean temperature between −40 ◦ C and −57 ◦ C, and a modern accumulation rate beween ∼21 and 80 mm per year (1 mm water
equivalent per year corresponds to 1 kg m−2 yr−1 ). Differences between the sites arise from their latitude (and insolation), elevation, distance to the nearest open ocean, and from
atmospheric heat and moisture advection.
In this section, we first describe the present day climatological context of different deep drilling sites in terms of precipitation regimes (Sect. 3.1), moisture origins (Sect. 3.2),
the importance of precipitation intermittency for the archiving of temperature variability in ice cores (Sect. 3.3), and
finally the ice flow contexts for the deep drilling sites
(Sect. 3.4). These characteristics will be used in Sect. 5
when assessing the different processes which can explain differences between deep ice core records from different East
Antarctic sectors.
3.1
Precipitation regimes of the ice core site locations
For a correct ice core interpretation it is highly important to
understand the precipitation regime of the drilling location.
In the interior of the continent on the majority of days, only
clear-sky precipitation (“diamond dust”) is observed. However, the amount of accumulation from diamond dust is extremely low. In recent years, increasing evidence has been
found that also on the high east Antarctic plateau precipitation events occur that yield precipitation amounts one or
two orders of magnitude larger than diamond dust. Although
such events occur only a few times per year, they can thus
bring a substantial part of the total yearly accumulation. In
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401
most cases, such events are connected to an amplification of
Rossby waves that leads to increased meridional flow patterns (e.g. Schlosser et al., 2010a). This means advection
of relatively warm and moist air from lower latitudes to the
continent, which is then orographically lifted and cooled, delivering high precipitation amounts.
Several studies were conducted for the EDML drilling site,
Kohnen Station (Schlosser et al., 2008, 2010a,b; Birnbaum et
al., 2006). Whereas Birnbaum et al. (2006) investigated only
a restricted number of cases observed during summer campaigns using ECMWF data, Schlosser et al. (2008, 2010a)
used data from the Antarctic Mesoscale Prediction System
(AMPS) (Powers et al., 2003) to investigate the characteristics of such “high-precipitation events” between 2001 and
2006. They found that only 20% of the events were directly
caused by frontal systems of passing cyclones in the circumpolar trough, the vast majority of the events being connected
to advection of warm air by amplified Rossby waves. They
estimated the ratio of diamond dust to synoptic precipitation
at the EDML site to be 40% to 54%.
At Dome Fuji, in eastern Dronning Maud Land (DML),
at an altitude almost 1000 m higher than Kohnen Station,
the same mechanisms have been observed. Enomoto et
al. (1998) and Hirasawa et al. (2000) studied meteorological conditions at Dome Fuji. In particular, they investigated
blocking anticyclones in winter, which were found to be able
to change meteorological conditions considerably by advection of warm air that led to cloud formation. This increased
the downward long-wave atmospheric radiation, destroying
the inversion layer and thus dramatically changing temperatures. However, it was not in all cases that humidity of the advected air was sufficient to produce precipitation. Fujita and
Abe (2006) carried out daily precipitation measurements at
Dome Fuji during a period of approximately 12 months during their wintering in 2003/2004. They estimated the amount
of diamond dust compared to synoptically induced precipitation to 52% of the total precipitation, which is in close agreement with Schlosser et al. (2010a), who estimated a value of
55% using AMPS archive data. The EDML drilling site and
Dome Fuji can get precipitation from the same blocking high
(Schlosser et al., 2010b), but usually, Dome Fuji would get
precipitation from a blocking situation linked with an anticyclone situated above the more eastern parts of DML. Suzuki
et al. (2008) investigated moisture sources for Dome Fuji
by calculating 5-day backward trajectories using ERA40 reanalysis data. They also found that snowfall conditions were
often connected to high-pressure ridges that force moist air
from the Atlantic and Indian Oceans to move over the continent to Dome Fuji.
Vostok is situated slightly further south than Dome Fuji,
but at an altitude about 300 m lower. The climatological mean annual temperatures are comparable, within 2 ◦ C.
Changes in accumulation rate and isotopic composition were
studied in the snow of Vostok (Ekaykin et al., 2004), using monthly accumulation from stake measurements and
Clim. Past, 7, 397–423, 2011
402
V. Masson-Delmotte et al.: A comparison of the present and last interglacial periods in six Antarctic ice cores
meteorological observations.
These authors compared
months with and without precipitation from clouds and thus
estimated the amount of diamond dust to approximately 75%
of the total precipitation, albeit with large uncertainties (59 to
91%). In spite of this relatively low fraction of synoptically
induced precipitation, they explain the observed changes in
accumulation rate and isotope ratio over several decades by
changes in cyclonic activity.
For Dome C, daily precipitation measurements are not
available yet. Being located at the same latitude as Kohnen
Station, it is nevertheless situated considerably farther away
from the coast and at an approximately 300 m higher altitude.
Massom et al. (2004) investigated the precipitation regime of
the Dome C and Law Dome areas using satellite imagery,
AWS data, and ECMWF model data. Their results show that
intermittent blocking-anticyclone events in the South Tasman Sea can cause significant precipitation events on the East
Antarctic ice sheet, but no firm conclusions have been drawn
about their contribution to the total precipitation amount.
They further note that not only blocking situations, but also
slowly moving cyclones offshore can cause significant individual precipitation events. They diagnosed a moisture origin
between 40◦ S and 35◦ S, which is consistent with moisture
trajectories (Sodemann and Stohl, 2009) discussed later in
our Sect. 3.2, and with Rayleigh isotopic modelling (Stenni
et al., 2001).
Talos Dome is by far the lowest drilling site, and it has
the shortest distance to the coast and also the highest accumulation rate (80 mm). (Scarchilli et al., 2010) investigated
precipitation conditions at Dome C and Talos Dome. They
also found that high-precipitation events are often caused by
blocking anticyclones, leading to increased moisture transport towards the drilling sites. Talos Dome is influenced by
moisture originating mainly from the Indian Ocean and secondarily from Pacific sectors of the Southern Ocean. Snow
precipitation originating from the Indian Ocean falls mainly
during winter (70%), whereas the snowfall events originating from the Pacific Ocean/Ross Sea are more homogenously
distributed during the year (50% in winter, 50% in summer).
The snowfall events originating from the Pacific Ocean arrive at Talos/Taylor Dome mainly via the Ross Sea, where
the extensive presence of sea ice also occurs during summer.
Longer and cooler distillation pathways of these air mass trajectories are expected to produce more negative δ 18 O precipitation values at TD compared to the ones originating from
the Indian Ocean. During the Last Glacial Maximum (LGM)
and most of the deglaciation, due to the presence of the Ross
Ice Sheet extending up to the continental margin, the transport of moisture from the Pacific sector via the Ross Sea was
very likely drastically reduced and compensated by an increase in the transport of moisture from the Indian Ocean
sector (Stenni et al., 2011; Scarchilli et al., 2010).
At the inter-annual scale, the occurrence of highprecipitation events is strongly connected to the Southern
Annular Mode (SAM), which is the dominant mode of
Clim. Past, 7, 397–423, 2011
climate variability at high latitudes in the Southern Hemisphere (Marshall, 2003). When the SAM is in its positive phase, the circumpolar westerlies are strong due to the
large north-south air pressure gradient, which means a highly
zonal flow with little meridional exchange of heat and moisture. A negative phase of the SAM, however, means weaker
westerlies above the polar ocean and a more meridional flow
due to amplified Rossby waves, which favours the formation of blocking highs that lead to the precipitation events
mentioned earlier. Generally, an East Antarctic cooling is
observed during periods with strongly positive SAM and a
warming when SAM is negative (Kwok and Comiso, 2002;
Marshall, 2007).
The East Antarctic ice core stable isotope records depict periods with stable isotope values less depleted than today, suggesting warmer conditions (Masson-Delmotte et al.,
2010b). If past warm conditions were marked by a negative SAM, one would expect a higher frequency of the previously described blocking events and a larger contribution of
high-precipitation events to the total accumulation. Climate
projections under scenarios of increased greenhouse gas concentrations and stratospheric ozone depletion point by contrast to a more positive SAM (IPCC, 2007) (Sect. 10.3.5.6).
While ice cores may offer the potential to depict past SAM
variations (Divine et al., 2009), information about SAM may
be needed for the quantitative interpretation of the ice core
data. The SAM itself is strongly connected to ENSO; however, the interaction between SAM and ENSO is highly nonlinear and not fully understood yet. There is evidence that
it varies temporally (Genthon and Cosme, 2003; Fogt and
Bromwich, 2006), since it additionally depends on tropical
forcings.
3.2
Moisture sources of precipitation
Backward trajectories calculated from atmospheric reanalysis products have been used in a number of studies to characterize the origin of East Antarctic air masses and water vapor for precipitation. A first comprehensive study
for several drilling locations focussed on snowfall days and
suggested a dominant (30% of precipitation) austral moisture source (50–60◦ S) together with significant seasonal and
inter-annual variability (Reijmer et al., 2002). However,
East Antarctic precipitation was seriously underestimated in
this approach. Field studies indeed have revealed a strong
contribution of clear sky precipitation to surface mass balance at Vostok (Ekaykin, 2003) and DF (Fujita and Abe,
2006). Moreover, isotope modelling studies and water tagging simulations in general circulation models both have suggested a more distant moisture origin (Delaygue et al., 2000a;
Masson-Delmotte et al., 2008; Werner et al., 2001).
More recently, a quantitative moisture source diagnostic
has been developed (Sodemann et al., 2008). Backward
calculations were conducted on long periods (20 days) as
compared to the commonly applied 5-day calculation period
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V. Masson-Delmotte et al.: A comparison of the present and last interglacial periods in six Antarctic ice cores
used in previous studies. Moisture sources from precipitating air parcels were diagnosed at a 6 h interval in a 100 km
radius around each drilling site. Monthly mean moisture
sources for each ice core site were then calculated from a
precipitation-weighted average of the evaporation-weighted
mean moisture source latitude and longitude diagnosed from
each tracked air parcel. This moisture source diagnostic was
applied in conjunction with a Lagrangian particle dispersion
model to Antarctica, covering a period from October 1999
to April 2005, using ECMWF analysis data (Sodemann and
Stohl, 2009). For the calculation setup, the global atmosphere was three-dimensionally subdivided into 1.4 million
particles of equal mass that were traced forward over the 5year calculation period. The subset of air parcels for which
moisture origin was considered here were selected when specific humidity anywhere over Antarctica (land mass south
of 60◦ S) decreased by more than 0.1 g kg−1 when relative
humidity was greater than 80% over Antarctica. Note that
parcels can be located at any altitude within the atmosphere.
Under these conditions it was assumed that a precipitating
cloud is present in the model atmosphere. For the particles selected by these criteria, moisture source regions were
detected as the regions where specific humidity increased
by more than 0.1 g kg−1 within the marine boundary layer,
weighted under consideration of the temporal sequence of
precipitation and evaporation events along the trajectory. The
method is described in further details in (Sodemann et al.,
2008) and (Sodemann and Stohl, 2009). This 5-year climatology of Antarctic moisture origin shows more distant moisture sources than from previous back-trajectory based studies, the results being consistent with water tagging simulations and isotope modeling in GCMs (Delaygue et al., 2000b;
Werner et al., 2001). In this analysis, we have focussed on
the average moisture source, calculated for the period 1999–
2005. Due to the length of analysis, we could not address the
drivers of inter-annual, decadal or longer time scale changes
in moisture origin, for instance in relationship with the SAM.
Figure 2 displays the results of this calculation in terms
of the mean and standard deviation of moisture source latitude and longitude for the three month periods associated
with maximum (August-September-October, ASO) and minimum (January-February-March, JFM) Antarctic sea ice extent for our six drilling locations. The central East Antarctic Plateau sites (Vostok, DF and EDC) show rather consistent moisture origin, with an annual mean moisture source
located around 42◦ S (not shown), shifting ∼2◦ S in summerautumn (JFM), possibly in relationship with the reduced
sea ice cover. The inter-site differences remain within the
limits of the seasonal mean variability spread of intra to
inter-annual moisture sources. Because sea surface temperature acts on kinetic fractionation at evaporation (Merlivat
and Jouzel, 1979), warmer (and more northward) moisture
sources are expected to produce higher deuterium excess levels in Antarctic snowfall. The highest modern deuterium
excess levels are encountered at Dome F (Uemura et al.,
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403
2004; Masson-Delmotte et al., 2008) and cannot be simply
explained by our analysis showing rather comparable moisture source latitudes for the central plateau. We note that deuterium excess is also affected by other evaporation conditions
such as relative humidity (Uemura et al., 2008), transport and
distillation effects (colder Dome F conditions are expected to
induce higher deuterium excess) and intermittency of precipitation. The moisture source calculation is also associated
with relatively large uncertainty and should be more firmly
established, e.g. by considering a longer climatology. Because this comparison between mean deuterium excess and
moisture source calculations is inconclusive (due to the overlap of the mean moisture sources for the central East Antarctic ice core sites), further investigations will need to compare
the isotopic composition of snowfall on an event basis (Fujita
and Abe, 2006) at different sites with daily moisture origin
calculations.
Differences clearly appear between the “highest elevation sites” (Vostok, DF and EDC) and the “lower elevation”
drilling sites (EDML, Taylor Dome and TALDICE). On the
annual mean, TALDICE and EDML share a moisture origin
at ∼45◦ S (not shown), while the moisture source of Taylor
Dome is located further south at ∼52◦ S. However, marked
seasonal differences appear: in winter (ASO), TALDICE has
a moisture source similar to the inland sites, but EDML exhibits a ∼45◦ S moisture origin, which contrasts with the
large Atlantic sector maximum sea-ice cover (Figs. 1 and 2a),
and Taylor Dome appears to receive moisture from ∼49◦ S.
In summer (JFM), TD and TALDICE are the exceptions,
showing the most southward moisture origins. In the Ross
Sea area, the dominant moisture source longitude appears
shifted westwards in summer with respect to winter, a feature
much stronger for TD than for TALDICE (Sodemann and
Stohl, 2009; Scarchilli et al., 2010). This anomaly could be
due to the topographic configuration of the Ross Sea and to
the cyclogenesis from the sea-ice free Ross Sea sector, with
the strongest distance of moisture transport for Taylor Dome.
At the scale of Antarctica, coastal/ice sheet margin areas undergo minimum seasonal shifts in moisture source
(Fig. 2c). By contrast, the inner sector of East Antarctica and
the Ross Sea sector (including TALDICE and Taylor Dome)
exhibit the strongest seasonality in moisture source location.
Both elevation and distance to the open ocean therefore seem
to affect the seasonal shifts in moisture origin. The precise
links between the seasonality of moisture origin and the isotopic records are difficult to assess due to the low accumulation at most of the ice core sites and the lack of seasonal
resolution (Tables 1 and 2).
Ice core records obtained from our different drilling sites
are therefore expected to reflect changes in local site characteristics (accumulation, condensation temperature sampled
through precipitation intermittency) and features linked with
the initial ocean basin where the moisture is formed. During
the last glacial periods and the last termination, the Antarctic Isotopic Maxima (AIM) have different shapes in EDC
Clim. Past, 7, 397–423, 2011
-150
-100
-50
0
50
100
150
Longitude
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-35
-35
a) JFM
-40
Latitude
Latitude
-40
b) ASO
-45
Dome F
EDC
EDML
TALDICE
Vostok
Taylor Dome
-50
-45
-50
-55
-55
-150
-100
-50
0
50
100
150
Longitude
-150
-100
-50
0
50
100
Longitude
(a)
150
(b)
-35
Figure 2ab. Moisture origin calculated from ECMWF b) ASO
moisture source diagnostic and extracted for two three m
-40
Latitude
and minimum Antarctic sea ice cover (a, JF, January‐Fe
-45
September‐October), for the six deep drilling locations s
TD, TALDICE and Vostok). The longitude of each drilling si
-50
the horizontal axis (the grey and red circles for TD and TA
-55
The mean moisture source latitude and longitude is displa
-150
-100
-50
0
50
100
150
(c)
depict the standard deviation of the source latitude and Longitude
Fig. 2. Moisture origin calculated
from ECMWF analysis data using a Lagrangian moisture source diagnostic and extracted for two
weighted) from the monthly mean values b
three- month periods linked
with
maximum
and minimum
Antarctic
sea
ice
– (a) ed by the di
JFM, January-February-March,
and (b)(themselves ASO,
cover
Figure 2
2c. Mean se
easonal amp
plitude of th
he moisture
e source shi
ifts, indicate
istance August-September-October, for the six deep drilling locations studied here (Dome F, EDC, EDML, TD, TALDICE and Vostok). The longietween the A
ASO and JFM mean mo
oisture sourrce location
n from the LLagrangian displayed with a filled circle on the horizontal
axis (the grey and red circles for TD and TALDICE are overlapping
tude of each drilling site is (km) be
each season, for the grid points within a 100 km radius ar
moistur
re source di
agnostic (So
odemann a
each other). The mean moisture
source
latitude
and longitude
isnd Stohl, 20
displayed009). with an open circle. Error bars depict the standard deviation of the
source
latitude
and
longitude
calculated
(precipitation-weighted)
from
the
monthly mean
values (themselves
based on a 6-hourly
trajectories),
Figure 2ab. Moisture origin calculated from ECMWF analysis data using Lagrangian points within a 100 km radius around each ice core site. (c) Missing mean seasonal amplitude of the moisture
for each season, for the grid
source shifts, indicated by the
distance (km) between the ASO and JFM mean moisture source location from the Lagrangian moisture source
52
moisture source diagnostic and extracted for two three month periods linked with maximum diagnostic (Sodemann and Stohl, 2009).
and minimum Antarctic sea ice cover (a, JF, January‐February‐March, and b, JJA, August‐
and TALDICE, receiving moisture from the Indo-Pacific sec-
3.3
Impact of precipitation intermittency on
September‐October), for the six drilling studied recording
here (Dome F, EDC, EDML, tor, and EDML, receiving moisture
from
thedeep Atlantic
sector locations temperature
(Stenni et al., 2010b; Stenni
et al., 2011). In Sect. 5, we will
Water stable isotope records from deep ice cores can only
TD, TALDICE and Vostok). The longitude of each drilling site is displayed with a filled circle on discuss if inter-site differences
in interglacial trends can be
archive climate information at times when precipitation ocattributed to regional changes
in moisture sources thanks to
curs. This section is focussed on the impact of precipitation
the horizontal axis (the grey and red circles for TD and TALDICE are overlapping each other). the available deuterium-excess
data.
intermittency on the temperature information archived in ice
cores, and makes use of atmospheric reanalyses to quantify
The mean moisture source latitude and longitude is displayed with an open circle. Error bars the regional differences in this potential bias.
depict the standard deviation of the source latitude and longitude calculated (precipitation‐
53
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405
ECMWF ERA40 reanalysis is considered reliable for both
temperature and precipitation across much of Antarctica
from 1980 onwards (Miles et al., 2008; Marshall, 2009).
Mean annual temperature and precipitation from 22 years
(1980–2002) of ECMWF ERA40 data are shown in Fig. 3a.
The ERA40 data can be used to characterize precipitation intermittency (at the synoptic to seasonal scales) and its impact
on the archiving of temperature information in stable isotope
records from deep ice cores. One study showed that reanalysis captures the seasonal cycle of coastal Antarctic precipitation well, but that it may overestimate the amount of central
East Antarctic summer precipitation (Marshall, 2009).
The effect of covariance between temperature and precipitation on the recorded ice core temperature can be investigated by calculating the precipitation-weighted temperature
(Steig et al., 1994; Krinner et al., 1997; Werner and Heimann,
2002; Krinner and Werner, 2003; Sime et al., 2008, 2009a,b).
This signal can moreover be decomposed into the sum of
a high-pass filtered component capturing synoptic scale coFigure 3. Panel a shows the mean annual 2 m air temperature (shaded) and precipitation variance (<60 days) and one low-pass filtered component
(contours show kg/m²/yr equal to mm/yr water equivalent) on a logarithmic interval scale) capturing seasonal scale covariance (60–375 days) (Sime et
from 22 years (1980‐2002) of ECMWF ERA40 data. Panels b‐d show the biasing anomaly, ie. al., 2008, 2009a).
the difference between temperature weighted by precipitation and mean annual Figure 3b–d shows the 1980–2002 temperature “biasing”
effect, that is the mean temperature minus the precipitationtemperature. Panel b is total biasing (covariance of all frequency temperature and weighted temperature due respectively to total, seasonal, and
precipitation), contoured at 0.5K intervals; panel c is the seasonal (60‐375 day biasing synoptic temperature and precipitation covariance (Sime et
signal); and panel d is synoptic (<60 days biasing signal). All data are presented using a 400 al., 2008, 2009b). The results are quite similar to those obkm averaging radius (see (Sime et al., 2008)). tained for present-day biasing from HadAM3 (Sime et al.,
e
Figure 3e. The regional coherency of ERA40 inter‐annual temperature and precipitation 2008). They allow us to quantify the degree to which AntarcFig. 3. Panel (a) shows the mean annual 2 m air temperature
−2 yr−1of equal
weighted (shaded)
temperature The colour scale displays the mnumber locations tic precipitation occurs under warmer-than-average condiandchanges. precipitation
(contours
show kg
to with −1
tions. Interesting features of the results are (1) the strong
mm
yr
water
equivalent)
logarithmic(shaded) intervaland scale)
of
correlated time‐series (R>0.75, 22 years) onof atemperature precipitation‐
seasonal biasing effect is largely restricted to central East
22 years (1980–2002) of ECMWF ERA40 data. Panels (b–d)
weighted temperature (contoured) for each grid point. show the biasing anomaly, i.e. the difference between temperature
Antarctic regions; (2) the synoptic biasing term is signifiweighted by precipitation and mean annual temperature. Panel (b)
cantly larger than the seasonal effect for most of Antarctica,
is total biasing (covariance of all frequency temperature and preand is very large in coastal regions; and (3) the total size
cipitation), contoured at 0.5 K intervals; panel (c) is the seasonal
of the biasing is quite large, between 6 and 8.5 K, for most
(60–375 day biasing signal); and panel (d) is synoptic (<60 days
of East Antarctica. This bias reflects warmer temperatures
biasing signal). All data are presented using a 400 km averag during snowfall events in East Antarctica. This is presum54
ing radius (see Sime et al., 2008). (e) The regional coherency of
ably due to the relationship between heat and moisture adERA40 inter-annual temperature and precipitation-weighted temvection, and perhaps also to local radiative feedbacks linked
perature changes. The colour scale displays the number of locawith increased moisture and cloudiness (Gallée and Gorodet tions with correlated time-series (R > 0.75, 22 years) of temperaskaya, 2008). The large size of the biasing terms means that
ture (shaded) and precipitation-weighted temperature (contoured)
rather small percentage changes in the biasing, during a clifor each grid point.
mate shift, can have rather large effects on the temperature isotope relationship across Antarctica. We note for the deep
grid spacing of 100 km. Cross-correlating all 2581 time seice core sites of interest here, that the seasonal bias seems
ries provided information on the regional coherency of interslightly larger for Dome C and Vostok than for Dome F, and
annual temperature and precipitation-weighted temperature
that the synoptic bias appears stronger for EDML than for
changes (Fig. 3e). The regional coherency is depicted by
TALDICE.
contouring the number of grid points which exceed an arbiAnnual mean temperature and annual mean precipitation trary correlation value, chosen to be R = 0.75 (Fig. 3). We
weighted temperature (using daily mean data) were calcu note that a similar pattern also emerges for higher correlation
lated across Antarctica from 1980–2002 data. Using only
threshold values. As such, this pattern seems to depict a rogrid points south of 60◦ S, this gives 2581 time series of
bust picture of regional coherency in inter-annual variations
22 years (where each year is represented by a single value),
in temperature.
on an approximately equal area grid, with an approximate
55
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406
V. Masson-Delmotte et al.: A comparison of the present and last interglacial periods in six Antarctic ice cores
Inter-annual changes in temperature and precipitationweighted temperature are highly coherent across the East
Antarctic plateau region (Fig. 3e). This coherency is
stronger in temperature than in precipitation-weighted temperature. Both temperature and precipitation-weighted temperature show a sharp decline in regional coherency in a band
which stretches across West Antarctica, extending across the
TALDICE region. There is a slight increase in coherency
across the Siple Dome area, and also across the west side of
the Antarctic Peninsula. We emphasise that these results are
obtained from a rather short 22 year ERA40 time-series, and
their validity at longer time scales remains to be explored.
The reanalyses suggests that temperature or precipitationweighted temperature in the TD/TALDICE sector may not
be expected to be coherent with the central East Antarctic
Plateau. Finally, we note that windy areas across Antarctica, including in the region of Talos Dome, are also subject
to high levels of surface snow sublimation (Frezzotti et al.,
2004). During past warm episodes, changes in sublimation
rates across core sites could introduce further additional regional variation into the ice core records. This is not pursued
further in this present work.
The importance of precipitation intermittency has been
highlighted on a variety of time scales (decadal variability,
response to orbital forcing, response to greenhouse gas concentration increases). Using an atmospheric general circulation model equipped with water stable isotopes, (Sime et
al., 2008) have shown that, in response to increased greenhouse gas concentrations, the simulated temperature-isotope
temporal slope is weaker than the spatial slope at Dome F or
EDML, and is about 40% of the spatial slope near Dome C
and Vostok. The low temporal slope is mostly caused by
changes in precipitation intermittency under warmer projected climates (Krinner et al., 2007) highlighted a stronger
increased greenhouse gas concentrations response in central versus coastal East Antarctica (possibly linked with an
increased frequency of cloud cover days), and a minimum
precipitation weighting bias near the Dome F area. Finally, Schmidt et al. (2007) explored the temporal isotopetemperature relationship in Antarctic precipitation using a
coupled ocean-atmosphere model equipped with the explicit
modelling of stable isotopes, at the inter-annual scale but also
during mid-Holocene, in response to changes in orbital forcing. In all of their simulations, the temporal East Antarctic slope (0.2–0.5‰ of δ 18 O per ◦ C) appears systematically
weaker than the present-day modern slope, but they did not
analyse the regional differences or the reasons for the weaker
temporal slope.
The available modelling framework therefore quantifies
how precipitation intermittency affects temperature reconstructions based on ice core records of precipitation isotopic
composition. The results suggest that, during interglacial
periods, this precipitation intermittency effect can be different among sites on the East Antarctic plateau, and between the East Antarctic plateau and the TD/TALDICE area.
Clim. Past, 7, 397–423, 2011
Over the time scales explored here (inter-annual variability,
mid Holocene, increased greenhouse gas concentrations), the
model results suggest rather coherent temperature anomalies
in the East Antarctic sector, as well as a weaker isotopetemperature temporal gradient than the modern spatial slope.
It was argued that using the spatial slope for the interpretation
of the ice core data may therefore lead to an underestimation
of past temperature during warmer-than-present interglacials
(Sime et al., 2008, 2009b).
3.4
Glaciological contexts
When drilling sites are located on modern domes, the ice at
depth is provided from the same geographical origin as today and the ice flow mostly results from vertical thinning if
the domes have stayed at the same locations. Recent studies conducted for EDC and TALDICE suggest that respective changes in advection and accumulation can induce local spatial and temporal accumulation changes and induce
migration of dome summits, even at decadal to centennial
scale (Urbini et al., 2008). At Vostok and EDML, located on
ice ridges, ice core records are formed upstream and transported downward towards the drilling site by ice flow, which
requires corrections for upstream effects (due to spatial gradients in accumulation or surface snow isotopic composition)
to be applied to these ice core records.
On the central Antarctic Plateau, changes in elevation are
expected to be primarily driven by changes in accumulation.
Glaciological dating of deep ice cores relies on the assumption of an exponential relationship between water stable isotopes and accumulation rate (due to the temperature dependence of saturation vapor pressure) (Parrenin et al., 2007a).
In the EDC3 age scale, modeled accumulation rates are
50 to 20% higher in the early-late LIG compared to the late
Holocene. Higher LIG accumulation rates seem supported
by ice core chemistry data (Wolff et al., 2010). Ice core data
clearly show that the interglacial accumulation rates are at
least twice as high as the glacial accumulation rates (Udisti
et al., 2004). As a result of this accumulation effect, the
central Antarctic Plateau gradually rises over the course of
interglacials. Glaciological models suggest rather similar elevation histories at EDC, Vostok or DF locations, within a
few tens of meters (Pollard and DeConto, 2009) (Fig. 7b),
also consistent with homogeneous changes in East Antarctic plateau accumulation rates simulated by climate models
(Sime et al., 2008, 2009b).
By contrast, the ice flow is more complex near TALDICE.
The ice can flow on two sides of TALDICE, either through
the small outlets located in the Transantarctic Mountains, or
through Wilkes Land, near an efficient ice stream. At this
place, shifts in the grounding line – linked with the local
sea level – affect upstream ice thickness. During the last
glacial period, changes in ice flow induced a ∼170 m thicker
ice sheet near TALDICE. Together with the increase of accumulation during the deglaciation, this feature is expected
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V. Masson-Delmotte et al.: A comparison of the present and last interglacial periods in six Antarctic ice cores
to result in a ∼100 ± 50 m higher elevation at the early
Holocene, reaching present day values around 7 kyr (Stenni
et al., 2011). It is not expected that MIS5.5 sea level high
stand would affect ice flow, as it would only induce a small
retreat of the grounding line. Changes in TALDICE elevation
are therefore expected to be rather smooth (typically tens of
meters per millennium). However, they remain difficult to
simulate with ice sheet models which cannot capture realistically the Mountain Transantarctic ice flow and fail to capture
the right location of the dome.
Ignoring flow effects and assuming that elevation changes
would be purely driven by accumulation, the present day
differences in accumulation between the different sites (Table 1) could account for elevation differences between Vostok, EDC and DF by a few meters/millennium, and, for
EDML or TALDICE, 40–60 m per millennium with respect
to the three central Plateau sites.
Differences in ice core records may provide constraints
on past relative elevation changes. The spatial Antarctic
surface air temperature – elevation slope is larger than the
dry adiabatic lapse rate, reaching respectively −11.3, −11.6
and −11.9 ◦ C per 1000 m for our drilling locations (without TD) (n = 5, R 2 = 0.91), for the whole Antarctic database
(n = 1280, R 2 = 0.81), and for the subset of sites located
above 2000 m (n = 587, R 2 = 0.61) (Masson-Delmotte et al.,
2008). Sensitivity studies conducted with climate models
can be used to estimate the local impact of changes in ice
sheet topography. Using the LMDZ model, LGM simulations were run with different estimates of glacial Antarctic topography, leading to different elevations at EDC and EDML
of 250 and 200 m, respectively. The simulated “temporal”
slopes are respectively of −14.5 and −10.8 ◦ C per 1000 m
(Masson-Delmotte et al., 2010b). In previous ice core works,
a correction of 9 ◦ C per 1000 m was used for correcting past
temperatures from changes in EDC elevation (Jouzel et al.,
2007). The spatial relationships between elevation and δD
(resp. δ 18 O) are −7.4‰ per 100 m, R 2 = 0.74 (−0.93‰ per
100 m, R 2 = 0.75) for the whole modern database (n = 1280),
and appear enhanced above 2000 m to −8.0‰ per 100 m,
R 2 = 0.56 (−1.13‰ per 100 m, R 2 = 0.64) (n = 587), possibly because of the stronger thermal inversion. Stable isotope – elevation relationships will be used in Sect. 5 for the
interpretation of anomalies among the deep ice core stable
isotope records, and for comparison with elevation changes
simulated by ice sheet models.
4
Data
For all the ice cores, isotopic measurements were conducted
on successive thin pieces of ice (with lengths varying here
between 0.1 and 5 m), producing continuous records albeit
with different temporal resolutions. Temporal resolution can
be enhanced by conducting isotopic measurements at higher
depth resolutions but it is limited by wind scouring mixing
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407
the initial snow layers and by firn and ice diffusion processes as clearly demonstrated for the deepest part of the
EDC ice core (Pol et al., 2011). Based on an intensive study
of stake array data, pits and shallow cores at Vostok, Ekaykin
et al. (2004) showed that, in central Antarctica, it is possible
to resolve ∼decadal variability using stable isotope records.
4.1
Isotopic records: resolution and age scales
Vostok: δ 18 O and δD were available from a shallow core
(138 m, BH8, with a sampling resolution of 5 m) and deep
ice cores with a sampling resolution of 5 m (down to 2083 m)
and 1 m (below 2083 m). Data were measured at LSCE with
an accuracy of ±0.5‰ for δD, ±0.05‰ for δ 18 O data down
to 1413 m and below 2083 m, and ±0.1‰ between 1413 and
2083 m (Vimeux et al., 1999, 2001b). Several glaciological
age scales have been released for Vostok (Petit et al., 1999;
Parrenin et al., 2001; Salamatin et al., 2009). On the initial GT4 age scale, the temporal resolution of the data is
220 ± 50 years (δD and δ 18 O) and 22 ± 5 years (BH8 δD)
for the Holocene and 290 ± 70 years for the last interglacial;
here, the error bar is not related to the dating uncertainty but
to the variability of the temporal resolution over the periods
of interest (linked with the ice core sampling). We have used
the published synchronisation of the Vostok ice core records
on the EDC3 age scale (Parrenin et al., 2007a).
EDC: δ 18 O and δD were available on the EDC96 core
(Holocene) (Jouzel et al., 2001; Stenni et al., 2001) and the
EDC99 core (last interglacial) (Jouzel et al., 2007; Stenni
et al., 2010a) with a depth resolution of 55 cm. δD data
were measured at LSCE with an accuracy of ±0.5‰ and
δ 18 O data in Trieste and Parma Universities with an accuracy of ±0.05‰. With the EDC3 timescale (Parrenin et al.,
2007a), this corresponds to a time step of 18 ± 3 years for the
Holocene and 40 ± 4 years for the last interglacial. Changes
in local elevation changes were estimated from the glaciological model and used to correct the temperature reconstructions. A slightly modified age scale spanning the Holocene
and late glacial has been produced (Lemieux-Dudon et al.,
2010) by improved synchronisation of Antarctic and Greenland records.
EDML: δ 18 O and δD were available with a depth resolution of 50 cm (EPICA-community-members, 2006; Stenni et
al., 2010b) and were measured by Alfred Wegener Institute
for Polar and Marine Research (AWI). δD data were measured with an accuracy ±0.5‰ and δ 18 O data with an accuracy of ±0.05‰. Data were not available from the main
core for the upper part (first 113 m corresponding to the past
1.2 ka) and the gap was filled using the nearby 148.84 m
deep B32 shallow ice core, sampled on a depth step of
6.2 ± 1.4 cm and spanning years 167 AD–1997 AD. A systematic 0.23‰ offset was identified from the overlapping
period between B32 and EDML main core (1.2–1.6 kyr)
and corrected before stacking the data. As the drilling site
is not located on a dome, upstream corrections had to be
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V. Masson-Delmotte et al.: A comparison of the present and last interglacial periods in six Antarctic ice cores
performed to account for spatial gradients in surface conditions (EPICA-community-members, 2006). The EDML1
age scale (Ruth et al., 2007) was built to be coherent with
EDC3 through volcanic and gas tie points and the synchronisation was recently updated (Lemieux-Dudon et al., 2010).
The depth sampling translates into a temporal resolution
of 0.8 ± 0.2 years for the past 1.2 ka, 9 ± 2 years for the
Holocene (1.2 to 12.2 kyr) and 85 ± 25 years for the last interglacial (118 to 131 kyr).
DF: δ 18 O data were available from the DF1 ice core with
a depth resolution varying between 0.05 and 0.6 m. For
the Holocene, the mean depth sampling was 0.27 ± 0.16 m,
and it is systematically 0.5 m for the last interglacial. Two
age scales were available for DF, a glaciological age scale
(Watanabe et al., 2003; Parrenin et al., 2007b) and an orbitally tuned age scale (Kawamura et al., 2007). On this
orbital DFO-2006 age scale, the temporal resolution of the
data is 9 ± 6 years for the Holocene and 44 ± 8 years for the
last interglacial. DF was also placed on EDC3 age scale by
peak-to-peak synchronisation of isotopic records, assuming
synchronous Antarctic Isotopic Maxima and transitions.
TD: δ 18 O data were available on a ∼0.15 m step for the
current interglacial and ∼0.22 m step for the last interglacial
(Table 2). The original TD age scale (Steig et al., 1998) has
been questioned for the last termination. The synchronisation
of calcium (dust) records from TD and EDC has shown that
the accumulation rate originally used for the TD age scale
was overestimated between 14.5 and 17.5 kyr, with implications for the modelling of gas age-ice age differences (Mulvaney et al., 2000; Stenni et al., 2011). It was suggested
that very low glacial accumulation rates were caused by enhanced wind scouring. This was confirmed by ice annual
layer thickness radar measurements combined with ice flow
modelling, and 10 Be concentrations (Morse et al., 2007) .
The last interglacial record has been tied to the Vostok GT4
age scale based on similarities in the stable isotope records.
A synchronisation with GICC05 and EDC3 is not yet available. With the original age scale, the temporal resolution of
the data was around 5 ± 5 years for the current interglacial
but 750 ± 480 years for the last interglacial, which is compressed within only 5 m of ice. We do not have sufficient
constraints to produce new age scales for TD. Because of the
very low resolution for the last interglacial and the age scale
uncertainties, we have decided to display TD data in Fig. 4a
and discuss the general structure of the present and last interglacial in this record but could not include this ice core
record in subsequent analyses of common variance.
TALDICE: δ 18 O data were available on a 1 m depth step
(Stenni et al., 2011). A glaciological age scale model specific
to TALDICE has been developed by methane synchronisation with GICC05 (Buiron et al., 2011; Stenni et al., 2011).
For the last interglacial, we used a preliminary age scale established using methane tie points with EDC3 and inverse
modelling as described in (Lemieux-Dudon et al., 2010).
The available 1 m record corresponds to a Holocene temporal
Clim. Past, 7, 397–423, 2011
resolution of 18 ± 7 years and a last interglacial temporal resolution of 490 ± 100 years. In order to improve the temporal
resolution, 980 additional measurements were conducted for
Termination II and the last interglacial on a 5 cm depth step
(from ∼146 to ∼116 kyr). The TALDICE age scale was then
extrapolated at this depth scale. The new temporal resolution
for the last Iinterglacial is therefore 25 ± 8 years.
In summary, the records offer temporal resolutions ranging
from ∼10 (EDML, DF) to ∼30 years (Vostok, TALDICE,
EDC) for the Holocene, and 20 (TALDICE) to ∼300 years
(Vostok) for the last interglacial. For the sake of comparisons
between records and between the Holocene and last interglacial, we choose to focus on the long-term trends and therefore select a 200 year time step to re-sample all the records
(Fig. 4a).
While Greenland Holocene ice cores can be accurately
dated using layer counting methods (Vinther et al., 2006),
this is not the case for dry central Antarctica where wind
scouring and diffusion erase seasonal signals. The dating
of the Antarctic Holocene records relies on glaciological
and accumulation modelling, together with absolute or relative age markers such as volcanic horizons, gas synchronisation with Greenland records, and alignment of 10 Be variations with 14 C cosmogenic isotope variations (Parrenin et
al., 2007a; Lemieux-Dudon et al., 2010). Altogether, the age
scale of Antarctic records for the Holocene is estimated to
be associated with a maximum uncertainty of ∼200 years
(Lemieux-Dudon et al., 2010). For this reason, assessing
the consistency of central Antarctic records on centennial or
shorter time scales is out of reach, and we focus on multicentennial and longer term trends.
The uncertainty linked with the last interglacial absolute
age is estimated to be ∼3 ka, and the uncertainty on its duration about 20% (Parrenin et al., 2007a; Kawamura et al.,
2007). The EDC3 age scale has recently been supported by
the absolute dating of Mount Moulton tephra (Dunbar et al.,
2008; Popp et al., 2004). We have therefore chosen to use
all the other records (Vostok and DF) on the EDC3 age scale.
This synchronisation lies on volcanic, methane and dust tie
points (EDML, TALDICE) for the Holocene and the last
deglaciation. For the previous interglacial, it mostly relies on
the hypothesis of synchronous climate and water stable isotope fluctuations. Marino et al. (2009) recently demonstrated
coherent geochemical dust composition at EDML and EDC,
confirming earlier results obtained at Vostok and EDC and
showing a common southern South American provenance
of glacial dust to East Antarctica. The available dust data
clearly confirm the synchronism of Eastern Antarctic stable
isotope variations.
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high resolution data have been artificially shifted by 1‰ for readability. The black arrows display the abrupt TALDICE and EDML δ18O decrease observed at ~118 ka. Figure 4b. Based on the smoothed records displayed in Figure 4a, comparison of the δ18O anomalies (‰ between the last millennium (reference), the early Holocene EH (warmest between 7 periods
and 13 ky), in
the Last Glacial Maximum LGM (coldest millennium V. Masson-Delmotte et al.: A comparison of the present andmillennium last interglacial
six
Antarctic
ice cores
409
between 18 and 30 ky), the optimum of the Last Interglacial LIG O (warmest millennium 18
δ O
18
δ O
-35
DF
EDC
VK
EDML
TALDICE
TD
-35
between 100 and 140 ky), the plateau of the Last Interglacial LIG P (mean conditions between 122 and 126 ky) and the previous glacial maximum MIS6 (coldest millennium between 130 and 150 ky). The Vostok and EDML record are affected by changes in upstream -40ice origin leading to a decreasing δ18O trend clearly visible for LIG O and MIS6. The published -40
estimates for the upstream effects are displayed as vertical arrows (without taking into -35
-35
-45
-45
account changes in ice sheet elevation with time). The same analyses were conducted on TD despite chronological uncertainties (grey bars). -40
-40
-45
-45
-50
-50
-55
-55
-60
-60
0
5
10
15
Age
20
25 30x10
3
115 120 125 130 135 140145x10
Age
3
(i)
(b) 56
57
(c)
Figure 4c. Sensitivity analysis of the δ18O geographical anomalies: tests conducted with respect to the definition of the “present‐day” (0‐1 ky, 0‐3 ky or 0‐6 ky mean, different Fig. 4. (i) Original water stable isotope records available from all ice core sites (‰), displayed on the EDC3 age scale for the current
symbols) and with respect to the smoothing length applied to the data (0.2 to 8 ka, (left panel) and last (right panel) interglacial periods with the exception of TD on its initial age scale (grey, left vertical axis with the same
horizontal axis). Percentage differences are relative to EDC and displayed for each time vertical scaling as for the right axis). All raw data (thin lines) and 5 point binomial filter 200 year re-sampled data are displayed (bold lines).
interval (a: Early Holocene, b: LGM, c: LIG maximum and d: LIG mean). For clarity, different y‐
Two series of measurements are available for the last interglacial period for TALDICE: bag samples (1 m) and fine samples (0.05 m). The
axis scales are used for the different panels. Note that the Vostok and EDML record are high resolution data have been artificially shifted by 1‰ for readability. The black18arrows display the abrupt TALDICE and EDML δ 18 O
affected by changes in upstream ice origin leading to a decreasing δ O trend for the oldest decrease observed at ∼118 kyr. (ii) Based on the smoothed records displayed in Fig. 4a, comparison of the δ 18 O anomalies (‰ between the
time periods. last millennium (reference), the early Holocene EH (warmest millennium between 7 and 13 kyr), the Last Glacial Maximum LGM (coldest
millennium between 18 and 30 kyr), the optimum of the last interglacial (LIG) O (warmest millennium between 100 and 140 kyr), the plateau
of the last interglacial LIG P (mean conditions between 122 and 126 kyr) and the previous glacial maximum MIS6 (coldest millennium
between 130 and 150 kyr). The Vostok
and EDML records are affected by changes in upstream ice origin leading to a decreasing δ 18 O
trend clearly visible for LIG O and MIS6. The published estimates for the upstream effects are displayed as vertical arrows, (without taking
58
into account changes in ice sheet elevation with time). The same analyses
were conducted on TD despite chronological uncertainties (grey
bars). (iii) Sensitivity analysis of the δ 18 O geographical anomalies: tests conducted with respect to the definition of the “present-day” (0–
1 kyr, 0–3 kyr or 0–6 kyr mean, different symbols) and with respect to the smoothing length applied to the data (0.2 to 8 ka, horizontal axis).
Percentage differences are relative to EDC and displayed for each time interval – (a) Early Holocene, (b) LGM, (c) LIG maximum and
(d) LIG mean. For clarity, different y-axis scales are used for the different panels. Note that the Vostok and EDML record are affected by
changes in upstream ice origin leading to a decreasing δ 18 O trend for the oldest time periods.
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4.2
V. Masson-Delmotte et al.: A comparison of the present and last interglacial periods in six Antarctic ice cores
Sea water and glaciological corrections
Sea water isotopic composition
Ice core δ 18 O data incorporate a signature of changes in sea
water δ 18 Osw , which varies during glacial-interglacial periods due to changes in ice volume. Classically, δ 18 Osw is extracted from paleoceanography data (Bintanja et al., 2005)
and synchronised on ice core age scales such as EDC3 (Parrenin et al., 2007a). δ 18 Osw affects the initial water vapour
isotopic composition and this initial anomaly is altered during distillation, leading to a final imprint δ 18 O on ice core
δ 18 O which is expressed by:
18 O
ice
1 + 11000
118 Ocorr = 118 Osw 18 O
sw
1 + 11000
Over the last 150 ka, our five ice cores have displayed δ 18 O
values ranging between −62 and −33‰, leading to minor
(∼0.03‰) differences in the classical sea water correction
applied to the different sites. This factor cannot explain any
significant differences between sites.
Glaciological corrections
VOSTOK: The ice below Vostok has been transported from
its surface upstream origin, towards Ridge B. Vimeux et
al. (2001a) indeed showed a marked change in Vostok deuterium excess prior to 250 kyr (at depths lower than 2820 m),
corresponding to a surface ice origin 230 km from Vostok.
Holocene ice was deposited 0–30 km from Vostok, while last
interglacial ice originates in an area located 110–130 km further in the direction of Ridge B. Changes in accumulation
gradients were discussed by Parrenin et al. (2004) and used
for improving the Vostok glaciological age scale. Based on
a linear interpolation of the modern spatial δ 18 O gradient
between Vostok and Dome B (2‰ over 280 km), an initial
deposition site 130 km upstream of Vostok is expected to
induce a progressive glaciological δ 18 O depletion reaching
∼1‰ during the last interglacial. However, glaciological investigations carried out in the interval 0–110 km from Vostok
towards Ridge B along the flow line showed no significant
trend in surface snow δ 18 O content (Ekaykin et al., 2010),
thus no correction was performed on stable isotope records.
Vostok ice core total air content data (V. Lipenkov, personal
communication, 2010) are coherent with an ∼80 m lower initial surface elevation during glacial maxima than at present
day.
EDML: Using a glaciological model forced by temperature and accumulation derived from ice core data, Huybrechts et al. (2007) simulated the EDML elevation history
and the upstream origin of EDML ice, expressed in elevation as well. The change in elevation of the surface ice was
then used to correct EDML δ 18 O using the present day local isotope-elevation slope (−0.96‰ per 100 m). Upstream
Clim. Past, 7, 397–423, 2011
of EDML, the ice sheet surface is expected to be higher during glacial periods and lower during interglacial periods in
response to changes in ice flow. The elevation effect is modelled to vary between −50 and −125 m during the last interglacial; the upstream origin effect is expected to add −190
to −220 m for the same time interval. Altogether, the glaciological effects are simulated to induce an elevation effect between −250 and −335 m from 120 to 130 kyr, corresponding to a δ 18 O “glaciological” depletion of 2.4 to 3.2‰, going back to ∼1.2‰ during MIS6 (because of a higher glacial
elevation in this area). In the next sections, raw data have
been used because of uncertainties on glaciological corrections (Stenni et al., 2010b).
EDC: A sub-product of the 1-D glaciological modelling
(Parrenin et al., 2007b) is the reconstruction of local ice
thickness, mainly responding to accumulation changes. This
was used by Jouzel et al. (2007) to correct EDC temperature
reconstructions. For EDC, a comparison of existing glaciological simulations (Huybrechts et al., 2007; Parrenin et al.,
2007b; Pollard and DeConto, 2009) (Fig. 7) shows comparable orders of magnitude, typically ∼100 m at the glacialinterglacial scale, but strong differences over the course of
the last interglacial. While elevation corrections remain secondary at the glacial-interglacial scale (about 10% of the
temperature amplitude), within an interglacial period they
can reach amplitudes comparable to temperature trends derived from stable isotope data (∼1 ◦ C).
Changes in Antarctic elevation have also been extracted
for Vostok, Dome F, EDML, and EDC from a long 3-D ice
sheet simulation (Pollard and DeConto, 2009) forced by marine data. This simulation did not use ice core data, thus
ignoring the impact of local accumulation or temperature
changes – which are however expected to be rather homogeneous on the East Antarctic plateau as discussed previously.
This has produced rather homogeneous elevation changes at
Vostok, EDC, and DF (Fig. 7c and d).
4.3
Comparison between the different ice core records
All records (Fig. 4a and b) depict comparable glacial (LGM)
to present-day (last millennium) δ 18 O increase, with a mean
amplitude of ∼5.2‰ and an inter-site standard deviation of
0.4‰. In this section, we will refer to differences between a
given past period and present day (average values over the
last millennium) as “anomalies”. We note that these results
are sensitive to the prescribed definition of “the present-day”
and also to the data filtering applied. We explore both of
these aspects in Sect. 4.4 and Fig. 4c.
All sites exhibit an early Holocene maximum between
∼12 and 9 kyr, albeit with different amplitudes: while it
reaches an anomaly of ∼2‰ above present-day at Vostok,
this anomaly is only ∼1.6‰ at DF, ∼1.1‰ at EDML and
0.9‰ at EDC. At TALDICE, early Holocene levels even
stay below present-day levels. After this optimum, Vostok and DF exhibit a decreasing trend towards present-day
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values, while EDC, EDML and TALDICE exhibit a midHolocene secondary maximum, most strongly imprinted in
TALDICE. There are therefore strong differences in glacialinterglacial amplitudes if one considers the reference period
as the last millennium or the early Holocene. The most
depleted sites (Vostok and DF) exhibit the weakest glacialpresent day magnitude, but strongest early Holocene optimum magnitude. This points to either different amplitudes
of temperature changes or different isotope-temperature relationships in the different Antarctic sectors (Vostok and DF
being more coherent than EDC and EDML). The Holocene
δ 18 O at TALDICE appears as an exception which could arise
from changes in local elevation and/or in moisture source and
pathways linked to the open Ross Sea during deglaciation
(Fig. 4).
The magnitude of MIS6 δ 18 O anomaly differs more
strongly between sites than for the LGM, plausibly as a consequence of upstream ice origin at EDML and Vostok imposing stronger depletion. At DC and DF, MIS6 appears
0.6–0.7‰ less depleted than the LGM (around 25 kyr). Assuming that the reference sites are DC and DF, this would
suggest an upstream effect reaching −0.5‰ at Vostok and
−2.5‰ at EDML, compatible with glaciological constraints
suggesting EDML upstream and elevation effects of ∼300 m
(Fig. 4).
The early optimum of the LIG appears systematically
stronger than the early Holocene maximum, leading to larger
deglacial amplitude (between the glacial minimum and the
interglacial maximum levels) during Termination II than during Termination I; this amplification varies from 20% (Vostok) to 32% (EDML). The LIG optimum anomaly (with
respect to present day) is strongest at DF (3.8‰), DC
(3.5‰), and Vostok (2.8‰), followed by EDML (2.0‰) and
TALDICE (1.8‰).
Interestingly, the initial isotopic profiles (Fig. 4) also highlight two abrupt decreases in the isotopic values at TALDICE
and EDML, around 118.5 kyr (black vertical arrows on
Fig. 4). These two rapid drops were not recorded at the more
inland sites (DF, EDC and Vostok). It is unlikely that both
result synchronously from an abrupt increase in the local ice
sheet topography, by about 200 m within <400 yr, in two different areas. It is likely that such a fast isotopic decrease
at the two sites most sensitive under interglacial conditions
to high latitude moisture transport (see Sect. 3) results either from changes in precipitation intermittency or from an
abrupt shift of moisture origin. During glacial inception, an
increased seasonal ice cover in the Ross Sea/Weddell Sea
sector may have closed a previous local moisture source, enlarging the distance between source and site. Such changes in
moisture transport could have contributed to a stronger final
isotopic depletion. This hypothesis needs further investigation, for instance using proxies of moisture origin (e.g. deuterium and oxygen 17 excess) and of regional sea ice potentially available from marine records or ice core aerosols.
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4.4
411
Sensitivity study of the inter-site differences
The simple comparison discussed in the previous section
highlights inter-site differences and suggests that they may
differ from one warm interval to the next. Here, we have
assessed the robustness of these inter-site δ 18 O differences
(Fig. 4b). For this purpose, we quantified the sensitivity of
the δ 18 O anomaly analysis examining the results for the same
past periods, except for the older MIS6 glacial maximum
which is not sufficiently resolved in the available TALDICE
and EDML records.
The sensitivity check has been carried out with respect to
(i) the smoothing of the data, and (ii) the reference “presentday” period. First, for each event, and for each ice core,
we calculated a mean value for a given past period (as defined in Fig. 4b, e.g. the Early Holocene δ 18 O maximum between 7–13 kyr). We repeated this analysis, using progressively more strongly smoothed (low-pass filtered) δ 18 O time
series. The low-pass filtering was carried out over a large
range of values between 0.2 and 8 kyr, using 0.2 ka filtering intervals. This provides forty sets of values, for each
ice core and for each past period, calculated using gradually
more strongly smoothed time-series. Secondly, we calculated “present-day” mean values for each ice core site using
three alternative specifications. These specifications are: a
last millennium mean (0–1 kyr mean); a late Holocene mean
(0–3 kyr mean); and a mid-Holocene mean (0–6 kyr mean).
For each ice core-site, each of the forty past period mean
values was subtracted from each of these three “present-day”
reference values. This has given 120 possible anomaly values for each core site and each period, providing a relatively
complete depiction of the dependency of Fig. 4b on the data
smoothing and the specification of “present-day”. Figure 4c
depicts these sensitivity test results. The results are presented
as “geographical percentage differences”, calculated as the
ratio between the individual ice core anomaly and the EDC
ice core anomaly. We used EDC as a reference because it appears to have an “average” behaviour in Fig. 4b and because
it has been drilled on a central East Antarctic plateau dome
(with therefore minimal glaciological biases).
A percentage difference of 0% means that the individual
site period anomaly is equal to the EDC anomaly, and therefore shows that there is no geographical difference between
the anomaly at this ice-core site and at EDC. A difference
of 100% would mean that the ice core site anomaly is twice
as large as the EDC anomaly, indicating a large geographical
difference.
The results are displayed in Fig. 4 as a function of the
smoothing length. They depict robust similarities between
the geographical anomalies of the EH and LIG, with the exception of the Vostok and EDML sites (which are likely affected by a negative glaciological bias, see Sect. 4.2). DF
appears to have generally ∼60% larger anomalies than EDC,
and TALDICE systematically −120 to −150% anomalies.
Our sensitivity study shows that the geographical percentage
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V. Masson-Delmotte et al.: A comparison of the present and last interglacial periods in six Antarctic ice cores
differences vary a lot for the EH depending on the filtering length, especially above 4 ka. For earlier periods, results are not very sensitive to the choice of the smoothing
length (Fig. 4). The geographical percentage differences are
affected (up to 50%) by the choice of the reference period
(e.g. 0–1, 0–3 or 0–6 kyr mean).
Our sensitivity tests show that the analysis of the “peak”
δ 18 O anomaly values can be sensitive to data smoothing and
to the specification of “present-day”. An alternative “relative rate of change” approach for examining geographical
patterns, was outlined in Sime et al. (2009b). This method
can alleviate the problems linked with glaciological trends
and may be worthwhile pursuing in the future.
The inter-site comparison is indeed complicated by glaciological biases for non-dome sites such as Vostok and EDML.
The analysis confirms that there are systematic differences
between EDC and DF East Antarctic Plateau sites, with a
stronger isotopic anomaly at DF than at EDC during warm
periods. With this type of analysis, we cannot reach a definitive conclusion about the stability of the VK or EDML patterns between the different warm periods, due to glaciological effects. Our analysis nevertheless suggests that EDC, DF,
and probably VK (if corrected for glaciological trends) δ 18 O
follow a similar geographical pattern of EH and LIG anomalies on the high East Antarctic Plateau. We note the complex
structure of EDML anomalies, which may display different
patterns of warm period isotopic responses during the EH
and LIG (Fig. 4a–c). Finally, the specificity of TALDICE is
confirmed, with a distinct site-specific pattern robust between
EH and LIG.
5
Analysis of the inter-site differences
The previous section clearly evidences that there are both
common signals and local specificities in the ice core records
of the present and last interglacials. This section is dedicated
to the analysis of the inter-site differences. We first specify the problems arising from changes in isotope-temperature
relationships (Sect. 5.1), and analyse the potential contribution of changes in moisture source to regional anomalies,
thanks to corrections using deuterium excess measurements
(Sect. 5.2). We then quantify the isotopic signal common to
the five high-resolution records using principal component
analyses (Sect. 5.3). We finally quantify elevation changes
which could account for local differences, and compare them
with ice sheet model results (Sect. 5.4).
5.1
Interpretation of isotopic deviations
For each of our ice core sites (i), and for each time period (t),
we have access to the local and instantaneous ice core isotopic composition δ 18 Oi,t . Changes in δ 18 Oi,t are expected
Clim. Past, 7, 397–423, 2011
to depend on local temperature Ti,t|z0 “at fixed elevation” but
also on local elevation changes zi,t :
δ 18 Oi,t = αi,t Ti,t|z0 + βi,t zi,t
(1)
Equation (1) relates δ 18 Oi,t to changes in local air temperature through a “temporal slope” αi,t varying with site (i) and
time (t). The modern Antarctic spatial slope is well established from a database of modern snowfall and surface snow
isotopic composition, with an average Antarctic slope of
0.8‰ per ◦ C (Masson-Delmotte et al., 2008). This isotopetemperature coefficient hides a variety of factors that add to
the well known Raleigh distillation effect, such as intermittency/ seasonality of snowfall, inversion strength, moisture
source effects and advection history (see Sect. 3).
Until recently, Antarctic temperature reconstructions have
assumed a linear and temporally constant αi,t , on the glacialpresent-day scale, a hypothesis supported by glacial climate
simulations conducted with isotopic atmospheric general circulation models (Jouzel et al., 2007; Sime et al., 2009b).
This implies that anomalies in δ 18 Oi,t are expected to mainly
depend on anomalies in local temperature and/or elevation.
However, modelling studies conducted under projected increased atmospheric CO2 concentrations (Sime et al., 2008)
have recently questioned whether the single linear αi,t hypothesis holds for climates warmer than present day, and suggested temporally and spatially varying αi,t . An analysis of
the relative rates of changes of δ 18 Oi,t between Vostok, EDC
and DF alongside with isotope-enabled model outputs has
been used to suggest a rather small αDF but a climatically
variable αEDV,t (Sime et al., 2009b) for Vostok and EDC.
Due to the lack of independent temperature and elevation constraints, Eq. (1) is under-constrained and cannot be
solved. Assuming a homogeneous temperature change on the
central Antarctic plateau immediately attributes anomalies in
δ 18 Oi to spatial differences in α 1 , defined as the average of
αi,t between the periods t and t0 :
δ 18 Oi,t − δ 18 Oi,t0
αl
∼
18
18
αj
δ Oj,t − δ Oj,t0
(2)
In order to explore these inter-site anomaly ratios, Fig. 4b
displays the early Holocene, MIS2, MIS6, LIG optimum and
LIG plateau anomalies for DF, TALDICE, Vostok, EDC and
EDML. Let us explore if we can identify a constant scaling between the various sites with the example of EDC/DF.
The EDC/DF anomaly ratio (as defined in Eq. 2) is slightly
higher than 1 during glacial maxima (resp. 1.12 and 1.13 for
MIS2 and MIS6), weak during the early Holocene optimum
(0.6) and LIG plateau (0.7), and again higher (0.9) during
the LIG optimum. Ignoring glaciological effects (e.g. due to
local elevation changes) and assuming a homogeneous temperature change at EDC and DF, the data suggest smaller
isotope-temperature slopes at EDC compared to DF for periods warmer than today. However, the results do not appear stable over the three warm periods analysed here. The
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processes accounting for changes in the isotope-temperature
slope, such as precipitation intermittency of moisture origin effects, may differ between these various warm episodes
(EH, LIG plateau, LIG optimum), which have very likely different causes (bipolar seesaw, response to orbital forcing...)
(Masson-Delmotte et al., 2010a).
For Vostok and EDML, this comparison does not fully
make sense because of uncertain upstream corrections (as
indicated by black vertical arrows on Fig. 4b). TALDICE
clearly appears as an outlier, with almost no change during
the EH or the LIG plateau and a small anomaly during the
LIG optimum. TALDICE shows coherent trends and amplitudes during glacial periods with Plateau cores, whereas it
exhibits a strong difference during the Early Holocene. This
different behaviour is coherent with the sensitivity of the site
to ice sheet retreat during the opening of the Ross Sea (Stenni
et al., 2011). However, it should be noted that despite its sensitivity to ice sheet retreat, TALDICE is qualitatively coherent with other ice cores at the LIG optimum.
Finally, the TD ice core appears quite peculiar. It exhibits
the strongest positive anomalies with respect to the present
day levels during the present and last interglacials, but the
smallest negative glacial amplitudes. Differences between
TD and TALDICE are particularly surprising given the proximity of the drilling sites, but they are consistent with large
differences in moisture sources (Sect. 3, Fig. 2). Given earlier findings regarding the possible accumulation hiatus during the last glacial (Morse et al., 1998), for this specific site
we cannot rule out that some of the characteristics arise from
site-specific precipitation intermittency and/or wind erosion
effects.
5.2
Changes in moisture origin
The analysis of moisture origin (Sect. 3) has demonstrated regional specificities in the modern seasonal moisture sources.
Changes in moisture origin and evaporation conditions are
known to affect the initial water vapour isotopic composition,
and therefore the isotopic composition of Antarctic snowfall
(Dansgaard, 1964). Second order isotopic parameters such as
deuterium excess and oxygen 17 excess are more sensitive to
evaporation conditions than δD or δ 18 O. These supplementary data can provide constraints on changes in moisture origin and correct their impacts of δD or δ 18 O. Methodologies
have been developed to quantify changes in source and site
temperatures. For our sites and periods of interest, estimates
of biases linked with changes in moisture sources have been
produced using deuterium excess for EDC, EDML, Dome F
and Vostok; this is not yet the case for TALDICE and TD.
Figure 5 displays the comparison between the δ 18 O
records and these published site temperature reconstructions,
which take into account deuterium excess constraints on
changes in moisture origin, as well as changes in sea water isotopic composition (see Sect. 3). Based on Rayleigh
models, these reconstructions do not take into account
www.clim-past.net/7/397/2011/
413
precipitation intermittency. Classical temperature reconstructions are linearly related to isotopic records and do not
take into account these source effects. The differences between the isotopic records (or the conventional reconstructions) (solid lines, Fig. 5) and the site temperature estimates
(dashed lines, Fig. 5) are used here to illustrate the possible
magnitude of moisture source biases.
For EDC, the site temperature estimate is up to ∼0.5 ◦ C
higher than the conventional temperature reconstruction
around 9.5 to 6 ka (Masson-Delmotte et al., 2004). For
the last interglacial, the site temperature estimate shows
∼0.5 ◦ C–1 ◦ C warmer conditions after the early interglacial
optimum, and enhanced by ∼1.5 ◦ C, the secondary temperature optimum (Stenni et al., 2010a). For EDML, the site
temperature estimate is up to 1 ◦ C lower than the conventional temperature reconstruction during the Early Holocene
optimum, ∼0.5 ◦ C during the secondary last interglacial optimum (Stenni et al., 2010a). At DF, a site temperature estimate has been published (Kawamura et al., 2007) using deuterium excess data (Uemura et al., 2004). The visual comparison between the site temperature and δ 18 O records (from
their Fig. 2) shows some differences. During the Holocene,
the moisture source correction leads to a 0.5 ◦ C weaker early
Holocene optimum, and removes the ∼1 ◦ C mid to late
Holocene decreasing trend derived from δ 18 O data. During the last interglacial, the moisture source correction also
slightly lessens the early optimum (by about 0.5 ◦ C), produces a secondary temperature optimum of ∼2 ◦ C and delays glacial inception cooling. Surprisingly, DF site temperature shows more similarity with DC temperature or isotopic
records than DF and DC isotopic records do.
Vostok site temperature estimates have been published
using deuterium excess data (Cuffey and Vimeux, 2001;
Vimeux et al., 2002). As for DF, the moisture source correction leads to a ∼1 ◦ C weaker early Holocene optimum and a
reduced late Holocene decreasing trend, a later and slightly
weaker early last interglacial optimum and a weaker cooling
trend during the last glacial inception. Temperature differences remain within ∼1.5 ◦ C.
Inspection of corrections introduced by the use of deuterium excess suggests limited albeit non-negligible influence on interglacial trends. We show that the available information (i) rules out that early interglacial or mid- interglacial
maxima and differences between interglacials are artefacts
caused by changes in the moisture source; (ii) rules out a
dominant contribution of changes in the moisture source on
isotopic trends over the course of interglacials. Figure 5
suggests that the relative amplitude of maxima can be affected by moisture source effects, with different amplitudes
for the different events and the different sites. This calls
for further examination of moisture source impacts on past
isotope-temperature changes, using both second order isotope data such as deuterium and oxygen 17 excess, and isotopic atmospheric general circulation models. The caveats
of our analysis lie in different quantification methods and
Clim. Past, 7, 397–423, 2011
Current interglacial, and b) Last interglacial. With this approach, strong increases in deuterium excess during interglacials at Vostok and DF (not shown here) appear to account for long term interglacial isotopic decreases. 414
V. Masson-Delmotte et al.: A comparison of the present and last interglacial periods in six Antarctic ice cores
18
δ O
δ O
18
DF
EDC
VK
EDML
4
4
2
2
0
-2
-4
-4
-6
-6
-8
-50
-50
4
4
-55
2
2
0
0
-2
-4
-4
-60
0
5
10
15
Age
20
-6
-6
-8
-8
-10
3
25 30x10
Tsite
Tsite
-2
-60
-8
-10
-10
-55
0
Tsite
-2
-45
Tsite
-45
-103
115 120 125 130 135 140145x10
Age
Fig. 5. Comparison between the initial isotopic records (‰, left axis, bold lines) and the site temperature (◦ C, right axis, same temperature
scaling for all records, dotted lines) estimated after correction for sea water
isotopic composition and moisture source correction taking
59
into account deuterium excess data, for EDC and EDML (Stenni et al., 2010b), Vostok (Vimeux et al., 2001a) and Dome F (Uemura et
al., 2004; Kawamura et al., 2007). The resolution of the Tsite records varies between 200 years (EDC and EDML), 300 years (DF) and
400 years (Vostok). They have been normalized with respect to the last millennium and smoothed using a binomial filter to highlight only
multi-millennial trends. (a) Current interglacial, and (b) last interglacial. With this approach, strong increases in deuterium excess during
interglacials at Vostok and DF (not shown here) appear to account for long- term interglacial isotopic decreases.
different temporal resolutions for site temperature reconstructions, and the lack of published deuterium excess information for TALDICE and TD. For this reason, we have not
performed the subsequent analyses on the site temperature
estimates, but on the δ 18 O records.
5.3
Principal component analyses
To explore the reasons for the inter-site differences, one can
assume that each site δ 18 Oi,t isotopic record is made of a
linear combination of a common Antarctic signal δ 18 Ot (expected to reflect a “mean Antarctic temperature history”) and
a local anomaly 118 Oi,t , which can then depend on (1) local elevation, (2) local temperature, (3) isotopic processes
linked with moisture origin, transport, and precipitation intermittency, (4) upstream glaciological effects, (5) errors on
ice core synchronisation:
δ 18 Oi,t = δ 18 Ot + 118 Oi,t
(3)
In this section, we first describe the extraction of the
mean signal, then the local residuals, and discuss the local
Clim. Past, 7, 397–423, 2011
glaciological or climate-isotopic processes that can be at
play. The five δ 18 Oi,t records are all placed on a common
age scale (here, EDC3) and re-sampled on a common time
step of 200 years. These re-sampled data are then smoothed
using a 5 point binomial filter. Figure 4a displays the raw
data (light lines) and therefore the differential initial temporal resolution of each time series, together with the smoothed
data (bold lines).
In order to extract the common δ 18 Ot signal, a principal
component analysis has been performed on the five smoothed
records using Analyseries software (Paillard et al., 1996).
For the current interglacial period, the first two EOF calculated on data from 0 to 15 kyr capture respectively 78% and
18% of common variance. For the last interglacial period, the
first two EOFs calculated on data from 135 to 116 kyr capture 91% and 5% of common variance (Table 3, Fig. 6). PC1
is interpreted to reflect the common δ 18 Ot signal, with PC2
expressing some of the site-specific differences. The magnitude of variance expressed in PC1 clearly demonstrates homogeneous stable isotope changes in the five East Antarctic
records.
www.clim-past.net/7/397/2011/
as displayed by variations in obliquity (thick black line) and reversed precession parameter (dashed grey line) (Laskar et al., 1993). d) Same as c) but for the Holocene. V. Masson-Delmotte
et al.: A comparison of the present and last interglacial periods in six Antarctic ice cores
118
a)
2
120
b)
Last interglacial age
122 124 126 128
415
3
130 132x10
2
1
0
0
-1
-1
PC
PC
1
-2
-60
d)
c)
24.5
-20
24.0
0
23.5
20
23.0
40
-3
60x10
Obliquity
Precession parameter
-40
-2
25.0
0
2
4
6
8
10
Current interglacial age
3
12 14x10
22.5
Fig. 6. (a) Common and residual signals highlighted by principal component analyses, showing the first (red) and second (blue) principal
the 5 δ 18 O records (TD not included) together with the percentage of variance explained, for the Holocene (expressed in
components of
‰ anomalies). (b) Same as (a) but for the last interglacial. (c) Last interglacial orbital context, as displayed by variations in obliquity (thick
reversed precession parameter (dashed grey line) (Laskar et al., 1993). (d) Same as (c) but for the Holocene.
black line) and
For the current interglacial period, PC1 shows a
Table 3. Principal component analysis of the five smoothed δ 18 O
∼1‰ early Holocene optimum from 11.5 to 10 kyr, followed
records for the present and last interglacial periods: percentage of
at 8 kyr (comparable to present day) and a
by a minimum
variance explained by the first and second EOF and their sum, and
secondary maximum
reaching ∼0.4‰ at 4 kyr (Fig. 6). PC2
linear coefficients applied to each record. The third EOF accounts
mostly accounts for a ∼1‰ decreasing trend from 12 to
for ∼2.5% of variance and is not displayed. TD is not included due
2 kyr. The long term trend seen in PC2 is most likely due
to its low resolution and dating accuracy for the last interglacial.
to long-term glaciological changes. For the previous interglacial, PC1 peaks at 128.5 kyr with a ∼2‰ strong early
Period
% of variance
DF
EDC TALDICE VK EDML
optimum, followed
by
a
minimum
at
126
kyr,
and
a
secCurrent
EOF1 78.4%
0.44
0.50
0.44 0.26
0.54
interglacial EOF2 17.8%
0.37
−0.11
−0.63 0.67
−0.01
ondary weak maximum (∼0.2‰) at 124 kyr (difficult to deTotal: 96.2%
tect from the millennial variability), and a progressive deLast
EOF1 91.1%
0.57
0.46
0.27 0.51
0.36
creasing trend. PC2 appears as a ∼2‰ decreasing trend (al5.2%
0.17 −0.003
−0.59 0.53
−0.58
beit with different rates of changes over time). We there- 60 interglacial EOF2
Total: 96.3%
fore observe rather comparable patterns of changes over the
course of the present and last interglacials, with about twice
larger magnitude during the last interglacial period in East
Antarctica. The larger magnitude of changes during MIS5.5
Hemisphere insolation on the deglacial freshwater flux and
may be linked with the different orbital contexts (Fig. 6, right
ocean circulation). With the EDC3 age scale, we can simpanel) marked by a larger eccentricity, stronger variations in
ply rule out a constant phase lag between early interglacial
the precession parameter, as well as a different phasing beoptima and the precession parameter as the MIS5.5 occurs
tween the precession and obliquity extrema than during the
prior to the precession parameter minimum, while the early
Holocene. The links between orbital forcing and AntarcHolocene occurs later. This finding obtained with the EDC3
tic temperature variations remain elusive, as both local seaage scale (associated with a ∼2–3 ka uncertainty) is further
sonal insolation (Renssen et al., 2004; Timmermann et al.,
supported by an alternative orbital dating of the Dome F ice
2009) and Northern Hemisphere insolation (Kawamura et al.,
core producing a ∼2 ka earlier onset of the LIG (Kawamura
2007) may be at play (e.g. through the impact of Northern
et al., 2007).
www.clim-past.net/7/397/2011/
Clim. Past, 7, 397–423, 2011
which are displayed. 416
V. Masson-Delmotte et al.: A comparison of the present and last interglacial periods in six Antarctic ice cores
Fig. 7. (a) Current interglacial and (b) last interglacial δ 18 O residuals between each record and the first PC of the five records (PC1 from
Fig. 5), expressed in ‰. The colour code is the same as in Fig. 2. 200 year data have been smoothed with a 3 point binomial filter and
centred to have zero values respectively at 0 and 120 kyr. (c) and (d) anomalies of elevation changes simulated at EDML (orange), EDC
(green), Vostok (blue) and Dome F (light blue) by Huybrechts et al. (2007) (long dashed line), Parrenin et al. (2007b) (solid line) and Pollard
and DeConto (2009) (dotted line). Anomalies have been calculated with respect to the present day elevation (also displayed with the zero
horizontal dashed black line). For EDML, the elevation change due to the geographical shift in surface ice origin (Huybrechts et al., 2007)
(“upstream” dashed orange curve) is also displayed. The sizes of the left (Holocene) and right (last interglacial) panels have been adjusted to
be proportional to the durations which are displayed.
5.4
Local residuals
In order to characterize the specificities of each ice core site,
we have extracted the residual between each ice core record
and the first principal component of the 5 records (Fig. 7, upper panels). For the Holocene, the EDML ice core exhibits
a small positive anomaly during the early and secondary optima (decreasing towards the present) and a negative anomaly
around 8 kyr; they all remain lower than 0.5‰. EDC shows
no significant anomaly during the early Holocene optimum,
a negative anomaly around 8 kyr, followed by an increasing trend towards the present; the anomalies also remain
small (lower than 0.5‰). By contrast, two sites have a tendency to display positive anomalies during the period 12–
5 kyr: Vostok and DF. For Vostok, the anomaly peaks during
the early Holocene (reaching 1.5‰) and subsequently decreases. For DF, the anomaly is rather stable (almost 1‰) until 6 kyr, followed by a decrease. Finally, TALDICE exhibits
Clim. Past, 7, 397–423, 2011
61
a generally negative anomaly, maximum (∼1‰) during the
early Holocene optimum, which progressively declines until around 4 kyr. The EDML and EDC records are clearly
those with minimum deviation from PC1 (from the sum of
the squared residuals).
For the last interglacial, the same features are observed;
note that upstream negative δ 18 O effects reaching about
1‰ are expected for both Vostok and EDML (see Fig. 4b
arrows). Again, (upstream corrected) EDML and Dome C
show smaller deviations from the average than each of the
other records. Again, Vostok and DF exhibit a positive residual (1 to 1.5‰), and TALDICE a negative residual (up to
1.5‰), particularly marked at the beginning of the Antarctic
warm period (132 to 124 kyr).
From present day spatial gradients (Sect. 4), δ 18 O anomalies may be translated into elevation anomalies with a slope
of 1‰ per 100 m (see previous section). The site residuals
can be compared with the local elevation changes as derived
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V. Masson-Delmotte et al.: A comparison of the present and last interglacial periods in six Antarctic ice cores
from 1-D (Parrenin et al., 2007b) or 3-D ice flow models
(Huybrechts et al., 2007; Pollard and DeConto, 2009) (Fig. 7,
lower panel). The latter model produces very small elevation anomalies during the late Holocene, positive elevation
anomalies (reaching at most a few tens of m) during the
early Holocene (by decreasing magnitudes, for DF, Vostok,
EDC and EDML) and a negative anomaly for TALDICE.
The patterns of elevation changes modelled for the last interglacial appear comparable to those of the Holocene, with
comparable magnitude; this may result from the simple marine record-based forcing of the Pollard and DeConto (2009)
ice sheet model, which does not account for the changes
in accumulation between the two periods as derived from
the ice core data. We note that this model suggests a midinterglacial change in anomaly at TALDICE (at 6 kyr and
124 kyr), which seems to occur later in the TALDICE residual. Finally, no modelling result can account for the observed
EDML anomaly at the beginning of the last interglacial, suggesting spurious ice flow features close to EDML bedrock
not accounted for by the current ice flow simulations, a feature previously noted by Stenni et al. (2010b).
The general features of the ice flow calculations support
the view that multi-millennial local residuals may embed a
small but significant local elevation signal, and that the ice
flow calculations may underestimate past changes in elevation, especially during the last interglacial in some areas
(e.g. TALDICE, Vostok, EDML). This points to a cautious
interpretation of local isotopic differences only in terms of
various isotope-temperature gradients. While the triplet (elevation, temperature, isotope-temperature slope) can be estimated in Greenland (Vinther et al., 2009), because both stable isotope and borehole temperature records are available
as well as peripheral dome records with minimum elevation
changes, the problem cannot be easily solved in Antarctica
due to the current lack of independent constraints on past
temperature and/or elevation changes.
6
Conclusion and perspectives
We have reviewed the modern glaciological and climatological contexts of these six East Antarctic sites. Meteorological
observations, atmospheric analyses, back-trajectory and atmospheric modelling point to site-specific properties in terms
of precipitation intermittency and moisture sources, but to
coherent central East Antarctic Plateau variations in annual
mean surface air temperature. We find very strong precipitation intermittency biasing effects across Antarctica; this implies that the “isotopic thermometer” is probably sensitive to
climatic changes affecting precipitation intermittency. The
difference in air mass trajectories and moisture sources for
adjacent sites such as TALDICE and Taylor Dome is particularly intriguing. Further work to confirm this finding,
for instance using moisture tagging in regional atmospheric
circulation models, would be of considerable interest. Ice
www.clim-past.net/7/397/2011/
417
sheet dynamics are expected to have different responses at
inland sites, where changes in local elevation appear to be
mostly driven by accumulation history, and at coastal locations, where ice flow response is important.
The comparison of the available ice core oxygen
18 records depicts a strong homogeneity in East Antarctica during the present and last interglacial, which is likely
due to a homogeneity of past temperature changes in the
central East Antarctic Plateau. “Coastal” sites such as TD
and TALDICE exhibit different characteristics. Both interglacials are marked by strong early and mid-period maxima.
Within the EDC3 age scale, different phase lags are observed
with respect to precession and obliquity maxima: the links
between the East Antarctic signal and the orbital context remain elusive.
An analysis of the site-specific residuals highlights systematic differences, such as larger magnitudes of changes at
Dome F than at EDC. Inter-site differences may arise because of differences in precipitation intermittency and covariance with temperature, resulting in different temporal
isotope-temperature slopes (Sime et al., 2008). It can also
be affected by changes in moisture origin as indicated by
temperature inversion taking into account deuterium excess.
Finally, small changes in elevation (<200 m) would be sufficient to account for the site-specific residuals. This possible
importance of glaciological effects will need to be further investigated. Ice core data should be combined with ice sheet
models to better estimate changes in ice volume of the East
Antarctic ice sheet during past interglacials.
Although detailed inspection of the different “warm” periods reveals a complex picture, with non-constant ratios between sites, considerable similarities exist between the intersite geographical pattern of isotopic change during the Early
Holocene and the last interglacial. A network of high spatial resolution ice core records is needed to document regional characteristics. Future work might investigate the differences between sites at shorter time scales, using improved
age scale synchronization. Regional differences could also
be tracked using an integrated perspective combining ice
core stable isotope and aerosol records with marine sediment records, a target of the ESF HOLOCLIP initiative
(http://www.holoclip.org/).
Further modelling investigations would be useful to better understand the causes of temporal and regional changes
in the isotope-temperature relationships. Our analysis of
inter-site differences, both during the instrumental period
and between ice core records, suggests that very high spatial resolution atmospheric modelling is needed. Further
work exploring and comparing the impacts on Antarctic
isotope-temperature relationships of various forcings, such
as changes in atmospheric greenhouse gas concentrations,
orbital forcing, freshwater perturbations and changes in
Antarctic ice sheet topography would be of high interest. Our
preliminary comparison of site temperature reconstructions,
taking into account deuterium excess constraints, suggests
Clim. Past, 7, 397–423, 2011
418
V. Masson-Delmotte et al.: A comparison of the present and last interglacial periods in six Antarctic ice cores
that changes in moisture origin may be at play, especially
for Dome F and Vostok. Documenting deuterium excess
and oxygen 17 excess along interglacials is therefore needed,
both in terms of observations and isotopic general circulation
model diagnoses.
The quantification of past regional temperature changes is
likely affected by changes in precipitation intermittency and
perhaps by changes in moisture sources. We note that the
ice core records do not offer any direct means of quantifying
the biases linked with precipitation intermittency. However,
independent observational constraints of temperature and accumulation rate during interglacial periods could be obtained
from δ 15 N in the air trapped in Antarctic ice cores. Indeed,
this parameter should give a faithful indication of the firn
depth during interglacial periods when convective zones at
the surface of the firn are negligible (Landais et al., 2006;
Dreyfus et al., 2010). Firnification models forced by temperature and accumulation rate were shown to be reliable
during this period (Landais et al., 2006). Hence, the combination of firnification models and δ 15 N measurements may
be a powerful tool to constraint past changes in temperature
and accumulation rate, with added value for inverse dating
methods (Parrenin et al., 2007a). However, in the meantime,
we will likely remain dependent on atmospheric circulation
modelling to help us explain the temporal and geographical patterns of isotopic change across multiple Antarctic ice
cores.
A priority of the International Partnership for Ice Core
Science (IPICS, http://www.pages-igbp.org/ipics/) is to improve the spatial coverage of Antarctic ice core records, in
order to improve the documentation of regional climate variability. In addition to homogeneous temperature changes on
the central East Antarctic Plateau, our comparison highlights
two abrupt decreases in the isotopic values at the two most
coastal TALDICE and EDML sites, around 118.5 kyr. These
changes could be linked with regional changes in sea ice extent. Documenting the last interglacial in West Antarctic ice
cores would complement our analysis, in an area where local
elevation effects are expected to be particularly strong (Siddall et al., 2011).
Acknowledgements. We thank Gaël Durand for constructive
discussions. H. S. acknowledges funding by the Norwegian
Research Council in the framework of the project WaterSIP. This
work is a contribution to EPICA, a joint European Science Foundation/European Commission scientific programme, funded by the
EU (EPICA-MIS) and by national contributions from Belgium,
Denmark, France, Germany, Italy, The Netherlands, Norway,
Sweden, Switzerland, and the UK. The main logistic support was
provided by IPEV and PNRA (at Dome C) and AWI (at Dronning
Maud Land). The Talos Dome Ice core Project (TALDICE), a joint
European programme, is funded by national contributions from
Italy, France, Germany, Switzerland and the United Kingdom.
Primary logistical support was provided by PNRA at Talos Dome.
The inter-site comparison of past interglacials was supported by the
ANR DOME A project (ANR-O7-BLAN-0125) and the analysis
Clim. Past, 7, 397–423, 2011
of modern climatic backgrounds by ANR VANISH. The research
leading to these results has received funding from the European
Union’s Seventh Framework programme (FP7/2007-2013) under
grant agreement 243908, “Past4Future. Climate change – Learning
from the past climate”, and is also a contribution to the ESF
HOLOCLIP project. The HOLOCLIP Project, a joint research
project of the European Science Foundation PolarCLIMATE
programme, is funded by national contributions from Italy, France,
Germany, Spain, Netherlands, Belgium and the United Kingdom. This is EPICA publication 274, TALDICE publication 12,
Past4Future publication 5, HOLOCLIP publication 2 and LSCE
publication 4488.
Edited by: V. Rath
The publication of this article is financed by CNRS-INSU.
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