Grounding-line retreat of the West Antarctic Ice Sheet from inner Pine Island Bay

Abstract

Ice loss from the marine-based, potentially unstable West Ant-arctic Ice Sheet (WAIS) contributes to current sea-level rise and may raise sea level by ≤3.3 m or even ≤5 m in the future. Over the past few decades, glaciers draining the WAIS into the Amundsen Sea Embay-ment (ASE) have shown accelerated ice fl ow, rapid thinning, and fast retreat of the grounding line (GL). However, the long-term context of this ice loss is poorly constrained, limiting our ability to accurately predict future WAIS behavior. Here we present a new chronology for WAIS retreat from the inner continental shelf of the eastern ASE, based on radiocarbon dates from three marine sediment cores. The ages document a retreat of the GL to within ~100 km of its modern position before ca. 10,000 calibrated (cal.) yr B.P. This early deglacia-tion is consistent with ages for GL retreat from the western ASE. Our new data demonstrate that, in contrast to the Ross Sea, WAIS retreat from the ASE shelf was largely complete by the start of the Holocene. Our results further suggest either slow GL retreat from the inner ASE shelf throughout the Holocene, or that any episodes of fast GL retreat must have been short-lived. Thus, today's rapid retreat may be excep-tional during the Holocene and may originate in recent changes in regional climate, ocean circulation, or ice-sheet dynamics. INTRODUCTION Pine Island Glacier, Thwaites Glacier, and Smith Glacier drain the West Antarctic Ice Sheet (WAIS) into Pine Island Bay in the eastern Amundsen Sea Embayment (ASE) (Fig. 1). Ice loss from this sector of the WAIS is currently raising global sea level at a rate of ~0.15–0.30 mm/yr, making it Antarctica's main contributor to present sea-level rise (Joughin and Alley, 2011, and references therein). Continued WAIS melting in the ASE sector has the potential to raise global sea level by ≤1.5 m, and thus to dominate sea-level change over coming centuries (Vaughan, 2008; Wing-ham et al., 2009). The current negative mass balance, which is mainly attributed to signifi cant sub–ice shelf melting by upwelling of relatively warm Circumpolar Deep Water (e.g., Rignot and Jacobs, 2002; Pritchard et al., 2012), is characterized by fast grounding line (GL) retreat (Pine Island Glacier, ~25 km from 1992 to 2009; Joughin et al., 2010; Thwaites Glacier, ≤14.5 km from 1992 to 2009; Tinto and Bell, 2011), accelerated ice dis-charge (Rignot, 2008; Joughin et al., 2010), and rapid thinning of grounded ice and ice shelves draining into the ASE (e.g., Joughin and Alley, 2011). However, it is unknown if the contemporary dynamic changes are simply part of long-term WAIS retreat since the Last Glacial Maximum (LGM, ca. 23,000–19,000 cal. yr B.P.), or solely recent phenomena. The deglacial history in the ASE sector since the LGM is poorly con-strained. Subglacial bedforms mapped by multibeam bathymetry and infor-mation from marine sediment cores revealed that Pine Island, Thwaites, and Smith Glaciers coalesced on the inner shelf during the LGM to form a major ice stream that advanced through a bathymetric trough to the outer shelf (Lowe and Anderson, 2002; Graham et al., 2010; Jakobsson et al., 2012). Radiocarbon ages from the sediment cores constrain deglaciation of the middle shelf at ~73°S to before 12,503 cal. yr B.P., while just a single 14 C date from core NBP99–02 PC41 (Fig. 1) constrains the timing of grounded ice retreat within ~250 km of the modern GL of Pine Island Glacier to before 10,256 cal. yr B.P. (Lowe and Anderson, 2002; Kirshner et al., 2012). Our study investigates whether rapid GL retreat similar to the modern GL retreat has typifi ed WAIS retreat from the ASE shelf during the Holocene. This knowledge will improve our understanding of current mass loss of West Antarctic glaciers and model-based predictions of future changes.

Full text

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Geology

doi: 10.1130/G33469.1
published online 19 October 2012;Geology

G. Vaughan
DavidRobert D. Larter, Johann P. Klages, Rachel Downey, Steven G. Moreton, Matthias Forwick and
Claus-Dieter Hillenbrand, Gerhard Kuhn, James A. Smith, Karsten Gohl, Alastair G.C. Graham,
Bay
Grounding-line retreat of the West Antarctic Ice Sheet from inner Pine Island

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1
ABSTRACT
Ice loss from the marine-based, potentially unstable West Ant-
arctic Ice Sheet (WAIS) contributes to current sea-level rise and may
raise sea level by ≤ ≤3.3 m or even ≤ ≤5 m in the future. Over the past few
decades, glaciers draining the WAIS into the Amundsen Sea Embay-
ment (ASE) have shown accelerated ice fl ow, rapid thinning, and fast
retreat of the grounding line (GL). However, the long-term context of
this ice loss is poorly constrained, limiting our ability to accurately
predict future WAIS behavior. Here we present a new chronology for
WAIS retreat from the inner continental shelf of the eastern ASE,
based on radiocarbon dates from three marine sediment cores. The
ages document a retreat of the GL to within ~100 km of its modern
position before ca. 10,000 calibrated (cal.) yr B.P. This early deglacia-
tion is consistent with ages for GL retreat from the western ASE. Our
new data demonstrate that, in contrast to the Ross Sea, WAIS retreat
from the ASE shelf was largely complete by the start of the Holocene.
Our results further suggest either slow GL retreat from the inner ASE
shelf throughout the Holocene, or that any episodes of fast GL retreat
must have been short-lived. Thus, today’s rapid retreat may be excep-
tional during the Holocene and may originate in recent changes in
regional climate, ocean circulation, or ice-sheet dynamics.
INTRODUCTION
Pine Island Glacier, Thwaites Glacier, and Smith Glacier drain the
West Antarctic Ice Sheet (WAIS) into Pine Island Bay in the eastern
Amundsen Sea Embayment (ASE) (Fig. 1). Ice loss from this sector of the
WAIS is currently raising global sea level at a rate of ~0.15–0.30 mm/yr,
making it Antarctica’s main contributor to present sea-level rise (Joughin
and Alley, 2011, and references therein). Continued WAIS melting in the
ASE sector has the potential to raise global sea level by ≤1.5 m, and thus to
dominate sea-level change over coming centuries (Vaughan, 2008; Wing-
ham et al., 2009). The current negative mass balance, which is mainly
attributed to signifi cant sub–ice shelf melting by upwelling of relatively
warm Circumpolar Deep Water (e.g., Rignot and Jacobs, 2002; Pritchard et
al., 2012), is characterized by fast grounding line (GL) retreat (Pine Island
Glacier, ~25 km from 1992 to 2009; Joughin et al., 2010; Thwaites Glacier,
≤14.5 km from 1992 to 2009; Tinto and Bell, 2011), accelerated ice dis-
charge (Rignot, 2008; Joughin et al., 2010), and rapid thinning of grounded
ice and ice shelves draining into the ASE (e.g., Joughin and Alley, 2011).
However, it is unknown if the contemporary dynamic changes are simply
part of long-term WAIS retreat since the Last Glacial Maximum (LGM,
ca. 23,000–19,000 cal. yr B.P.), or solely recent phenomena.
The deglacial history in the ASE sector since the LGM is poorly con-
strained. Subglacial bedforms mapped by multibeam bathymetry and infor-
mation from marine sediment cores revealed that Pine Island, Thwaites,
and Smith Glaciers coalesced on the inner shelf during the LGM to form
a major ice stream that advanced through a bathymetric trough to the outer
shelf (Lowe and Anderson, 2002; Graham et al., 2010; Jakobsson et al.,
2012). Radiocarbon ages from the sediment cores constrain deglaciation of
the middle shelf at ~73°S to before 12,503 cal. yr B.P., while just a single 14C
date from core NBP99–02 PC41 (Fig. 1) constrains the timing of grounded
ice retreat within ~250 km of the modern GL of Pine Island Glacier to
before 10,256 cal. yr B.P. (Lowe and Anderson, 2002; Kirshner et al., 2012).
Our study investigates whether rapid GL retreat similar to the modern GL
retreat has typifi ed WAIS retreat from the ASE shelf during the Holocene.
This knowledge will improve our understanding of current mass loss of
West Antarctic glaciers and model-based predictions of future changes.
MATERIALS
During R/V Polarstern expedition ANT-XXVI/3 in 2010, we col-
lected three sediment cores (PS75/160, PS75/167, and PS75/214) at inner
shelf locations in Pine Island Bay to reconstruct the history of post-LGM
WAIS retreat (Table DR1 in the GSA Data Repository1). The three core
sites are located on shallow ridges fl anking the paleo–ice stream trough
*E-mail: hilc@bas.ac.uk.
Grounding-line retreat of the West Antarctic Ice Sheet from inner
Pine Island Bay
Claus-Dieter Hillenbrand1*, Gerhard Kuhn2, James A. Smith1, Karsten Gohl2, Alastair G.C. Graham1, Robert D. Larter1,
Johann P. Klages2, Rachel Downey1, Steven G. Moreton3, Matthias Forwick4, and David G. Vaughan1
1British Antarctic Survey, Natural Environment Research Council, High Cross, Madingley Road, Cambridge CB3 0ET, UK
2 Alfred Wegener Institute for Polar and Marine Research (AWI), Helmholtz Association, Am Alten Hafen 26, D-27568 Bremerhaven, Germany
3NERC Radiocarbon Laboratory (Environment), Scottish Enterprise Technology Park, Rankine Avenue, East Kilbride G75 0QF, UK
4University of Tromsø, Department of Geology, N-9037 Tromsø, Norway
GEOLOGY, January 2013; v. 41; no. 1; p. 1–4; Data Repository item 2013012 | doi:10.1130/G33469.1 | Published online XX Month 2012
!
105°W
0 200 100
km
120°W
Amundsen
Sea
72°S
75°S
PIB
[1.19]
Getz Ice Shelf
WAIS
PS75/160PS75/129
Pine
Island
Glacier
PS75/215
Dotson
Ice
Shelf
Thwaites
Glacier
A
C
E
F
Smith
Glacier
11.7
9.2
11.9
10.3
9.0
11.8
6.7
13.0
10.8
14.6
9.9
15.3
9.415.9
3.6
13.0
3.4
B
D
10.3
9.6
[1.46]
[1.14]
[1.55]
[2.85]
[1.43]
[2.34]
PS69/251
PS75/235
TC22
PS69/255
PS69/275
BC448
VC419
BC451
PC41
PC23 PC26
PS75/167PS75/214
WAIS
Figure 1. Map of Amundsen Sea Embayment, Antarctica, showing
shelf break (black dashed line), locations of paleo–ice stream trough
systems (dark blue), ice shelves (gray shaded), modern grounding
line (GL; black continuous line, from Rignot et al., 2011), sediment
core sites from the shelf, core-top 14C dates (from calcareous micro-
fossils) used for establishing the regional marine reservoir effect (in
uncorrected 14C kyr B.P.; black numbers in brackets), minimum ages
for GL retreat (in calibrated kyr B.P.; black numbers), and maximum
rates for GL retreat from core sites to modern GL of the West Ant-
arctic Ice Sheet (WAIS) (in m/yr; white numbers). Core locations pre-
sented here for fi rst time are highlighted by orange symbols (cores
discussed in detail are highlighted by ages and retreat rates given in
italics), while those from previously published studies are marked by
green symbols (details and references in Table DR1; see footnote 1).
Distances of core sites from present GL were measured from furthest
landward GL point upfl ow from each site (black crosses) along tra-
jectories marked by black dotted lines. Inset map shows location of
study area within wider context of Antarctica. PIB—Pine Island Bay.
1GSA Data Repository item 2013012, Table DR1 (radiocarbon dates from the
ASE cores) and Appendix DR1 (methods and laboratory techniques), is available on-
line at www.geosociety.org/pubs/ft2013.htm, or on request from editing@geosociety
.org or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, USA.
as doi:10.1130/G33469.1Geology, published online on 19 October 2012

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2 www.gsapubs.org | January 2013 | GEOLOGY
and are 110, 112, and 93 km offshore from the modern GLs of Pine
Island Glacier (PS75/160, PS75/214) and Thwaites Glacier (PS75/167),
respectively (Fig. 1). As in other paleo–ice stream troughs around West
Antarctica (e.g., Livingstone et al., 2012), grounded ice and subglacial
meltwater eroded a deep basin in the most landward trough section over
numerous glacial cycles (Lowe and Anderson, 2003). Consequently, inner
Pine Island Bay is characterized by rugged seafl oor topography with only
localized patches of sediment cover, and water depths of ≤1600 m. Crys-
talline bedrock is exposed over most of its seafl oor (Lowe and Anderson,
2003); however, we identifi ed small sediment pockets suitable for gravity
coring on highs above 800 m water depth, where sediments contain bio-
genic carbonate (Hauck et al., 2012). Calcareous microfossils are usually
absent from Antarctic shelf sediments, but provide the most reliable 14C
ages (e.g., Anderson et al., 2002; Domack et al., 2005).
RESULTS AND DISCUSSION
Facies Analysis
All three cores contained soft, predominantly fi ne-grained, muddy
to sandy sediments with common abundances of foraminifera (Fig. 2). In
accordance with previous studies on sediments from ice-marginal settings,
we investigated their lithological composition, sedimentary structures,
grain-size distribution, and physical properties (Appendix DR1 in the
Data Repository) to assign them to six different glaciomarine facies types
(Table 1). We attributed coarse-grained sediments to a depositional setting
proximal to the GL and fi ne-grained sediments to a depositional setting
distal from the GL, following previously published facies analyses on
Antarctic shelf sediments (e.g., Lowe and Anderson, 2002, 2003; McKay
et al., 2009; Hillenbrand et al., 2010; Passchier et al., 2011). Near the core
top, the sediments at all three sites consist of bioturbated mud (facies M)
with some diatom frustules, likely deposited by hemipelagic settling in a
seasonal open-marine environment distal from the GL, although perennial
coverage by fl oating ice (e.g., Domack et al., 2005) cannot be ruled
out. Physical property values of facies M resemble those of underlying
terrigenous muds interlaminated with thin silt layers (facies MSi) present
at site PS75/214 (Fig. 2), which are interpreted as meltwater plume
sediments deposited at some distance from grounded ice (cf. Lowe and
Anderson, 2002). In contrast, a massive sandy gravel unit (facies GS) at
site PS75/167, a slightly bioturbated sand unit (facies S) at site PS75/160,
and muds interstratifi ed with sandy layers (facies MSa) recovered in all
three cores are characterized by both lower water contents and higher
values of wet-bulk density, shear strength, and magnetic susceptibility
(Fig. 2). The coarse-grained intervals of facies S and MSa are occasionally
characterized by fi ning upward and erosional basal contacts suggesting
their formation by gravity fl ows, turbidity currents, meltwater plumes,
and/or current winnowing in a setting proximal to the GL (Table 1). The
deposition of facies GS probably resulted from similar processes, or a high
accumulation of ice-rafted debris combined with current winnowing.
The sedimentary sequences retrieved at sites PS75/214 and PS75/167
document a transition from a setting proximal to the GLs of Pine Island and
Thwaites Glaciers to a distal seasonal open-marine environment (Fig. 2;
Table 1). The more complex stratigraphy in core PS75/160 also contains
facies MC. Facies MC is characterized by deformed and sheared pebble-
sized muddy to sandy soft sediment clasts that are randomly orientated
in a muddy matrix. This facies probably results from iceberg furrowing,
because the modern average iceberg keel depth at the fl oating part of Pine
Island Glacier is ~500 m (e.g., Jenkins et al., 2010), greatly exceeding the
337 m water depth at site PS75/160. Alternatively, facies MC may have
been formed by debris fl ows or slumps or melt-out of soft sediment clasts
from icebergs or the base of an ice shelf. We rule out that facies MC formed
as a glaciotectonite (i.e., a subglacially deformed and sheared deposit
consisting of till and reworked ice-marginal sediments that have retained
some of the structural characteristics of the parent material; e.g., Ó Cofaigh
et al., 2011), because facies MC lacks some key features of glaciotectonites,
such as interbedding with subglacial diamicton, orientated fabric of soft-
sediment clasts, and clear evidence for erosional unconformities.
Notably, the three cores recovered exclusively foraminifera-bearing
muddy to sandy sediments that are normally consolidated and show a range
8680
8354
8410
11,664
8034
PS75/214
water content
(wt%)
15
35
25
wet-bulk density
(g cm–3)
1.22.01.6 1.41.8 55 45
κ
0 24001200
shear strength
(kPa)
02
4
68 10
gravel//
(wt%)
40
sand mud
0 10020 60 80
0
100
200
300
400
500
600
700
M
MSi
MSa
h
t
p
e
D
(cmbsf)
facies
type
PS75/167
water content
(wt%)
1525 1.22.0 1.6 1.41.855
35
45
wet-bulk density
(g cm–3)
gravel//
(wt%)
40
sand mud
0 100 2060
κ
(10–6 SI units)
0 2400 1200
shear strength
(kPa)
02
4
6810
80
h
t
p
e
D
(cmbsf)
0
100
200
300
400
500
600
700
800
900
M
GS
MSa
8283
8564
8945
9048
9034
9529
9707
9142
9479
9735
9656
10,348
facies
type
14C age
(cal. yr
B.P.)
100
1188
1665
4381
6959
8166
9196
8391
7970/
7999
PS75/160
water content
(wt%)
15
35
25
wet-bulk density
(g cm–3)
1.22.01.6
1.41.8 5545
κ
(10–6 SI units)
024001200
shear strength
(kPa)
02
4
68 10
gravel//
(wt%)
40
sand mud
0206080
0
100
200
300
400
500
600
M
MC
M
MC
M
S
MC
MSa
MC
MSa
h
t
p
e
D
(cmbsf)
facies
type
Lithofacies:
M
S
MC
MSa
GS
MSi
(10–6 SI units)
14C age
(cal. yr
B.P.)
14C age
(cal. yr
B.P.)
Figure 2. Facies (described in detail in Table 1), physical properties (wet-bulk density, volume-specifi c magnetic susceptibility κ κ, water
content, shear strength), grain-size distribution (gravel/sand/mud ratios) and calibrated (cal.) accelerator mass spectrometry 14C ages on
calcareous microfossils in sediment cores PS75/167, PS75/160, and PS75/214 (oldest age at each site is underscored; cmbsf—centimeters
below seafl oor). Examples of X-ray radiographs (negatives) for six facies types are also shown (yellow scale bars in upper left corners =
5 cm). X-ray radiograph examples for facies M (bioturbated mud), MSa (muds interstratifi ed with sandy layers), and MSi (terrigenous muds
interlaminated with thin silt layers) are from core PS75/214, those for facies S (slightly bioturbated sand) and MC (deformed, sheared pebble-
sized muddy to sandy soft sediment clasts randomly orientated in muddy matrix) are from core PS75/160, and example for facies GS (mas-
sive sandy gravel) is from core PS75/167.
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3
of sedimentary structures (Table 1). We do not observe massive terrigenous
diamictons resembling subglacial till recovered in other cores from the
ASE (Lowe and Anderson, 2002; Smith et al., 2011; Kirshner et al., 2012)
and other parts of the West Antarctic shelf (e.g., Anderson et al., 2002;
Livingstone et al., 2012). Therefore, we are certain that the sedimentary
sequences recovered in our cores are of glaciomarine origin and that their
deposition must postdate the last GL advance across the core sites. However,
the occurrence of microfossils in the sediments does not necessarily exclude
the presence of an ice shelf (e.g., Domack et al., 2005) that may have
covered inner Pine Island Bay until the 1930s (Steig et al., 2012).
Radiocarbon Chronology
A unique feature of our cores is the common abundance of
calcareous microfossils, mainly benthic and planktonic foraminifera
tests (Table DR1). We obtained accelerator mass spectrometry 14C dates
on calcareous microfossils taken from all facies types (Fig. 2; Appendix
DR1). In accordance with uncorrected 14C ages from core-top sediments
in the ASE (Fig. 1; Table DR1) and previous Antarctic studies (e.g.,
Livingstone et al., 2012, and references therein), we corrected the 14C
dates by subtracting a marine reservoir effect of 1100 ± 200 yr (Berkman
and Forman, 1996; Domack et al., 2005). The 14C ages of core PS75/160
span the time from 1188 to 9196 cal. yr B.P. (Fig. 2; Table DR1). The 14C
dates of core PS75/214 range from 8034 to 11,664 cal. yr B.P., and those
of core PS75/167 range from 8283 to 10,348 cal. yr B.P., suggesting that a
hiatus in sedimentation may have occurred at both sites during the mid-late
Holocene, or that sediments accumulated at very low rates. We observe
minor 14C age reversals at sites PS75/160 and PS75/214 below ~5 m core
depth (Fig. 2) that probably result from sediment reworking by downslope
processes, strong current activity, and/or iceberg scouring (Table 1),
but do not affect our deglacial chronology or our paleoenvironmental
interpretation. All other age reversals are within the uncertainty of the
reservoir correction and the analytical error (Table DR1).
TIMING OF INNER SHELF DEGLACIATION AND RATE OF
HOLOCENE GL RETREAT
Regardless of their subsequent redeposition from nearby, shallower
shelf areas by gravitational downslope transport, the calcareous microfossils
can only have lived near a core site after grounded ice had retreated further
landward. Therefore, our 14C dates constitute reliable minimum ages for
GL retreat. Consequently, the GL must have retreated before 9196 cal. yr
B.P. from site PS75/160, before 11,664 cal. yr B.P. from site PS75/214,
and before 10,348 cal. yr B.P. from site PS75/167 (Fig. 2). Combining
our reconstruction of GL retreat from inner Pine Island Bay with existing
minimum deglaciation ages from near-coastal core locations in the western
ASE indicates consistent grounded ice-sheet retreat across the inner shelf
before at least ~10,000 cal. yr B.P. (Fig. 1; Table DR1). In contrast to the
steady pattern of GL retreat from the Ross Sea shelf (Conway et al., 1999),
the WAIS had already retreated to the inner ASE shelf before the start of the
Holocene, achieving a confi guration close to the modern remarkably early.
Given that there is currently no evidence from the ASE sector of the
WAIS for a Holocene GL position landward of the modern position or
a Holocene readvance of the GL onto the inner shelf, the calculation of
retreat rates from the core sites to the present-day GL position allows us to
characterize the long-term context of WAIS retreat. Our calculated retreat
rates are maxima (they are based on minimum deglaciation ages) and range
from 3.4 ± 0.1 m/yr to 11.9 ± 0.7 m/yr; the highest rate is observed in Pine
Island Bay (Fig. 1). It is signifi cant that all retreat rates are more than two
orders of magnitude lower than the recent GL retreat of Pine Island and
Thwaites Glaciers (Joughin et al., 2010; Tinto and Bell, 2011), and thus
indicate that today’s rapid retreat is exceptional in a Holocene context.
TABLE 1. SUMMARY OF FACIES IDENTIFIED IN THE STUDIED SEDIMENT CORES, AND INFERRED PROCESSES AND PALEOENVIRONMENTS
yr a tnem i deS
structures
yg o l oht i Lse i caF Physical properties Processes and paleoenvironments
M mud and/or sandy mud,
with diatom frustules and/
or calcareous microfossils;
occasionally with a few
dispersed gravel grains
mud alternating with layers and
lenses of silt and/or sandy
silt; coarse-grained layers
occasionally enriched in
calcareous microfossils
slightly to strongly bioturbated,
slightly to moderately
laminated and/or stratifi ed
high water content, low shear
strength, low wet bulk density,
low to medium magnetic
susceptibility
hemipelagic suspension settling with deposition of
IRD in an open-marine setting or under sea-ice
and/or thin ice shelf distal from the grounding
line (references A, B, C, D, F, G, H)
MSi
moderately to strongly
laminated and/or stratifi ed,
slightly bioturbated
high water content, low shear
strength, low wet bulk density,
low magnetic susceptibility
hemipelagic suspension settling alternating with
settling from meltwater plumes under sea-ice
and/or ice-shelf cover in a setting distal from the
grounding line (references D, F, G)
GS sandy gravel (with a few
pebbles), enriched in
calcareous microfossils
massive, with coarsening
upward
low water content, low shear
strength, high wet bulk density,
high magnetic susceptibility
high-energy gravity fl ows (references C, F, H)
or meltwater fl ows (reference E), high IRD
accumulation (reference G), high-energy current
winnowing of glaciomarine sediment (reference
D) under sea-ice and/or ice-shelf cover proximal
to the grounding line
high-density gravity fl ows (references C, D, F, G,
H), current winnowing of glaciomarine sediment
(references D, G) under sea-ice and/or ice-shelf
cover proximal to the grounding line
hemipelagic suspension settling alternating
with turbidity currents (references C, F, G, H),
settling from meltwater plumes (reference F),
current winnowing of glaciomarine sediment
(reference D) under sea-ice and/or ice-shelf
cover proximal to the grounding line
iceberg turbation of glaciomarine sediments in an
open-marine setting distal from the grounding
line (references D, H), melt-out of clasts from
an ice-shelf base in a proximal sub–ice shelf
setting (references A, B), debris fl ow or slump
(references C, D, F, G)
S sand (with a few gravel grains),
enriched in calcareous
microfossils
slightly bioturbated, moderately
stratifi ed
low water content, high shear
strength, medium wet bulk
density, medium magnetic
susceptibility
low water content, high shear
strength, high wet bulk density,
medium to high magnetic
susceptibility
MSamud alternating with layers and
lenses of sand and/or gravelly
sand; coarse-grained layers
occasionally enriched in
calcareous microfossils
moderately to strongly stratifi ed
and/or laminated; coarse-
grained layers occasionally
massive with fi ning upward
and erosional base
MCsoft sediment clasts of mud
and/or sandy mud and sand
and/or muddy sand in a
muddy matrix, occasionally
with calcareous microfossils
randomly orientated deformed
and sheared mottles of
massive to laminated and/or
stratifi ed soft sediment clasts
medium to high water
content, variable shear
strength, medium wet bulk
density, variable magnetic
susceptibility
Note: IRD— ice-rafted debris. References: A—Anderson et al., 2002; B—Domack et al., 2005; C—Hillenbrand et al., 2010; D—Lowe and Anderson, 2002; E—Lowe and
Anderson, 2003; F—McKay et al., 2009; G—Passchier et al., 2011; H—Smith et al., 2011.
as doi:10.1130/G33469.1Geology, published online on 19 October 2012

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4 www.gsapubs.org | January 2013 | GEOLOGY
However, our rates are averaged over thousands of years. The presence
of grounding-zone wedges and radiocarbon dates from sediment cores in
the mid-shelf part of the Pine Island paleo–ice stream trough document
that post-LGM ice-sheet retreat there was episodic (Graham et al., 2010;
Jakobsson et al., 2012; Kirshner et al., 2012). Similarly, the recent episode
of rapid GL retreat of Pine Island Glacier, attributed to its decoupling from
a transverse seafl oor ridge at some time between A.D. 1975 and 1982,
may have followed an extended period of GL stability (Jenkins et al.,
2010; Joughin et al., 2010). Even if the Holocene deglaciation of inner
Pine Island Bay was characterized by long-term GL stability interrupted
by brief (i.e., ~25–30 yr) episodes of rapid retreat comparable to those
recently recorded for Pine Island Glacier, our results would imply that
no more than 3–4 such episodes could have occurred between ~10,000
cal. yr B.P. and A.D. 1992, because otherwise the GL would have retreated
landward of its modern position. Sedimentary wedges deposited during
potential long-term still-stands of the GL are not present between our core
sites and the modern calving fronts of Pine Island and Thwaites Glaciers
(e.g., Lowe and Anderson, 2003), suggesting that the present rapid retreat
may be unprecedented during the Holocene.
We conclude that the current rapid deglaciation of the ASE sector
is probably a recent phenomenon driven by changes in glacier-bed
interaction (Jenkins et al., 2010), atmospheric warming (Ding et al.,
2011), and a strengthening of Circumpolar Deep Water infl ow (Jacobs et
al., 2011), which may have been forced by an increase of westerly wind
stress (Steig et al., 2012), rather than being a continued ice-sheet response
to much earlier changes. Our data provide the fi rst geological constraints
on past ice retreat from the coastal vicinity of a key drainage sector of the
WAIS, and reveal for the fi rst time a pattern of deglaciation that forms the
critical context for present and future ice-sheet changes.
ACKNOWLEDGMENTS
We thank the captain and crew participating in R/V Polarstern expedition
ANT-XXVI/3, and H. Blagbrough, D. Baqué, R. Fröhlking, M. Gutjahr, P. Jernas,
N. Lensch, I. MacNab, M. Seebeck, S. Wiebe, and S. Wiers for their assistance. This
work was fi nancially supported by the UK Natural Environment Research Council
and the Alfred Wegener Institute research program Polar Regions and Coasts in
a Changing Earth System. We thank A. Rodger, J. Johnson, and two anonymous
reviewers for their constructive comments.
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Manuscript received 29 March 2012
Revised manuscript received 3 July 2012
Manuscript accepted 3 July 2012
Printed in USA
as doi:10.1130/G33469.1 Geology, published online on 19 October 2012