have been sustained for several decades, then the
ice surface must have been considerably higher
when it started. However, if the ice surface was
even a few meters above present for a substantial
period of the late Holocene, then we would observe erratics with much younger exposure ages at
sites adjacent to the modern ice surface than those
at higher elevations. Given that this is not the case,
if the ice surface was above these samples between
7.9 ka and recent decades, it can only have been so
for a time comparable with the precision of the
exposure ages (~100 years). Thus, the most likely
scenario consistent with our data is that the ice
surface was near its present elevation (or possibly
lower, because our observations cannot detect periods of thinner ice) between 7.9 ka and the onset
of contemporary thinning.
The high thinning rates determined from our
exposure ages imply an ice-dynamic change because
drivers such as a decrease in accumulation rate or
increase in atmospheric temperature would produce
a slower response. Marine geological and geophysical studies show that the PIG grounding line
had retreated to within, but had not stabilized at,
112 km of its present position (the core site is
shown in Fig. 1B and fig. S5) by 11.7 T 0.7 ka
(21), and that a ridge beneath PIG ice shelf (Fig.
1B and fig. S5) acted as a pinning point for the
grounding line before the 1970s (7). Therefore,
potential hypotheses for the mechanism of an
early Holocene ice-dynamic change could be (i)
rapid migration of the PIG grounding line resulting from decoupling from a topographic high or (ii)
reduction in ice shelf buttressing.
First, we discuss the effect of subglacial
topography. This can influence the style of ice
stream retreat—for example, by providing topographic highs on which pinning can occur (25)
and by constraining ice stream width (26, 27).
Although the marine geological data constrain
retreat of the grounding line landward from the
core site to the sub–ice shelf ridge only to sometime between 11.7 ka and the 1970s, they do not
preclude that the retreat was associated with inland thinning at ~8 ka. However, there are no
topographic highs seaward of the sub–ice shelf
ridge where the grounding line might have been
pinned after 11.7 ka (fig. S5) and from which
detachment could have triggered the dynamic
thinning inland. Therefore, although grounding
line retreat may have been associated with the
early Holocene thinning, decoupling of the PIG
grounding line from a topographic high [hy-pothesis (i)] is unlikely to have been the trigger
Alternatively, thinning may have been the
consequence of reduction in buttressing by an ice
shelf. Marine sediments have been used to infer
the presence of an ice shelf across the middle
shelf of the Amundsen Sea before ~10.6 T 0.3 ka
(fig. S5A) (19). Although the available chrono-
logical data cannot resolve when that ice shelf
finally retreated into inner Pine Island Bay, one
study suggests that it persisted there until ~7 ka
(19). Glaciers in the Amundsen Sea Embayment
and elsewhere in Antarctica have responded to
recent ice shelf thinning with acceleration of
flow, grounding line retreat, and thinning (14).
Similarly, subsequent retreat or weakening (such
as by thinning) of a buttressing ice shelf in Pine
Island Bay could have triggered the dynamic
thinning in the Hudson Mountains at ~8 ka.
Reduction in ice shelf buttressing would most
likely have been initiated by enhanced basal
melting in response to inflow of warm Circum-
polar Deep Water, as is suggested to account for
present thinning (11). We favor hypothesis (ii) as
the most likely mechanism for early Holocene
ice-dynamic change, but we cannot rule out more
complicated mechanisms. For example, it is pos-
sible that thinning of the outlet glacier may be
related to its separation from PIG.
These results have implications for understanding how the Pine Island–Thwaites sector of
the WAIS is likely to evolve in coming decades
to centuries. The knowledge that PIG has previously undergone sustained dynamic thinning,
followed by relative stabilization over several
millennia before the onset of contemporary thinning, suggests that the PIG system can respond
quickly to environmental change by abrupt, discontinuous, and stepwise retreat. Continued thinning may lead to an even more dramatic response
if a dynamic threshold, such as a critical ice shelf
thickness or ice flow rate, is exceeded. In addition, the rate and magnitude of early Holocene
thinning is consistent with model-based estimates
of future PIG thinning sustained over the coming
century (28, 29), a time scale over which the magnitude of sea-level rise most concerns policy-makers. In a wider context, the pattern of abrupt
past thinning of PIG contrasts with evidence for
slower and steadier Holocene deglaciation of other regions of the WAIS (16, 30), hinting that a
considerable part of any WAIS contribution to
sea-level rise in the early Holocene may have
come from its Amundsen Sea sector.
The data presented here demonstrate that thinning of PIG at a rate comparable with that over
the past two decades is rare but not unprecedented
in the Holocene. Moreover, in contrast to previous glacial-geological work in Antarctica that
has provided average thinning rates only over millennial time scales, our data are precise enough to
show that rapid thinning of PIG was sustained for
at least 25 years, and most likely for much longer.
These data provide a long-term context for contemporary thinning of PIG, suggesting that on-going ocean-driven melting of PIG ice shelf can
result in continued rapid thinning and grounding line retreat for several more decades or even
References and Notes
1. M. A. King et al., Nature 491, 586–589 (2012).
2. A. Shepherd et al., Science 338, 1183–1189 (2012).
3. E. Rignot, Geophys. Res. Lett. 35, L12505 (2008).
4. H. D. Pritchard, R. J. Arthern, D. G. Vaughan,
L. A. Edwards, Nature 461, 971–975 (2009).
5. D. Wingham, D. W. Wallis, A. Shepherd, Geophys. Res.
Lett. 36, L17501 (2009).
6. J. W. Park et al., Geophys. Res. Lett. 40, 2137–2142
7. A. Jenkins et al., Nat. Geosci. 3, 468–472 (2010).
8. I. Joughin, R. B. Alley, Nat. Geosci. 4, 506–513
9. R. Thomas et al., Science 306, 255–258 (2004).
10. A. Shepherd, D. J. Wingham, J. A. D. Mansley, H. F. Corr,
Science 291, 862–864 (2001).
11. S. S. Jacobs, A. Jenkins, C. F. Giulivi, P. Dutrieux,
Nat. Geosci. 4, 519–523 (2011).
12. A. J. Payne, A. Vieli, A. Shepherd, D. J. Wingham,
E. Rignot, Geophys. Res. Lett. 31, L23401 (2004).
13. A. Shepherd, D. Wingham, E. Rignot, Geophys. Res. Lett.
31, L23402 (2004).
14. H. D. Pritchard et al., Nature 484, 502–505 (2012).
15. Intergovernmental Panel on Climate Change, in Climate
Change 2007: The Physical Science Basis. Contribution
of Working Group I to the Fourth Assessment Report
of the Intergovernmental Panel on Climate Change,
S. Solomon, et al., Eds. (Cambridge Univ. Press,
16. J. O. Stone et al., Science 299, 99–102 (2003).
17. A. L. Lowe, J. B. Anderson, Quat. Sci. Rev. 21,
18. A. G. C. Graham et al., J. Geophys. Res. 115, F03025
19. A. E. Kirshner et al., Quat. Sci. Rev. 38, 11–26
20. M. Jakobsson et al., Quat. Sci. Rev. 38, 1–10 (2012).
21. C.-D. Hillenbrand et al., Geology 41, 35–38 (2013).
22. J. S. Johnson, M. J. Bentley, K. Gohl, Geology 36,
23. Materials and methods are available as supplementary
materials on Science Online.
24. J. M. Schaefer et al., Science 324, 622–625 (2009).
25. K. J. Tinto, R. E. Bell, Geophys. Res. Lett. 38, L20503
26. S. S. R. Jamieson et al., Nat. Geosci. 5, 799–802
27. G. H. Gudmundsson, J. Krug, G. Durand, L. Favier,
O. Gagliardini, Cryosphere 6, 1497–1505 (2012).
28. I. Joughin, B. E. Smith, D. M. Holland, Geophys. Res. Lett.
37, L20502 (2010).
29. R. M. Gladstone et al., Earth Planet. Sci. Lett. 333-334,
30. M. J. Bentley et al., Geology 38, 411–414 (2010).
31. E. Rignot, J. Mouginot, B. Scheuchl, MEaSUREs
InSAR-Based Antarctica Ice Velocity Map (National Snow
and Ice Data Center, Boulder, CO, 2011).
Acknowledgments: The data presented here are archived in
the supplementary materials. The project was conceived
and developed by M.J.B. and R.D.L. Fieldwork and sampling
were planned and undertaken by M.J.B., J.A.S., and J.S.J.
K.G. led the cruise (RV Polarstern Expedition ANT-XXVI/3). J.S.J.
processed the samples and interpreted the data, with direction
from J.M.S., and analyses were performed by R.C.F. and
D.H.R. G.B. developed the Monte Carlo simulations for Fig. 2
and fig. S4. M.J.B. and J.S.J. wrote the first draft, and all
authors contributed to the interpretation and writing of the
paper. This work forms part of the British Antarctic Survey
program “Polar Science for Planet Earth,” funded by the
Natural Environment Research Council, and was made
possible by a Marie Tharp Fellowship in Earth, Environmental,
and Ocean Sciences at Columbia University Earth Institute/
Lamont-Doherty Earth Observatory, awarded to J.S.J. The
fieldwork was supported by the research program PACES, Topic
3 “Lessons from the Past” of the Alfred Wegener Institute.
This is LDEO publication 7577.
Materials and Methods
Figs. S1 to S5
Tables S1 to S3
18 October 2013; accepted 5 February 2014
Published online 20 February 2014;