We report cosmogenic-nuclide measurements from two isolated groups of nunataks in West Antarctica: the Pirrit Hills, located midway between the grounding line and the divide in the Weddell Sea sector, and the Whitmore Mountains, located along the Ross–Weddell divide. At the Pirrit Hills, evidence of glacial-stage ice cover extends
Our knowledge of past thickness changes of the West Antarctic Ice Sheet (WAIS) is largely derived from geologic evidence collected from the continental shelf seafloor and from sites near the margin of the present-day ice sheet. Less is known about changes in the high-elevation WAIS interior where outcropping mountains and thus geologic evidence are sparse. The only geological constraints come from exposure dating at the Ohio Range and Mt. Waesche (Fig.
Map of West Antarctica. Hillshade of ice-sheet surface topography
At the end of the last ice age in West Antarctica, the three processes that likely exerted the greatest influence on ice thickness were (i) the retreat of the margin, (ii) the increase in the accumulation rate, and (iii) warming of the ice surface
The combined effect of these processes can result in a complex history of ice-thickness change at a given site, with thickening and thinning potentially both occurring over the course of the deglaciation as the balance between the different processes shifts
Exposure ages from nunataks near the present-day WAIS margin indicate progressive surface lowering as the grounding line neared
The data we report in this paper are consistent with this hypothesis. We find that at the Pirrit Hills, the WAIS stood at a highstand early in the deglaciation and thinned monotonically through the Holocene, similar to previously published records from sites near the ice-sheet margin. In contrast, at the Whitmore Mountains the WAIS appears to have (i) been no thicker than present, and possibly thinner, during the LGM when snowfall rates were lowest and (ii) reached a highstand sometime in the last
The Pirrit Hills emerge from the WAIS at an elevation of
WorldView satellite imagery (copyright 2012 DigitalGlobe, Inc.) of the Pirrit Hills
Although the Pirrit Hills were carved by mountain glaciers, this is a relic alpine landscape unrelated to the present-day or Pleistocene WAIS. The glacial deposits establish that the ice sheet here was at least 320–340 m thicker than present at least once in the past. There is no evidence that ice has reached above the depositional limit, and the absence of any glacial debris on the Axtell bench, along with the difference in bedrock weathering between the bench and the ridge below, suggests that ice has not been more than
The northwest ridge of Mt. Seelig (the only site we visited in the Whitmore Mountains) divides two partially buried cirques and climbs from the ice-sheet surface at
Views facing south of Mts. Axtell and Tidd
Unlike the Pirrit Hills, we found no glacially transported cobbles or boulders perched on bedrock surfaces. Glacially transported rock is less likely here at the divide because there is little area from which to source debris. The only glacial deposit we found was a small patch (several square meters) of indurated and weathered till
At the Pirrit Hills, we sampled elevation transects of glacial deposits to determine the age and height of the most recent highstand and to chronicle the subsequent thinning (Figs.
On the northwest ridge of Mt. Seelig, where recent glacial deposits are absent, we collected an elevation transect of bedrock samples from stable surfaces to identify past highstands and compare exposure and ice cover at different altitudes. Despite targeting sites unlikely to have been snow covered in the past, in places the only exposed bedrock was located within meters of snowfields or the summit ice cap (e.g., Fig.
At both sites, we measured sample elevations using drift-corrected barometric measurements, calibrated with geodetic GPS measurements. Accuracy is estimated to be
To determine the history of exposure and ice cover on million-year timescales, we measured the long-lived cosmogenic nuclides
Quartz was separated from crushed rock samples, sieved to 0.25–0.5 mm, and purified using surfactant separation, flotation in heavy liquids, and repeated etching in 2 % HF. Beryllium was extracted from quartz aliquots at the University of Washington Cosmogenic Nuclide Lab by addition of
Uncertainties in
Quartz aliquots for
Repeat measurements of the CRONUS-A quartz standard
Some replicate
We compute production rates for
Glacial deposits at the Pirrit Hills have apparent exposure ages that range from 1 Myr to
The exposure age of a cobble from the depositional limit on Mt. Axtell,
Below this level, deposits are more sparse (Figs. S1 and S2), suggesting that thinning from the highstand occurred relatively rapidly and that samples were exposed in the ablation zone only briefly before being deposited. The thinning is constrained by only two samples from Mt. Tidd (Fig.
This result is similar to thinning chronologies from the Heritage Range and from the Pensacola Mountains, sites in the Weddell Sea sector that are more seaward than the Pirrit Hills (Fig.
At Mt. Seelig, four bedrock samples have
The other three samples with
While snow cover is the only simple explanation for
As discussed above, the
The results of this calculation can be explained graphically by considering a diagram like Fig.
Evaluation of ice-sheet models at the Pirrit Hills and Whitmore Mountains. Panels
The
At the WAIS Divide ice-core site, the accumulation rate was lowest during the LGM and then doubled to near-modern values between 18 and 15 kyr BP
At the Pirrit Hills, ice levels appear to have lowered monotonically following the LGM (Fig.
Figure
These considerations strongly suggest that the less-than-saturated
Thinning to the modern ice level at Mt. Seelig therefore could not have occurred before 7 kyr ago (i.e., before modern ice levels were reached on lower Reedy Glacier). If the two lowest samples emerged 7 kyr ago, their
These constraints demonstrate that the WAIS at the Whitmore Mountains was the same thickness or thinner than present prior to the most recent highstand and that this highstand was reached sometime in the last
Our data provide an opportunity to evaluate the performance of Antarctic ice-sheet models in the WAIS interior, where there are few other constraints on past ice thickness. We compare our results from the Pirrit Hills and the Whitmore Mountains to five thermomechanical ice-sheet models as well as the ICE-6G_C reconstruction of ice-sheet history. Two of the thermomechanical models
Figure
At the Pirrit Hills, the best-performing model is that of
At the Whitmore Mountains, two of the simulations are ruled out because they depict ice considerably more than 190 m thicker than at the present, which is the upper limit on the highstand imposed by the
The overall best-performing model at both the Pirrit Hills and the Whitmore Mountains is that of
We present cosmogenic-nuclide constraints on ice-thickness changes since the LGM from the Pirrit Hills and Whitmore Mountains, located on the flank and the divide of the WAIS, respectively. At the Pirrit Hills, monotonic thinning occurred after accumulation rates had risen from their ice-age low, implying that the dominant control on ice thickness was the retreat of the ice-sheet margin downstream. In contrast, at the Whitmore Mountains, the WAIS appears to have initially thickened following the LGM due to the increased snowfall, and it only thinned once the dynamic effects of margin retreat began to outpace the thickening from snowfall. We compare our ice-thickness constraints to several recently published models of the Antarctic ice sheet over the last deglaciation and find that while most of the models poorly capture the timing and/or magnitude of thickness changes at the Pirrit Hills and Whitmore Mountains, the model of
Sample information and cosmogenic-nuclide data are available in the ICE-D: ANTARCTICA database (
The supplement related to this article is available online at:
PS and JS conducted the fieldwork. BG made the carbon-14 measurements. PS made the beryllium-10 measurements, analyzed all data, and wrote the paper.
The authors declare that they have no conflict of interest.
Support for this work was provided by the United States Antarctic Program. Perry Spector received support from the NSF Graduate Research Fellowship Program. We thank Trevor Hillebrand, Mika Usher, Taryn Black, and Maurice Conway for assistance in the field; Kier Nichols for lab assistance; Greg Balco and Eric Steig for insightful discussions; and David Pollard, Torsten Albrecht, Jonathan Kingslake, and Michelle Tigchelaar for providing ice-sheet simulations. Geospatial support for this work was provided by the Polar Geospatial Center under NSF-OPP awards 1043681 and 1559691.
This research has been supported by the U.S. National Science Foundation (grant nos. 1142162 and 1341728).
This paper was edited by Pippa Whitehouse and reviewed by two anonymous referees.