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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 ∼320 m above the present ice surface. Subsequent thinning mostly occurred after ∼14 kyr BP, and modern ice levels were established some time after ∼4 kyr BP. We infer that, like at other flank sites, these changes were primarily controlled by the position of the grounding line downstream. At the Whitmore Mountains, cosmogenic 14C concentrations in bedrock surfaces demonstrate that ice there was no more than ∼190 m thicker than present during the past ∼30 kyr. Combined with other constraints from West Antarctica, the 14C data imply that the divide was thicker than present for a period of less than ∼8 kyr within the past ∼15 kyr. These results are consistent with the hypothesis that the divide initially thickened due to the deglacial rise in snowfall and subsequently thinned in response to retreat of the ice-sheet margin. We use these data to evaluate several recently published ice-sheet models at the Pirrit Hills and Whitmore Mountains. Most of the models we consider do not match the observed timing and/or magnitude of thickness change at these sites. However, one model performs relatively well at both sites, which may, in part, be due to the fact that it was calibrated with geological observations of ice-thickness change from other sites in Antarctica. [ABSTRACT FROM AUTHOR]

We measured concentrations of Ne isotopes in quartz in a 27.6-m sandstone core from a low-erosion-rate site at 2183 m elevation at Beacon Heights in the Antarctic Dry Valleys. Surface concentrations of cosmogenic 21Ne indicate a surface exposure age of at least 4.1 Ma and an erosion rate no higher than ca. 14 cm Myr−121Ne concentrations in the upper few centimeters of the core show evidence for secondary spallogenic neutron escape effects at the rock surface, which is predicted by first-principles models of cosmogenic-nuclide production but is not commonly observed in natural examples. We used a model for 21Ne production by various mechanisms fit to the observations to distinguish cosmic-ray-produced 21Ne from nucleogenic 21Ne produced by decay of trace U and Th present in quartz, and also constrain rates of subsurface 21Ne production by cosmic-ray muons. Core samples have a quartz (U-Th)/Ne closure age, reflecting cooling below approximately 95 °C, near 160 Ma, which is consistent with existing apatite fission-track data and the 183 Ma emplacement of nearby Ferrar dolerite intrusions. Constraints on 21Ne production by muons derived from model fitting are consistent with a previously proposed value of 0.79 mb at 190 GeV for the cross-section for 21Ne production by fast muon interactions, but indicate that 21Ne production by negative muon capture is likely negligible. [ABSTRACT FROM AUTHOR]

The Holocene history of outlet glaciers affords information on the behavior and mechanisms controlling the extent of the East Antarctic Ice Sheet. Here, we present both new radiocarbon and recalculations of previously published cosmogenic exposure-age data that constrain Holocene ice dynamics along upper Reedy Glacier in the southernmost Transantarctic Mountains. Ice remained at or close to its last glacial maximum position until the early Holocene, at which time it underwent thinning. A period of apparent relative stability in the mid-Holocene led to the formation of ice-dammed proglacial ponds, as well as of moraines located roughly two-thirds of the distance from the maximum position to the present-day ice margin. Renewed thinning began after 3600 yr BP, with ice reaching present-day levels by 2400 yr BP. Ice variations along upper Reedy Glacier likely reflect the balance between upstream propagation of mechanical thinning events at the glacier mouth and regional accumulation changes. [ABSTRACT FROM AUTHOR]

Many glacial deposits in the Quartermain Mountains, Antarctica present two apparent contradictions regarding the degradation of unconsolidated deposits. The glacial deposits are up to millions of years old, yet they have maintained their meter-scale morphology despite the fact that bedrock and regolith erosion rates in the Quartermain Mountains have been measured at 0·1-4·0 m Ma. Additionally, ground ice persists in some Miocene-aged soils in the Quartermain Mountains even though modeled and measured sublimation rates of ice in Antarctic soils suggest that without any recharge mechanisms ground ice should sublimate in the upper few meters of soil on the order of 10 to 10 years. This paper presents results from using the concentration of cosmogenic nuclides beryllium-10 (Be) and aluminum-26 (Al) in bulk sediment samples from depth profiles of three glacial deposits in the Quartermain Mountains. The measured nuclide concentrations are lower than expected for the known ages of the deposits, erosion alone does not always explain these concentrations, and deflation of the tills by the sublimation of ice coupled with erosion of the overlying till can explain some of the nuclide concentration profiles. The degradation rates that best match the data range 0·7-12 m Ma for sublimation of ice with initial debris concentrations ranging 12-45% and erosion of the overlying till at rates of 0·4-1·2 m Ma. Overturning of the tills by cryoturbation, vertical mixing, or soil creep is not indicated by the cosmogenic nuclide profiles, and degradation appears to be limited to within a few centimeters of the surface. Erosion of these tills without vertical mixing may partially explain how some glacial deposits in the Quartermain Mountains maintain their morphology and contain ground ice close to the surface for millions of years. Copyright © 2010 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Abstract: Fresh deposits above the margins of Reedy Glacier show that maximum ice levels during the last glaciation were several hundred meters above present near the glacier mouth and converged to less than 60 m above the present-day surface at the head of the glacier. Exposure ages of samples from five sites along its margin show that Reedy Glacier and its tributaries thickened asynchronously between 17 and 7 kyr BP At the Quartz Hills, located midway along the glacier, maximum ice levels were reached during the period 17–14 kyr BP. Farther up-glacier the ice surface reached its maximum elevation more recently: 14.7–10.2 kyr BP at the Caloplaca Hills; 9.1–7.7 kyr BP at Mims Spur; and around 7 kyr BP at Hatcher Bluffs. We attribute this time-transgressive behavior to two different processes: At the glacier mouth, growth of grounded ice and subsequent deglaciation in the Ross Sea embayment caused a wave of thickening and then thinning to propagate up-glacier. During the Lateglacial and Holocene, increased snow accumulation on the East Antarctic Ice Sheet caused transient thickening at the head of the glacier. An important result of this work is that moraines deposited along Reedy Glacier during the last ice age cannot be correlated to reconstruct a single glacial maximum longitudinal profile. The profile steepened during deglaciation of the Ross Sea, thinning at the Quartz Hills after 13 kyr BP while thickening upstream. Near its confluence with Mercer Ice Stream, rapid thinning beginning prior to 7–8 kyr BP reduced the level of Reedy Glacier to close to its present level. Thinning over the past 1000 years has lowered the glacier by less than 20 m. [Copyright &y& Elsevier]

Surface exposure ages of glacial deposits in the Ford Ranges of western Marie Byrd Land indicate continuous thinning of the West Antarctic Ice Sheet by more than 700 meters near the coast throughout the past 10,000 years. Deglaciation Lagged the disappearance of ice sheets in the Northern Hemisphere by thousands of years and may still be under way. These results provide further evidence that parts of the West Antarctic Ice Sheet are on a long-term trajectory of decline. West Antarctic melting contributed water to the oceans in the late Holocene and may continue to do so in the future. [ABSTRACT FROM AUTHOR]

Abstract: Glacial deposits preserved adjacent to Scott Glacier, southern Transantarctic Mountains, provide a record of past fluctuations in the thickness of the West Antarctic Ice Sheet. Geologic mapping of these deposits, in conjunction with emerging 10Be surface-exposure data, indicate that the most recent expansion of Scott Glacier occurred during the last glacial maximum in response to grounding of ice in the Ross Sea Embayment. At that time, the ice surface at the confluence of Scott Glacier and the West Antarctic Ice Sheet lay at ∼1100 m elevation. While this ice-surface reconstruction is in accord with other geologic estimates from throughout the Ross Sea Embayment, it contrasts with most computer-based simulations, which tend to overestimate former ice thickness in the southern Ross Sea. Together with recently modelled estimates of Antarctica''s contribution to sea level, this finding calls into question an Antarctic source for meltwater pulse 1A. [Copyright &y& Elsevier]

Abstract: Deposits corresponding to multiple periods of glaciation are preserved in ice-free areas adjacent to Reedy Glacier, southern Transantarctic Mountains. Glacial geologic mapping, supported by 10Be surface-exposure dating, shows that Reedy Glacier was significantly thicker than today multiple times during the mid-to-late Cenozoic. Longitudinal-surface profiles reconstructed from the upper limits of deposits indicate greater thickening at the glacier mouth than at the head during these episodes, indicating that Reedy Glacier responded primarily to changes in the thickness of the West Antarctic Ice Sheet. Surface-exposure ages suggest this relationship has been in place since at least 5Ma. The last period of thickening of Reedy Glacier occurred during Marine Isotope Stage 2, at which time the glacier surface near its confluence with the West Antarctic Ice Sheet was at least 500m higher than today. [Copyright &y& Elsevier]

Abstract: In the coastal mountains of Marie Byrd Land, Antarctica, there is a juxtaposition of ice moulded landforms at lower altitudes and upstanding summits with weathered surfaces bearing tors. This paper uses geomorphological mapping and exposure dating to test two hypotheses commonly used to explain such a landscape contrast: either the pattern reflects contrasts in glacial erosion related to the basal thermal regime beneath a former ice sheet or it represents a periglacial trimline marking the upper limit of ice cover during the Last Glacial Maximum (LGM). Cosmogenic nuclide measurements on erratics show that the mountains were covered by ice on several occasions, most recently during the LGM. Similar measurements on bedrock show that fragile landforms on weathered surfaces survived many glaciations. Nuclide concentrations in bedrock indicate both that subglacial erosion is insignificant and that ice-free periods of periglacial weathering are longer at higher elevations. We conclude that the pattern of landscape modification reflects both topographically controlled contrasts in the basal thermal regime of overriding ice and the longer exposure to periglacial conditions at higher elevations. In addition, the combination of nuclide data and the wider pattern of striations shows that the LGM ice cover was thinner than predicted by ice sheet models assuming equilibrium conditions, that ice sheet thinning has occurred from 10,400 years ago to the present, and that regional flow by overriding ice has been replaced by radial ice flow from local glaciers centred on individual mountain massifs. [Copyright &y& Elsevier]

We analyze published cosmogenic ³He depth profiles through the till that covers relict glacier ice in Beacon Valley, Antarctica, in order to derive rigorous constraints on the till thickness history, and on the amount and rate of ice loss by sublimation. The till is a residue of debris-laden ice that sublimed. The ³He profiles show that the lower 80% of the till formed in the past 310-43 k.y. under sublimation rates averaging >7 m·m.y.-1 (meters per million years). Such rapid recent growth of the till contradicts previous interpretations that it is older than 8.1 Ma at an adjacent site, where it encloses volcanic ash of this age. We question whether the ash provides a valid age constraint for the ice. Cosmogenic nuclide analysis of the till where the ash was collected for dating should resolve this question. [ABSTRACT FROM AUTHOR]