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Geological records from the Antarctic margin offer direct evidence of environmental variability at high southern latitudes and provide insight regarding ice sheet sensitivity to past climate change. The early to mid-Miocene (23-14 Mya) is a compelling interval to study as global temperatures and atmospheric CO2 concentrations were similar to those projected for coming centuries. Importantly, this time interval includes the Miocene Climatic Optimum, a period of global warmth during which average surface temperatures were 3-4 °C higher than today. Miocene sediments in the ANDRILL-2A drill core from the Western Ross Sea, Antarctica, indicate that the Antarctic ice sheet (AIS) was highly variable through this key time interval. A multiproxy dataset derived from the core identifies four distinct environmental motifs based on changes in sedimentary facies, fossil assemblages, geochemistry, and paleotemperature. Four major disconformities in the drill core coincide with regional seismic discontinuities and reflect transient expansion of grounded ice across the Ross Sea. They correlate with major positive shifts in benthic oxygen isotope records and generally coincide with intervals when atmospheric CO2 concentrations were at or below preindustrial levels (∼280 ppm). Five intervals reflect ice sheet minima and air temperatures warm enough for substantial ice mass loss during episodes of high (∼500 ppm) atmospheric CO2 These new drill core data and associated ice sheet modeling experiments indicate that polar climate and the AIS were highly sensitive to relatively small changes in atmospheric CO2 during the early to mid-Miocene.

Atmospheric, oceanic, and subglacial forcing scenarios from the Sea-level Response to Ice Sheet Evolution (SeaRISE) project are applied to six three-dimensional thermomechanical ice-sheet models to assess Antarctic ice sheet sensitivity over a 500 year timescale and to inform future modeling and field studies. Results indicate (i) growth with warming, except within low-latitude basins (where inland thickening is outpaced by marginal thinning); (ii) mass loss with enhanced sliding (with basins dominated by high driving stresses affected more than basins with low-surface-slope streaming ice); and (iii) mass loss with enhanced ice shelf melting (with changes in West Antarctica dominating the signal due to its marine setting and extensive ice shelves; cf. minimal impact in the Terre Adelie, George V, Oates, and Victoria Land region of East Antarctica). Ice loss due to dynamic changes associated with enhanced sliding and/or sub-shelf melting exceeds the gain due to increased precipitation. Furthermore, differences in results between and within basins as well as the controlling impact of sub-shelf melting on ice dynamics highlight the need for improved understanding of basal conditions, grounding-zone processes, ocean-ice interactions, and the numerical representation of all three. Key Points Sensitivity study of Antarctica to atmospheric, oceanic and subglacial forcings Different sectors of Antarctica are vulnerable to the forcings Atmospheric forcing lead to a growth, but dynamic forcing lead to a mass loss ©2013. American Geophysical Union. All Rights Reserved.

Ten ice-sheet models are used to study sensitivity of the Greenland and Antarctic ice sheets to prescribed changes of surface mass balance, sub-ice-shelf melting and basal sliding. Results exhibit a large range in projected contributions to sea-level change. In most cases, the ice volume above flotation lost is linearly dependent on the strength of the forcing. Combinations of forcings can be closely approximated by linearly summing the contributions from single forcing experiments, suggesting that nonlinear feedbacks are modest. Our models indicate that Greenland is more sensitive than Antarctica to likely atmospheric changes in temperature and precipitation, while Antarctica is more sensitive to increased ice-shelf basal melting. An experiment approximating the Intergovernmental Panel on Climate Change's RCP8.5 scenario produces additional first-century contributions to sea level of 22.3 and 8.1 cm from Greenland and Antarctica, respectively, with a range among models of 62 and 14 cm, respectively. By 200 years, projections increase to 53.2 and 26.7 cm, respectively, with ranges of 79 and 43 cm. Linear interpolation of the sensitivity results closely approximates these projections, revealing the relative contributions of the individual forcings on the combined volume change and suggesting that total ice-sheet response to complicated forcings over 200 years can be linearized.

Retreat of the Antarctic ice sheet (AIS) is likely to be a major contributor to future sea-level rise (SLR). Current projections of SLR due to ice-sheet mass loss remain highly uncertain. Better understanding of how ice sheets respond to future climate forcing and variability is essential for assessing long-term risk of SLR. However, predictability of future climate is limited by uncertainties from emission scenarios, model structural differences, and internal climate variability (ICV) that is inherently generated within the fully coupled climate system. Among those uncertainties, the impact of ICV on the AIS changes has not been explicitly assessed. Here we quantify the effects of ICV on the AIS evolutions by employing climate fields from two large-ensemble experiments using the Community Earth System Model to force a three-dimensional ice-sheet model. We find that ICV of climate fields among ensemble members leads to significantly different AIS responses, and that most of the effect is due to atmospheric variability compared to oceanic. Our results show that ICV can cause about 0.08 m differences of AIS contribution to SLR by 2100 compared to the ensemble-mean AIS contribution of 0.38–0.45 m. Moreover, using ensemble-mean climate forcing fields as the forcing in an ice-sheet model significantly delays retreat of the West AIS for up to 20 years and significantly underestimates the AIS contribution to SLR by 0.07–0.11 m in 2100 and up to 0.34 m in the 2250's. This study highlights the need to take internal climate variability into account in assessing uncertainty associated with the AIS contribution in sea-level rise projections. [ABSTRACT FROM AUTHOR]

A gravitationally self-consistent, global sea level model with 3D viscoelastic Earth structure is interactively coupled to a 3D dynamic ice sheet model, and the coupled model is applied to simulate the evolution of ice cover, sea level changes, and solid Earth deformation over the last deglaciation, from 40 ka to the modern. The results show that incorporating lateral variations in Earth's structure across Antarctica yields local differences in the modeled ice history and introduces significant uncertainty in estimates of both relative sea level change and modern crustal motions through the last deglaciation. An analysis indicates that the contribution of glacial isostatic adjustment to modern records of sea level change and solid Earth deformation in regions of Antarctica underlain by low mantle viscosity may be more sensitive to ice loading during the late Holocene than across the last deglaciation. [ABSTRACT FROM AUTHOR]

The response of the Antarctic ice sheet (AIS) to changing climate forcings is an important driver of sea-level changes. Anthropogenic climate change may drive a sizeable AIS tipping point response with subsequent increases in coastal flooding risks. Many studies analyzing flood risks use simple models to project the future responses of AIS and its sea-level contributions. These analyses have provided important new insights, but they are often silent on the effects of potentially important processes such as Marine Ice Sheet Instability (MISI) or Marine Ice Cliff Instability (MICI). These approximations can be well justified and result in more parsimonious and transparent model structures. This raises the question of how this approximation impacts hindcasts and projections. Here, we calibrate a previously published and relatively simple AIS model, which neglects the effects of MICI and regional characteristics, using a combination of observational constraints and a Bayesian inversion method. Specifically, we approximate the effects of missing MICI by comparing our results to those from expert assessments with more realistic models and quantify the bias during the last interglacial when MICI may have been triggered. Our results suggest that the model can approximate the process of MISI and reproduce the projected median melt from some previous expert assessments in the year 2100. Yet, our mean hindcast is roughly 3/4 of the observed data during the last interglacial period and our mean projection is roughly 1/6 and 1/10 of the mean from a model accounting for MICI in the year 2100. These results suggest that missing MICI and/or regional characteristics can lead to a low-bias during warming period AIS melting and hence a potential low-bias in projected sea levels and flood risks. [ABSTRACT FROM AUTHOR]

Geological records from the Antarctic margin offer direct evidence of environmental variability at high southern latitudes and provide insight regarding ice sheet sensitivity to past climate change. The early to mid-Miocene (23-14 Mya) is a compelling interval to study as global temperatures and atmospheric CO2 concentrations were similar to those projected for coming centuries. Importantly, this time interval includes the Miocene Climatic Optimum, a period of global warmth during which average surface temperatures were 3-4 °C higher than today. Miocene sediments in the ANDRILL-2A drill core from the Western Ross Sea, Antarctica, indicate that the Antarctic ice sheet (AIS) was highly variable through this key time interval. A multiproxy dataset derived from the core identifies four distinct environmental motifs based on changes in sedimentary facies, fossil assemblages, geochemistry, and paleotemperature. Four major disconformities in the drill core coincide with regional seismic discontinuities and reflect transient expansion of grounded ice across the Ross Sea. They correlate with major positive shifts in benthic oxygen isotope records and generally coincide with intervals when atmospheric CO2 concentrations were at or below preindustrial levels (~280 ppm). Five intervals reflect ice sheet minima and air temperatures warm enough for substantial ice mass loss during episodes of high (~500 ppm) atmospheric CO2. These new drill core data and associated ice sheet modeling experiments indicate that polar climate and the AIS were highly sensitive to relatively small changes in atmospheric CO2 during the early to mid-Miocene. [ABSTRACT FROM AUTHOR]

Ocean-ice interactions have exerted primary control on the Antarctic Ice Sheet and parts of the Greenland Ice Sheet, and will continue to do so in the near future, especially through melting of ice shelves and calving cliffs. Retreat in response to increasing marine melting typically exhibits threshold behavior, with little change for forcing below the threshold but a rapid, possibly delayed shift to a reduced state once the threshold is exceeded. For Thwaites Glacier, West Antarctica, the threshold may already have been exceeded, although rapid change may be delayed by centuries, and the reduced state will likely involve loss of most of the West Antarctic Ice Sheet, causing >3 m of sea-level rise. Because of shortcomings in physical understanding and available data, uncertainty persists about this threshold and the subsequent rate of change. Although sea-level histories and physical understanding allow the possibility that ice-sheet response could be quite fast, no strong constraints are yet available on the worst-case scenario. Recent work also suggests that the Greenland and East Antarctic Ice Sheets share some of the same vulnerabilities to shrinkage from marine influence. [ABSTRACT FROM AUTHOR]

Geological data indicate that global mean sea level has fluctuated on 10 3 to 10 6 yr time scales during the last ∼25 million years, at times reaching 20 m or more above modern. If correct, this implies substantial variations in the size of the East Antarctic Ice Sheet (EAIS). However, most climate and ice sheet models have not been able to simulate significant EAIS retreat from continental size, given that atmospheric CO 2 levels were relatively low throughout this period. Here, we use a continental ice sheet model to show that mechanisms based on recent observations and analysis have the potential to resolve this model–data conflict. In response to atmospheric and ocean temperatures typical of past warm periods, floating ice shelves may be drastically reduced or removed completely by increased oceanic melting, and by hydrofracturing due to surface melt draining into crevasses. Ice at deep grounding lines may be weakened by hydrofracturing and reduced buttressing, and may fail structurally if stresses exceed the ice yield strength, producing rapid retreat. Incorporating these mechanisms in our ice-sheet model accelerates the expected collapse of the West Antarctic Ice Sheet to decadal time scales, and also causes retreat into major East Antarctic subglacial basins, producing ∼17 m global sea-level rise within a few thousand years. The mechanisms are highly parameterized and should be tested by further process studies. But if accurate, they offer one explanation for past sea-level high stands, and suggest that Antarctica may be more vulnerable to warm climates than in most previous studies. [ABSTRACT FROM AUTHOR]

Reconstructing the historical behaviour of the Antarctic Ice Sheet is essential for understanding and predicting climatic interactions, ice sheet sensitivities, and sea level contributions. Furthermore, to interpret such reconstructions with any degree of confidence, meaningful uncertainty estimates are required. Working toward this goal, this article presents results from a large-ensemble data-constrained study of Antarctic evolution over the Last Glacial cycle. Ice sheet chronologies have been generated using a 3-D glacial systems model that includes sheet/stream/shelf flow; a parametrized basal drag coefficient (accounting for sub-grid topographic roughness, sediment likelihood, and systematic model-to-observation thickness misfits); a sub-grid grounding-line flux parametrization; a visco-elastic bedrock response component; parametrized climate forcing; separate shelf and tidewater calving treatments; and a physically-motivated, empirically derived, sub-shelf melt component. There are 31 ensemble parameters to capture uncertainties in the glacial cycle climate, mass-balance processes, and ice dynamics. Once generated, the ensemble of model runs is constrained using a suite of observational data (including constraints on relative sea level, past ice thickness and grounding line retreat data-points, as well as present-day ice volumes and configuration) and an evaluation methodology that produces a misfit-to-observations score for each run. Assuming variants of a Gaussian error model, the scores are used to generate probability distributions for the past evolution of the ice sheet. In our ensemble-based analysis the Last Glacial Maximum (LGM) of Antarctic ice volume occurs at ∼25–24 ka. The LGM ice volume excess relative to present-day is likely between 5.6 and 14.3 m equivalent sea level (mESL), and with less confidence >10 mESL. There is little change in the grounded ice volume from 24 ka until 17–16 ka at which time widespread deglaciation commences. The Ross and Weddell Sea sectors are the largest sources of ice mass loss. The major period of grounding line retreat in the Weddell Sea sector, resulting in the present-day Ronne–Filchner shelf system, occurs after 10 ka; the grounding line in the Ross Sea sector start its major retreat phase after 12 ka. There is no significant contribution (<1.8 mESL) to meltwater pulse 1a. During the Eemian, the modelled Antarctic Ice Sheet has a minimal ice volume at 114 ka contributing ∼2 mESL relative to present-day. The mass loss is principally due to a major retreat of the grounding line in the Siple Coast sector of the Ross Sea. We present standard deviation plots of the ensemble results for present day and at the LGM. These plots highlight geographic areas that would most benefit from additional constraint and therefore offer priorities and guidance for future field campaigns. [ABSTRACT FROM AUTHOR]

Abstract: We present results from a three-dimensional ice sheet-shelf model of Antarctica, coupled to a gravitationally self-consistent global sea-level model that incorporates (Maxwell) viscoelastic deformation of the solid Earth. The coupled model captures complex post-glacial changes in sea level associated with the gravitational, deformational and rotational effects of the evolving surface mass (ice plus ocean) load over the global ocean, including at the grounding lines of marine-based ice. The simulations are initiated at 40 ka and we focus on ice distributions and sea levels from the Last Glacial Maximum to present. Our results extend and confirm the key conclusions of earlier work using a simplified, 1-D ice-sheet model, by demonstrating that the sea-level coupling has a significant stabilizing influence on marine ice-sheet grounding lines. The feedback of sea-level changes into the ice-sheet model acts to slow down the retreat and advance of the grounding line relative to simulations in which the full coupling is not incorporated. Differences in ice thickness between these simulations can reach ∼1 km close to the grounding line. Finally, we perform preliminary comparisons of our results to relative sea level (RSL) histories and GPS-derived present-day uplift rates at sites near the margins of Antarctica. We find that the coupling improves fits to uplift rates in several regions, and that the RSL predictions of the coupled model yield a fit to the observations that is comparable to recent, uncoupled simulations in which the underlying Earth model was varied to obtain a best-fit to the RSL histories. [Copyright &y& Elsevier]

Abstract: Marine Isotope Stage 31 (MIS-31) is one of the major interglacials of the early Pleistocene ~1.08 to 1.06Ma. Data from proximal sediment cores around several sectors of Antarctica indicate strong sea surface warming and ice shelf and sea ice retreat. Benthic deep-sea-core δ18O values at this time are some of the lowest of the Pleistocene, indicating both deep sea warming and reduced global ice volume. A coeval alignment of orbital parameters produces one of the strongest high-latitude summer insolation anomalies of the last several million years. Here we use a 3-D ice sheet-shelf model to simulate the evolution of Antarctic ice sheets through the event, and a global climate model to simulate temperatures and sea ice during peak Antarctic warmth. The ice model predicts nearly complete collapse and subsequent recovery of marine ice in West Antarctica, and the ice and climate model results agree well with proximal sediment core data in the Ross Embayment recovered by the ANDRILL and Cape Roberts drilling projects. The dominant forcing is found to be variations in sub-ice-shelf oceanic melting, with insignificant surface melting of terrestrial ice flanks even during peak warmth. Implications are noted in light of other observations and theories of Pliocene–Pleistocene Antarctic ice sheet variability that do involve surface melt. [Copyright &y& Elsevier]

Abstract: Marine Isotope Stage 31 (MIS-31) is one of the major interglacials of the early Pleistocene ~1.08 to 1.06Ma. Data from proximal sediment cores around several sectors of Antarctica indicate strong sea surface warming and ice shelf and sea ice retreat. Benthic deep-sea-core δ18O values at this time are some of the lowest of the Pleistocene, indicating both deep sea warming and reduced global ice volume. A coeval alignment of orbital parameters produces one of the strongest high-latitude summer insolation anomalies of the last several million years. Here we use a 3-D ice sheet-shelf model to simulate the evolution of Antarctic ice sheets through the event, and a global climate model to simulate temperatures and sea ice during peak Antarctic warmth. The ice model predicts nearly complete collapse and subsequent recovery of marine ice in West Antarctica, and the ice and climate model results agree well with proximal sediment core data in the Ross Embayment recovered by the ANDRILL and Cape Roberts drilling projects. The dominant forcing is found to be variations in sub-ice-shelf oceanic melting, with insignificant surface melting of terrestrial ice flanks even during peak warmth. Implications are noted in light of other observations and theories of Pliocene–Pleistocene Antarctic ice sheet variability that do involve surface melt. [Copyright &y& Elsevier]

Abstract: The sensitivity of long-term Cenozoic variations of the East Antarctic ice sheet to geothermal heat flux is investigated, using a coupled climate–ice sheet model with various prescribed values and patterns of geothermal heat flux. The sudden growth of major ice across the Eocene–Oligocene boundary (∼34 Ma) is used as a test bed for this sensitivity. A suite of several million year-long simulations spanning the transition is performed, with various geothermal heat flux magnitudes and spatial distributions reflecting current uncertainty. The climate–ice sheet model simulates the Eocene–Oligocene transition realistically as a non-linear ice-sheet response to orbital perturbations and a long-term gradual decline of atmospheric CO2. It is found that reasonable variations of geothermal heat flux have very little effect on overall ice volumes and extents, and on the timing of major ice transitions. However, they cause large changes in basal areas at the pressure melting point at a given time, which could strongly influence other aspects of Cenozoic Antarctic evolution such as basal hydrology, sediment deformation and discharge, subglacial lakes, and basal erosional forms. [Copyright &y& Elsevier]

Abstract: A coupled global climate–Antarctic ice sheet model is run for 10 million years across the Eocene–Oligocene boundary ∼34 Ma. The model simulates a rapid transition from very little ice to a large continental ice sheet, forced by a gradual decline of atmospheric CO2 and higher-frequency orbital forcing. The structure of the transition is explained in terms of height mass balance feedback (HMBF) inherent in the intersection of the ice-sheet surface with the climatic pattern of net annual accumulation minus ablation, as found in earlier simple ice sheet models. Hysteresis effects are explored by running the model in reverse, starting with a full ice sheet and gradually increasing CO2. The effects of higher-frequency orbital forcing on the non-linear transitions are examined in simulations with and without orbital variability. Similar effects are demonstrated with a much simpler one-dimensional ice-sheet flowline model with idealized bedrock topography and parameterized mass balance forcing. It is suggested that non-linear Antarctic ice-sheet transitions and hysteresis have played important roles in many of the observed fluctuations in marine δ 18O records since 34 Ma, and that the range of atmospheric CO2 variability needed to induce these transitions in the presence of orbital forcing is ∼2× to 4× pre-industrial level. [Copyright &y& Elsevier]

A model of deforming sediment is added to a climate–ice sheet model, and applied to the Eocene–Oligocene transition in Antarctic ice volume around 34 Ma. The coupling between the global climate and ice sheet models is asynchronous, with a climate simulation performed once every 10 000 years, and only for the first 40 000 years. These global climate model solutions are re-used to perform runs of 400 000 years in length, with orbital forcing and different levels of CO2. The sediment model includes bulk transport under ice, generation of sub-ice till, and river transport, and predicts the continental-scale evolution of sediment thickness and coastal discharge. For the first few 10 000’s of years after the onset of substantial ice (triggered by CO2 falling below ∼3× pre-industrial level), sediment discharge to the ocean is relatively uniform around the coast, derived from nearby pre-existing regolith. After that, most discharge is concentrated into ∼4 sites at the mouths of major trough systems, with orbitally paced pulses on 104-yr scales. These sites and the average magnitudes of the fluxes agree generally with those deduced from offshore seismic and core data, although longer (10 my) integrations are needed to model the real sediment evolution. [Copyright &y& Elsevier]

The sudden, widespread glaciation of Antarctica and the associated shift toward colder temperatures near the Eocene–Oligocene boundary (∼34 Ma) represents one of the most fundamental reorganizations of the global climate system recognized in the geologic record. This glacial inception and the subsequent evolution of the early East Antarctic Ice Sheet (EAIS) are simulated using a new, coupled global climate–dynamical ice sheet model accounting for the paleogeography, greenhouse gas concentrations, changing orbital parameters, and varying ocean heat transport. Suites of long (105 yr) climate–ice sheet simulations are used to investigate the effects of declining atmospheric CO2, compared to those of the tectonic opening of Southern Ocean gateways and the timing of mountain uplift in the Antarctic interior. In contrast to the established paradigm for the glaciation of Antarctica, which centers on the opening of the Southern Ocean gateways and the ‘thermal isolation’ of the continent, our results show that declining Cenozoic pCO2 may have played the dominant role. First, small isolated ice caps formed on the highest Antarctic plateaus. Then, as a CO2 threshold between ∼3× and 2× pre-industrial level (PAL) was crossed, height–mass balance feedbacks were initiated during orbital periods with cold austral summers, triggering much larger, highly dynamic terrestrial ice sheets. As CO2 continued to decline, these isolated ice caps eventually merged into a permanent continental-scale EAIS. In our model, neither the opening of the Southern Ocean gateways nor mountain uplift significantly affected the timing of the major ice sheet transition, given a scenario of gradually declining CO2 from 4× to 2× PAL over 10 million years around the Eocene–Oligocene boundary. [Copyright &y& Elsevier]

The Antarctic ice sheet – storing ∼27 million cubic kilometres of ice – has the potential to contribute greatly to future sea level rise; yet its past evolution and sensitivity to long-term climatic drivers remain poorly understood and constrained. In particular, a long-standing debate questions whether Antarctic climate and ice volume respond mostly to changes in global sea level and atmospheric greenhouse gas concentrations or to local insolation changes. So far, long-term Antarctic simulations have used proxy-based parameterizations of climatic drivers, presuming that external forcings are synchronous and spatially uniform. Here for the first time we use a transient, three-dimensional climate simulation over the last eight glacial cycles to drive an Antarctic ice sheet model. We show that the evolution of the Antarctic ice sheet was mostly driven by CO 2 and sea level forcing with a period of about 100,000 yr, synchronizing both hemispheres. However, on precessional time scales, local insolation forcing drives additional mass loss during periods of high sea level and CO 2 , enhancing the Antarctic interglacial and putting northern and southern ice sheet variability temporarily out of phase. In our simulations, partial collapses of the West Antarctic ice sheet during warm interglacials are only simulated with unrealistically large Southern Ocean subsurface warming exceeding ∼4 °C. Overall, our results further elucidate the complex interplay of global and local forcings in driving Late Quaternary Antarctic ice sheet evolution, and the unique and overlooked role of precession therein. [ABSTRACT FROM AUTHOR]

• Deglacial history of Pope Glacier reconstructed using surface exposure dating. • 560 m of early Holocene thinning at an average rate of 0.13 ± 0.09/0.04 m yr−1. • Thinning coincides with enhanced upwelling of warm water onto continental shelf. • Reduction in ice shelf buttressing may have triggered widespread glacier thinning. The Amundsen Sea sector of the Antarctic ice sheet presently dominates the contribution from Antarctica to sea level rise. Several large ice streams that currently drain the sector have experienced rapid flow acceleration, grounding line retreat and thinning during the past few decades. However, little is known of their longer-term – millennial-scale – retreat history, despite the reliance of several ice sheet and glacial-isostatic adjustment models on such data for improving sea level prediction from this critical region. This study investigates the timing and extent of surface lowering of one of those ice streams, Pope Glacier, since the Last Glacial Maximum (LGM), using glacial-geological evidence for former ice cover. We present a new deglacial chronology for the glacier, derived from surface exposure dating of glacially-deposited cobbles and ice-scoured bedrock from Mount Murphy and its surrounding peaks. Cosmogenic 10Be exposure ages from 44 erratic cobbles and 5 bedrock samples, and in situ 14C exposure ages from one erratic and 8 bedrock samples are predominantly in the range 5.5-16 ka. Although 10Be inheritance from prior exposure is prevalent in some erratics and probably all bedrock samples, none of the ages pre-date the LGM. From these results we infer that the surface of Pope Glacier lowered by 560 m during the early- to mid-Holocene (9-6 ka), at an average rate of 0.13 ± 0.09/0.04 m yr−1. The lowering coincided with a period of enhanced upwelling of warm Circumpolar Deep Water onto the continental shelf in the region. A reduction in buttressing − facilitated by such upwelling − by an ice shelf that is thought to have spanned the embayment until 10.6 cal kyr BP could have triggered simultaneous early Holocene thinning of Pope Glacier and glaciers elsewhere in the Amundsen Sea Embayment. [ABSTRACT FROM AUTHOR]