Polygonal terrain results from thermal contraction, is commonly found in periglacial environments, and serves as a valuable proxy for interpreting subsurface ice distribution and climate history on Earth and Mars. In this study, we investigate the morphology of polygons in Beacon Valley, Antarctica, to assess the relationship between polygon size and ice table depth and to evaluate whether existing models for ice-cemented soils apply to polygons overlying massive ice. Using high-resolution remote sensing imagery and lidar datasets, we mapped over 1500 polygons and compared their sizes to measured ice table depth. Statistical analyses indicate that polygons formed over massive ice are generally smaller than those formed on ice-cemented soils. This observation contradicts current models, which predict larger polygons on massive ice due to a reduction in stress through viscous flow. We propose that sediment within the glacial ice inhibits viscous deformation and lowers tensile strength, thereby reducing the size of the observed polygons. Our statistical analysis also reveals two distinct subpopulations of polygons, for both polygons on ice-cemented soil and massive ice, which we interpret as evidence of polygon time evolution. This observation is supported by a crack propagation model that illustrates how polygon size decreases with time. Our findings challenge prevailing models of polygon formation and evolution and offer a revised conceptual framework with implications for interpreting subsurface ice availability on both Earth and Mars.

This study describes 16 well-dated, terrestrial glacial sedimentary cycles deposited during astronomically paced climate cycles from the termination of the Miocene Climatic Optimum (MCO) through the middle Miocene Climate Transition (MMCT) (15.1−13.8 Ma) in the Friis Hills, Transantarctic Mountains, Antarctica. Three locations were continuously cored (79% recovery) to a maximum depth of 50.48 m through a succession of interbedded till sheets and fossil-bearing, fluvio-lacustrine sediments. A composite chronostratigraphic framework is presented for the cores based on the previous mapping, a seismic refraction survey that defines basin geometry, and a new, integrated age model based on paleomagnetic stratigraphy that is constrained by radioisotopic 40Ar/39Ar numeric ages on two newly identified silicic tephra. The paleoecologic and sedimentologic characteristics of organic-rich lithologies are relatively consistent up-section, which implies that successively younger interglacial deposits during the MMCT represented broadly similar environmental and climatic conditions. During these interglacials, the Friis Hills hinterland was likely ice-free. Major disconformities in the section suggest a transition to colder climates, and after ca. 14.6 Ma, thicker, more extensive and erosive ice cover occurred across the Friis Hills during glacial episodes. Diamictites in the upper three cycles suggest that climate cooled and became drier after ca. 14.2 Ma. However, cyclical retreat of the ice and a return to warm climate conditions during interglacials continued through ca. 13.9 Ma. These direct records reflect a highly variable East Antarctic Ice Sheet margin but show that the ice margin became progressively more extensive during successive glacial intervals, which is consistent with a cooling trend toward more glacial values in the far-field benthic foraminifera δ18O proxy ice volume and temperature record. Age constraints show that glacial-interglacial variability at the terrestrial margin of the East Antarctic Ice Sheet was primarily paced by astronomical precession (∼23 k.y.) through the onset of the MMCT (15−14.7 Ma). Precession-driven cycles are modulated by short-period (∼100 k.y.) eccentricity cycles. Intervals of maximum eccentricity (high seasonality) coincide with sedimentary cycles comprising thin diamictites and relatively thick interglacial sandstone and mudstone units. Intervals of minimum eccentricity (low seasonality) coincide with sedimentary cycles comprising thick diamictites and relatively thin interglacial sedimentary deposits. Major disconformities in the Friis Hills succession that span more than ∼100 k.y. reflect episodes of expansion of erosive ice across, and well beyond, the Transantarctic Mountains and coincide with nodes in eccentricity (∼400 k.y.). These relationships suggest that during relatively warm intervals in the middle Miocene, the East Antarctic Ice Sheet expanded and contracted over 100 k.y. cycles, while its margins continued to fluctuate at higher (∼23 k.y.) frequency. After 14.5 Ma, obliquity is the dominant frequency in δ18O records, marking a period during which large regions of the Antarctic Ice Sheet grounded in marine environments.

In the Ross Sea sector of Antarctica, periodic large-scale marine ice-sheet fluctuations since the mid-Miocene are recorded by drill core and seismic data, revealing a dynamic ice-sheet response to past increases in temperature and atmospheric CO2. In the adjacent, predominantly ice-free McMurdo Dry Valleys (MDVs), preserved terrestrial landscapes reflect persistent cold conditions and have been interpreted as indicators of a stable polar ice sheet, implying that the Antarctic Ice Sheet was largely insensitive during past warm periods. These disparate data-based perspectives highlight a long-standing debate around the past stability of the Antarctic Ice Sheet, with direct implications for the future ice-sheet response to ongoing climate warming. We reconcile marine records of dynamic ice-sheet behavior and episodic open-marine conditions with nearby ancient terrestrial landscapes recording consistent cold-polar conditions. Coupled ice-sheet and regional climate models nested at a high resolution are used to investigate surface temperatures in the MDVs during past warm periods. We find that high-elevation regions of the MDVs remain below freezing even when ice-free conditions prevail in the nearby Ross Sea. We compare observed landscapes with the spatial extent of modeled persistent cold conditions required for preservation of these ancient features, demonstrating that frozen MDVs landscapes could have coexisted with receded or collapsed ice sheets during past warm periods.

Antarctic ice sheet and climate evolution during the mid-Miocene has direct relevance for understanding ice sheet (in)stability and the long-term response to elevated atmospheric CO2 in the future. Geologic records reconstruct major fluctuations in the volume and extent of marine and terrestrial ice during the mid-Miocene, revealing a dynamic Antarctic ice-sheet response to past climatic variations. We use an ensemble of climate - ice sheet - vegetation model simulations spanning a range of CO2 concentrations, Transantarctic Mountain uplift scenarios, and glacial/interglacial climatic conditions to identify climate and ice-sheet conditions consistent with Antarctic mid-Miocene terrestrial and marine geological records. We explore climatic variability at both continental and regional scales, focusing specifically on Victoria Land and Wilkes Land Basin regions using a high-resolution nested climate model over these domains. We find that peak warmth during the Miocene Climate Optimum is characterized by a thick terrestrial ice sheet receded from the coastline under high CO2 concentrations. During the Middle Miocene Climate Transition, CO2 episodically dropped below a threshold value for marine-based ice expansion. Comparison of model results with geologic data support ongoing Transantarctic Mountain uplift throughout the mid-Miocene. Modeled ice sheet dynamics over the Wilkes Land Basin were highly sensitive to CO2 concentrations. This work provides a continental-wide context for localized geologic paleoclimate and vegetation records, integrating multiple datasets to reconstruct snapshots of ice sheet and climatic conditions during a pivotal period in Earth's history. (C) 2021 Elsevier B.V. All rights reserved.

Over the last 5 Million years, outstanding warm interglacial periods (i.e. ‘super-interglacials’) occurred under low atmospheric CO2 levels that may feature extensive Antarctica ice sheet collapse. Here, we focus on the extreme super-interglacial known as Marine Isotope Stage 31 (MIS31) that took place 1.072 million years ago and is the subject of intense debate. Our Southern Ocean organic biomarker based paleotemperature reconstructions show that the surface ocean was warmer by ~5 °C than today between 50 °S and the Antarctic ice margin. We used these ocean temperature records to constrain the climate and ice sheet simulations to explore the impact of ocean warming on the Antarctic ice sheets. Our results show that low amplitude short term oceanic modifications drove the collapse of the West Antarctic Ice Sheet (WAIS) and deflation of sectors of the East Antarctic Ice Sheet (EAIS) resulting in sustained sea-level rise of centimeters to decimeters per decade. We suggest the WAIS retreated because of anomalously high Southern Hemisphere insolation combined with the intrusion of Circumpolar Deep Water onto the continental shelf under poleward-intensified winds leading to a shorter sea ice season and ocean warming at the continental margin. Under this scenario, the extreme warming we observe likely reflects the extensively modified oceanic and hydrological circulation patterns following ice sheet collapse. Our work highlights the sensitivity of the Antarctic ice sheets to relatively minor oceanic and/or atmospheric perturbations that could be at play in the near future.

Over the last 5 million years, the Earth's climate has oscillated between warm (interglacial) and cold (glacial) states. Some particularly warm interglacial periods (i.e. 'super-interglacials') occurred under low atmospheric CO2 and may have featured extensive Antarctic ice sheet collapse. Here we focus on an extreme super-interglacial known as Marine Isotope Stage 31 (MIS31), between 1.085 and 1.055 million years ago and is the subject of intense discussion. We reconstructed the first Southern Ocean and Antarctic margin sea surface temperatures (SSTs) from organic biomarkers and used them to constrain numerical ice sheet-shelf simulations. Our SSTs indicate that the ocean was on average 5 degrees C (+/- 1.2 degrees C) warmer in summer than today between 50 degrees S and the Antarctic ice margin. Our most conservative ice sheet simulation indicates a complete collapse of the West Antarctic Ice Sheet (WAIS) with additional deflation of the East Antarctic Ice Sheet. We suggest the WAIS retreated because of anomalously high Southern Hemisphere insolation coupled with the intrusion of Circumpolar Deep Water onto the continental shelf under poleward-intensified winds leading to a shorter sea ice season and ocean warming at the continental margin. In this scenario, the extreme warming we observed likely reflects the extensively modified oceanic and hydrologic system following ice sheet collapse. Our work highlights the sensitivity of the Antarctic ice sheets to minor oceanic perturbations that could also be at play for future changes. (C) 2019 Elsevier Ltd. All rights reserved.

The geometry of Antarctic ice sheets during warm periods of the geological past is difficult to determine from geological evidence, but is important to know because such reconstructions enable a more complete understanding of how the ice-sheet system responds to changes in climate. Here we investigate how Antarctica evolved under orbital and greenhouse gas conditions representative of an interglacial in the early Pliocene at 4.23 Ma, when Southern Hemisphere insolation reached a maximum. Using offline-coupled climate and ice-sheet models, together with a new synthesis of high-latitude palaeoenvironmental proxy data to define a likely climate envelope, we simulate a range of ice-sheet geometries and calculate their likely contribution to sea level. In addition, we use these simulations to investigate the processes by which the West and East Antarctic ice sheets respond to environmental forcings and the timescales over which these behaviours manifest. We conclude that the Antarctic ice sheet contributed 8.6 ± 2.8 m to global sea level at this time, under an atmospheric CO2 concentration identical to present (400 ppm). Warmer-than-present ocean temperatures led to the collapse of West Antarctica over centuries, whereas higher air temperatures initiated surface melting in parts of East Antarctica that over one to two millennia led to lowering of the ice-sheet surface, flotation of grounded margins in some areas, and retreat of the ice sheet into the Wilkes Subglacial Basin. The results show that regional variations in climate, ice-sheet geometry, and topography produce long-term sea-level contributions that are non-linear with respect to the applied forcings, and which under certain conditions exhibit threshold behaviour associated with behavioural tipping points.

Atmospheric warming is projected to increase global mean surface temperatures by 0.3 to 4.8 degrees Celsius above pre-industrial values by the end of this century. If anthropogenic emissions continue unchecked, the warming increase may reach 8-10 degrees Celsius by 2300 (ref. 2). The contribution that large ice sheets will make to sea-level rise under such warming scenarios is difficult to quantify because the equilibrium-response timescale of ice sheets is longer than those of the atmosphere or ocean. Here we use a coupled ice-sheet/ice-shelf model to show that if atmospheric warming exceeds 1.5 to 2 degrees Celsius above present, collapse of the major Antarctic ice shelves triggers a centennial- to millennial-scale response of the Antarctic ice sheet in which enhanced viscous flow produces a long-term commitment (an unstoppable contribution) to sea-level rise. Our simulations represent the response of the present-day Antarctic ice-sheet system to the oceanic and climatic changes of four representative concentration pathways (RCPs) from the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. We find that substantial Antarctic ice loss can be prevented only by limiting greenhouse gas emissions to RCP 2.6 levels. Higher-emissions scenarios lead to ice loss from Antarctic that will raise sea level by 0.6-3 metres by the year 2300. Our results imply that greenhouse gas emissions in the next few decades will strongly influence the long-term contribution of the Antarctic ice sheet to global sea level.

ABSTRACT To assess the role of thermal contraction‐crack polygons (sublimation polygons) in modulating sublimation of buried glacier ice in Antarctica, we applied a 2D numerical model using COMSOL Multiphysics that calculates the rate and spatial variability of vapour diffusion through porous media. Specifically, we examined vapour transport through Granite drift, a dry supraglacial till marked with thermal contraction‐crack polygons that rests on glacier ice reportedly ≥8‐million years in age. The model results show that sublimation varies with drift texture and surface topography. Initially, the rates are highest beneath relatively coarse‐grained sand‐wedge deposits at polygon margins, creating deep, surface troughs. As troughs approach ~1‐m depth, the cooler atmospheric and soil temperatures that arise from solar shielding reduce the rates of ice sublimation to levels below that at polygon centres, preventing runaway ice loss at polygon margins. Including the effects of a salt‐cemented horizon at 10 − 15‐cm depth (porosity 20%) and a rocky surface pavement (75% ground coverage), our modelled ice loss at polygon centres, for example, is 0.022 mm a −1 , an order of magnitude lower than previous estimates (0.14 mm a −1 ). This finding highlights the importance of including field‐based data for drift texture, topography and microclimate variation in modelling ice sublimation. The results also suggest that stable conditions (no ice loss) at polygon centres are possible with either a 1.9°C decrease in mean annual atmospheric temperature or a 12 per cent increase in mean annual relative humidity. These results indicate that the preservation of buried, multi‐million‐year‐old ice is plausible in the coldest and driest regions of Antarctica. Copyright © 2011 John Wiley & Sons, Ltd.

Continental drift processes such as major gateway openings have been historically advocated to explain the distribution of marine benthic taxa in the Southern Ocean (SO). The separation between Antarctic Peninsula and the southern tip of South America together with the onset of the Antarctic Circumpolar Current (ACC) represent the final step for the complete isolation of the Antarctic region. However, there is still controversy concerning the timing and mode of this process, and especially about the role of the Scotia Arc geodynamics in the development of a fully deep and intensified ACC circulation. Based on mitochondrial Cytochrome c Oxidase Subunit I (COI) sequences obtained from different taxa, we performed molecular comparisons between Antarctic and South American relatives to provide independent time estimations of Antarctica's isolation. We include in the analyses congeneric Antarctic and Patagonian near-shore marine benthic invertebrates including indirect developers (Nacella, Yoldia, Sterechinus, and Parbolasia) and brooders (Xymenopsis and Trophonella). Considering the levels of genetic differentiation between relatives from both regions and assuming the molecular clock hypothesis, we estimated the onset of their respective divergence. On one hand, similar levels of genetic distance in broadcast–spawners (7%–8.3%) support the hypothesis that the development of an effective barrier between Antarctica and South America occurred almost simultaneously for these groups. Divergence time estimations based on specific substitution rates indicate that the separation occurred near the Mio-Pliocene transition, long after the physical separation of both continents. Genetic distance and divergence time estimation in direct developers indicate an older separation time, close to the mid-Miocene. Even when the analyzed groups included both broadcast–spawners and brooder organisms, the divergence between Antarctic and South America lineages rather than being related to processes of continental drift, seems to be associated more to major changes in the Southern Ocean such as the evolution of the Scotia Arc and the deepening of the Drake Passage. Accordingly, these results support a genetic continuity between Antarctica and South America, probably along the Scotia Ridge, until the middle Miocene and a late ACC intensification at the Mio-Pliocene boundary.

Marine Isotope Stage 31 (MIS-31) is one of the major interglacials of the early Pleistocene ~ 1.08 to 1.06 Ma. 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.