Ice Sheet Stability Research Articles

Grounding line transient response in marine ice sheet models

Abstract. Marine ice-sheet stability is mostly controlled by the dynamics of the grounding line, i.e. the junction between the grounded ice sheet and the floating ice shelf. Grounding line migration has been investigated within the framework of MISMIP (Marine Ice Sheet Model Intercomparison Project), which mainly aimed at investigating steady state solutions. Here we focus on transient behaviour, executing short-term simulations (200 yr) of a steady ice sheet perturbed by the release of the buttressing restraint exerted by the ice shelf on the grounded ice upstream. The transient grounding line behaviour of four different flowline ice-sheet models has been compared. The models differ in the physics implemented (full Stokes and shallow shelf approximation), the numerical approach, as well as the grounding line treatment. Their overall response to the loss of buttressing is found to be broadly consistent in terms of grounding line position, rate of surface elevation change and surface velocity. However, still small differences appear for these latter variables, and they can lead to large discrepancies (> 100%) observed in terms of ice sheet contribution to sea level when cumulated over time. Despite the recent important improvements of marine ice-sheet models in their ability to compute steady state configurations, our results question the capacity of these models to compute short-term reliable sea-level rise projections.

Sea‐surface hydrographical conditions off South Faeroes and within the North‐Eastern North Atlantic through MIS 2: the response of dinocysts

ABSTRACTThe last glacial period, showing the progressive development of large boreal ice sheets, was punctuated by large climatic excursions. These excursions were triggered mainly by atmosphere–ocean–ice coupled dynamics and are thus exemplary case studies of natural climate variability. To characterize the sea‐surface palaeohydrographical changes accompanying these oscillations along the European margin, we have integrated new palynological data (dinocysts) acquired on core MD99‐2281 (60°N, Faeroe Margin) during Marine Isotope Stage 2 in a latitudinal transect including published cores MD95‐2002 (47°N, Celtic Margin) and MD95‐2010 (66°N, Vöring Plateau). This transect is superimposed on the modern North Atlantic Drift pathway, but also at the outskirts of glacial European ice sheets, thus ideally located to track sea‐ice extent and ice‐sheet instabilities through time. The results show a coherent and sensitive response of sea‐surface environments to the complex chain of abrupt events that punctuated the end of the last glacial period. The Last Glacial Maximum was marked by large seasonal contrasts of temperatures, whereas Heinrich events (HE) were characterized by a sharp cooling and sea‐ice development. A tripartite structure is identified within HE1, with indices of melting at 19k cal a BP, followed by a temperate phase synchronous of a relative stability of ice sheets, and a terminal phase (17.5–15k cal a BP) characteristic of the ‘conventional Laurentian’ HE1. Copyright © 2013 John Wiley & Sons, Ltd.

A review of geomorphic processes and landforms in the Dry Valleys of southern Victoria Land: implications for evaluating climate change and ice-sheet stability

Abstract The Dry Valleys are subdivided into three microclimate zones on the basis of summertime measurements of atmospheric temperature, soil moisture, relative humidity and wind-speed/ direction. Subtle variations in these climate parameters result in considerable differences in process geomorphology and in the development of unique landforms within each zone. The mapped zones include a coastal thaw zone, an inland mixed zone and a stable upland zone. Landforms within each zone are subdivided into macroscale features (e.g. valleys, slopes and gullies), mesoscale features (e.g. polygons and viscous-flow features) and microscale features (e.g. rock and near-surface soil features, including the effects of salt weathering, wind erosion and pitting). We present a review of landscape development in the Dry Valleys with implications for long-term climate change and ice-sheet stability. Chronological control is afforded by 40 Ar/ 39 Ar dating of volcanic ash-fall deposits and cosmogenic nuclide analyses of surface boulders. Collectively, the data call for persistent cold and dry conditions in the stable upland zone for approximately the last 14 Ma, although some level of climatic amelioration and landform modification may have occurred within low-lying regions and in the inland mixed zone.

Using distributed temperature sensors to monitor an Antarctic ice shelf and sub-ice-shelf cavity

AbstractMonitoring of ice-shelf and sub-ice-shelf ocean temperatures represents an important component in understanding ice-sheet stability. Continuous monitoring is challenging due to difficult surface access, difficulties in penetrating the ice shelf, and the need for long-term operation of non-recoverable sensors. We aim to develop rapid lightweight drilling and near-continuous fiber-optic temperature-monitoring methods to meet these challenges. During November 2011, two instrumented moorings were installed within and below the McMurdo Ice Shelf (a sub-region of the Ross Ice Shelf, Antarctica) at Windless Bight. We used a combination of ice coring for the upper portion of each shelf borehole and hot-point drilling for penetration into the ocean. The boreholes provided temporary access to the ice-shelf cavity, into which distributed temperature sensing (DTS) fiber-optic cables and conventional pressure/temperature transducers were installed. The DTS moorings provided near-continuous (in time and depth) observations of ice and ocean temperatures to a depth of almost 800 m beneath the ice-shelf surface. Data received document the presence of near-freezing water throughout the cavity from November through January, followed by an influx of warmer water reaching ∼150 m beneath the ice-shelf base during February and March. The observations demonstrate prospects for achieving much higher spatial sampling of temperature than more conventional oceanographic moorings.

History of the grounded ice sheet in the Ross Sea sector of Antarctica during the Last Glacial Maximum and the last termination

Abstract Knowledge of variations in the extent and thickness of the Antarctic Ice Sheet is key for understanding the behaviour of Southern Hemisphere glaciers during the last ice age and for addressing issues involving global sea level, ocean circulation and climate change. Insight into past ice-sheet behaviour also will aid predictions of future ice-sheet stability. Here, we review terrestrial evidence for changes in ice geometry that occurred in the Ross Sea sector of Antarctica at the Last Glacial Maximum (LGM) and during subsequent deglaciation. During the LGM, a thick grounded ice sheet extended close to the continental shelf edge in the Ross Embayment. This ice reached surface elevations of more than 1000 m along the coast of the central and southern Transantarctic Mountains and Marie Byrd Land. The local LGM occurred by 18 ka on the coast, but as late as 7–10 ka inland. The first significant thinning took place at roughly 13 ka, with most ice loss happening in the Holocene. This history makes it unlikely that the Ross Sea sector was a major contributor to meltwater pulse 1A (MWP 1A). Resolution of a possible Antarctic origin for MWP 1A awaits detailed reconstructions from all sectors of the ice sheet.

Ice sheets viewed from the ocean: the contribution of marine science to understanding modern and past ice sheets

Over the last two decades, marine science, aided by technological advances in sediment coring, geophysical imaging and remotely operated submersibles, has played a major role in the investigation of contemporary and former ice sheets. Notable advances have been achieved with respect to reconstructing the extent and flow dynamics of the large polar ice sheets and their mid-latitude counterparts during the Quaternary from marine geophysical and geological records of landforms and sediments on glacier-influenced continental margins. Investigations of the deep-sea ice-rafted debris record have demonstrated that catastrophic collapse of large (10(5)-10(6) km(2)) ice-sheet drainage basins occurred on millennial and shorter time scales and had a major influence on oceanography. In the last few years, increasing emphasis has been placed on understanding physical processes at the ice-ocean interface, particularly at the grounding line, and on determining how these processes affect ice-sheet stability. This remains a major challenge, however, owing to the logistical constraints imposed by working in ice-infested polar waters and ice-shelf cavities. Furthermore, despite advances in reconstructing the Quaternary history of mid- and high-latitude ice sheets, major unanswered questions remain regarding West Antarctic ice-sheet stability, and the long-term offshore history of the East Antarctic and Greenland ice sheets remains poorly constrained. While these are major research frontiers in glaciology, and ones in which marine science has a pivotal role to play, realizing such future advances will require an integrated collaborative approach between oceanographers, glaciologists, marine geologists and numerical modellers.

The stability of grounding lines on retrograde slopes

Abstract. The stability of marine ice sheets grounded on beds that slope upwards in the overall direction of flow is investigated numerically in two horizontal dimensions. We give examples of stable grounding lines on such retrograde slopes illustrating that marine ice sheets are not unconditionally unstable in two horizontal dimensions. Retrograde bed slopes at the grounding lines of marine ice sheets, such as the West Antarctic Ice Sheet (WAIS), do not per se imply an instability, nor do they imply that these regions are close to a threshold of instability. We therefore question those estimates of the potential near-future contribution of WAIS to global sea level change based solely on the notion that WAIS, resting on a retrograde slope, must be inherently unstable.

Complex interaction of subglacial hydrology and deforming bed processes and its implications for ice marginal stability: the example of the Middle Pleistocene British Ice Sheet from eastern England

Subglacial drainage is known to have a profound influence on glacier and ice sheet stability. Many scientists consider that such processes acted to cause abrupt climatic shifts in the North Atlantic region during Termination 1, and believe that meltwater-induced instabilities in the modern-day Greenland and Antarctic ice sheets could have similar climatic impacts in the future. Understanding the role played by porewater content and/or pressure, coupled with its temporal and spatial distribution within the subglacial bed, appear to be critical factors. These act to control the rheology and style of deformation within the subglacial deforming bed, and in-turn, the amount and distribution of ‘stick-slip’ at the ice-bed interface which facilitates ice motion. Within this paper, we present a model based upon sedimentological and structural evidence from Middle Pleistocene glacial deposits in Eastern England, which illustrate the symbiotic relationship between subglacial deforming bed processes, subglacial drainage within a pressurised channel system, and ice advance. The model shows the development of large-scale subglacial folds and thrusts within a subglacial melange that relate to temporal changes in porewater content and/or pressure within the subglacial bed. These acted to control the development and stability of subglacial drainage beneath the margins of the British Ice Sheet, ultimately leading to an increase in the efficiency of subglacial drainage and the subsequent collapse of the subglacial shear zone. This potentially slowed or even stopped ice advance leading to the part-collapse of the ice margin.

Pliocene Ice Sheet Modelling Intercomparison Project (PLISMIP) – experimental design

Abstract. During the mid-Pliocene warm period (3.264 to 3.025 million years ago), global mean temperature was similar to that predicted for the next century and atmospheric carbon dioxide concentrations were slightly higher than today. Sea level was also higher than today, implying a reduction in the extent of the ice sheets. Thus, the mid-Pliocene warm period (mPWP) provides a unique testing ground to investigate the stability of the Earth's ice sheets and their contribution to sea level in a warmer-than-modern world. Climate models and ice sheet models can be used to enhance our understanding of ice sheet stability; however, uncertainties associated with different ice-sheet modelling frameworks mean that a rigorous comparison of numerical ice sheet model simulations for the Pliocene is essential. As an extension to the Pliocene Model Intercomparison Project (PlioMIP; Haywood et al., 2010, 2011a), the Pliocene Ice Sheet Modelling Intercomparison Project (PLISMIP) will provide the first assessment as to the ice sheet model dependency of ice sheet predictions for the mPWP. Here we outline the PLISMIP experimental design and initialisation conditions that have been adopted to simulate the Greenland and Antarctic ice sheets under present-day and warm mid-Pliocene conditions. Not only will this project provide a new benchmark in the simulation of ice sheets in a past warm period, but the analysis of model sensitivity to various uncertainties could directly inform future predictions of ice sheet and sea level change.

Eccentricity-paced late Paleozoic climate change

Cyclic sedimentary deposits characterize low-latitude late Paleozoic successions and preserve evidence of dynamic climate change on the Pangaean supercontinent. Although their orbitally paced glacioeustatic origins are widely accepted, their climatic signatures are open to interpretation. In this study, we utilize the GENESIS general circulation model (GCM) coupled to dynamic ice sheet and ecosystem components, to explore the response of low-latitude continental climate and high-latitude ice sheets to orbital and atmospheric pCO2 forcing. Our results suggest that atmospheric pCO2 concentration exerts the primary control over low-latitude continental climate and high-latitude glaciation. Our experiments constrain the atmospheric pCO2 window within which late Paleozoic climate was amenable to orbitally-driven glacial-interglacial fluctuations. The results suggest that both high-latitude ice-sheet accumulation and ablation and low-latitude climate change were paced by the eccentricity of Earth's orbit. Periods of high eccentricity amplified precession-driven changes in insolation and promoted high-latitude ice sheet volume fluctuations as well as increased low-latitude precipitation variability. When eccentricity was low, the amplification of precessionally-driven insolation fluctuations was reduced, which promoted high-latitude continental ice sheet stability and less variable low-latitude precipitation. Based on these modeling results we discuss the implications of eccentricity paced precessional-scale climatic changes on low-latitude Pangaean depositional environments.

Marine ice sheet stability

AbstractWe examine the stability of two-dimensional marine ice sheets in steady state. The dynamics of marine ice sheets is described by a viscous thin-film model with two Stefan-type boundary conditions at the moving boundary or ‘grounding line’ that marks the transition from grounded to floating ice. One of these boundary conditions constrains ice thickness to be at a local critical value for flotation, which depends on depth to bedrock at the grounding line. The other condition sets ice flux as a function of ice thickness at the grounding line. Depending on the shape of the bedrock, multiple equilibria may be possible. Using a linear stability analysis, we confirm a long-standing heuristic argument that asserts that the stability of these equilibria is determined by a simple mass balance consideration. If an advance in the grounding line away from its steady-state position leads to a net mass gain, the steady state is unstable, and stable otherwise. This also confirms that grounding lines can only be stable in positions where bedrock slopes downwards sufficiently steeply.

Evolution of a coupled marine ice sheet–sea level model

We investigate the stability of marine ice sheets by coupling a gravitationally self‐consistent sea level model valid for a self‐gravitating, viscoelastically deforming Earth to a 1‐D marine ice sheet‐shelf model. The evolution of the coupled model is explored for a suite of simulations in which we vary the bed slope and the forcing that initiates retreat. We find that the sea level fall at the grounding line associated with a retreating ice sheet acts to slow the retreat; in simulations with shallow reversed bed slopes and/or small external forcing, the drop in sea level can be sufficient to halt the retreat. The rate of sea level change at the grounding line has an elastic component due to ongoing changes in ice sheet geometry, and a viscous component due to past ice and ocean load changes. When the ice sheet model is forced from steady state, on short timescales (<∼500 years), viscous effects may be ignored and grounding‐line migration at a given time will depend on the local bedrock topography and on contemporaneous sea level changes driven by ongoing ice sheet mass flux. On longer timescales, an accurate assessment of the present stability of a marine ice sheet requires knowledge of its past evolution.

The deglacial history of NW Alexander Island, Antarctica, from surface exposure dating

Recent changes along the margins of the Antarctic Peninsula, such as the collapse of the Wilkins Ice Shelf, have highlighted the effects of climatic warming on the Antarctic Peninsula Ice Sheet (APIS). However, such changes must be viewed in a long-term (millennial-scale) context if we are to understand their significance for future stability of the Antarctic ice sheets. To address this, we present nine new cosmogenic 10Be exposure ages from sites on NW Alexander Island and Rothschild Island (adjacent to the Wilkins Ice Shelf) that provide constraints on the timing of thinning of the Alexander Island ice cap since the last glacial maximum. All but one of the 10Be ages are in the range 10.2–21.7ka, showing a general trend of progressive ice-sheet thinning since at least 22ka until 10ka. The data also provide a minimum estimate (490m) for ice-cap thickness on NW Alexander Island at the last glacial maximum. Cosmogenic 3He ages from a rare occurrence of mantle xenoliths on Rothschild Island yield variable ages up to 46ka, probably reflecting exhumation by periglacial processes.

Geometry and dynamics of an East Antarctic Ice Sheet outlet glacier, under past and present climates

[1] The behavior of East Antarctic Ice Sheet (EAIS) outlet glaciers both at present and during the recent geological past is of considerable interest both from the point of view of understanding the mechanics of contemporary glacier dynamics, and also with regard to epoch-scale ice sheet stability during Plio-Pleistocene climate transitions. Here we use a glacier flowline model that incorporates the effects of longitudinal stresses to numerically simulate Ferrar Glacier, first under present-day environmental conditions, and subsequently under both colder and warmer climate regimes representing the Last Glacial Maximum (LGM) and mid-Pliocene peak warmth respectively. Using airborne radar profiles, InSAR-derived surface velocities, ice core and geological data for empirical constraint, we present a diagnostic simulation that uses an iterative method to closely reproduce observed dynamics. Our model suggests that the glacier is largely cold-based under present conditions, flows predominantly by way of internal deformation, and ‘cascades’ over bedrock ridges due to the combined action of changes in cross-sectional valley geometry, local steepening of the glacier surface, and the non-local effects of longitudinal coupling. Time-dependent (evolutionary) simulation of a lower-profile glacier under a colder, drier, LGM climate, predicts flow velocities lower than present with minimal bedrock erosion or basal till flux. Conversely, the warmer-than-present mid-Pliocene climate produces a more dynamic glacier that is warm-based and sliding along much of its bed. We propose that EAIS outlet glaciers, such as the Ferrar, respond dynamically along their length in response to changing environmental forcings, with most significant changes taking place in their lower reaches. Adjustment to perturbations in upper catchments is more muted.

A Reason to Surge

Climate Science Heinrich events—massive, episodic iceberg discharges from the Laurentide Ice Sheet into the north Atlantic Ocean during the last glacial period—once were thought to be the consequence of internal ice sheet stability: the inability of the ice sheet to remain intact after having grown to a particularly large size. Support for this view has eroded over the past decade, however, and the suggestion that the events are instead the consequence of external causes, such as solar variability or the addition of glacial meltwater into the adjacent surface ocean, have gained favor. Marcott et al. add to that line of thought, with a combination of data from benthic foraminifera and results from a climate-model simulation, which suggest that basin-wide subsurface warming occurred at the same time as large reductions in Atlantic deep circulation, with subsurface temperature increasing by approximately 2° C during the 1000- to 2000-year interval before a Heinrich event. Such a temperature rise would erode the floating ice shelves that buttress terrestrial ice streams, and produce massive iceberg discharges. This finding could have important implications about how these ice sheets might behave in the future in our warming climate. Proc. Natl. Acad. Sci. U.S.A. 108 , 10.1073/pnas.1104772108 (2011).

The Mid‐Brunhes Event and West Antarctic ice sheet stability

AbstractThe complex cyclical nature of Pleistocene climate, driven by the evolving orbital configuration of the Earth, is well known but not well understood. A major climatic transition took place at the Mid‐Brunhes Event (MBE), ca. 430 ka ago after which the amplitude of the ca.100 ka climate oscillations increased, with substantially warmer interglacials, including periods warmer than present. Recent modelling has indicated that while the timing of these warmer‐than‐present transient (WPT) events is consistent with southern warming due to a deglaciation‐forced slowdown of the Atlantic Meridional Overturning Circulation, the magnitude of warming requires a local amplification, for which a candidate is the feedback of significant West Antarctic Ice Sheet (WAIS) retreat. We here extend this argument, based on the absence of WPTs in the early ice core record (450–800 ka ago), to hypothesize that the MBE could be a manifestation of decreased WAIS stability, triggered by ongoing subglacial erosion. Copyright © 2011 John Wiley & Sons, Ltd.

Assessing the catastrophic break-up of Briksdalsbreen, Norway, associated with rapid climate change

Abstract: Recent research has raised concerns about the potential influence of rapid climate change on the stability of major ice sheets. The behaviour of glaciers is determined largely by the processes and conditions operating at their base. Technological advances have allowed these factors to be examined and their contribution to ice flow constrained. This study investigated the rapid disintegration of an aquatic based Norwegian glacier, through the study of boreholes, video, ground-penetrating radar, differential global positioning system, bathymetry and Glacsweb wireless probes. Briksdalsbreen retreated dramatically between 2000 and 2007, with c . 56 × 10 5 m 3 of ice lost from the glacier tongue, equivalent to a rate of 70 m a −1 . This was due to the combined effect of higher summer temperatures, decreased precipitation (resulting from a negative phase of the North Atlantic Oscillation) and increased fracturing of the glacier tongue. The enlargement of a proglacial lake played a key role in Brikdalsbreen's rapid retreat, allowing calving events and promoting crevassing and fluctuating water contents at the glacier margin. We suggest that hydro-fracturing was the dominant mechanism responsible for generating more crevasses each year, which facilitated the development of an efficient englacial drainage system. This fed increasing quantities of water to the bed, where it was stored in subglacial cavities and transferred through a distributed (‘slow') drainage system. However, despite this increase in subglacial water content, ice velocities remained constant during the break-up. Comparisons are made between the processes observed at Briksdalsbreen and those associated with the acceleration and rapid retreat of Greenland's tidewater glaciers.

Departures from eustasy in Pliocene sea-level records

Proxy data suggest that atmospheric CO2 levels during the middle of the Pliocene epoch (about 3 Myr ago) were similar to today, leading to the use of this interval as a potential analogue for future climate change. Estimates for mid-Pliocene sea levels range from 10 to 40 m above present, and a value of +25 m is often adopted in numerical climate model simulations. A eustatic change of such magnitude implies the complete deglaciation of the West Antarctic and Greenland ice sheets, and significant loss of mass in the East Antarctic ice sheet. However, the effects of glacial isostatic adjustments have not been accounted for in Pliocene sea-level reconstructions. Here we numerically model these effects on Pliocene shoreline features using a gravitationally self-consistent treatment of post-glacial sea-level change. We find that the predicted modern elevation of Pliocene shoreline features can deviate significantly from the eustatic signal, even in the absence of subsequent tectonically-driven movements of the Earth’s surface. In our simulations, this non-eustatic sea-level change, at individual locations, is caused primarily by residual isostatic adjustments associated with late Pleistocene glaciation. We conclude that a combination of model results and field observations can help to better constrain sea level in the past, and hence lend insight into the stability of ice sheets under varying climate conditions. Estimates for sea level three million years ago, a period with similar atmospheric CO2 levels to today, vary from 10 to 40 m above present. Glacial isostatic adjustment modelling suggests that variations in the height of palaeoshorelines result from the residual adjustment of continental flexure following recent glaciations.

Sensitivity of Pliocene ice sheets to orbital forcing

The stability of the Earth's major ice sheets is a critical uncertainty in predictions of future climate and sea level change. One method of investigating the behaviour of the Greenland and the Antarctic ice sheets in a warmer-than-modern climate is to look back at past warm periods of Earth history, for example the Pliocene. This paper presents climate and ice sheet modelling results for the mid-Pliocene warm period (mPWP; 3.3 to 3.0 million years ago), which has been identified as a key interval for understanding warmer-than-modern climates ( Jansen et al., 2007). Using boundary conditions supplied by the United States Geological Survey PRISM Group (Pliocene Research, Interpretation and Synoptic Mapping), the Hadley Centre coupled ocean–atmosphere climate model (HadCM3) and the British Antarctic Survey Ice Sheet Model (BASISM), we show large reductions in the Greenland and East Antarctic Ice Sheets (GrIS and EAIS) compared to modern in standard mPWP experiments. We also present the first results illustrating the variability of the ice sheets due to realistic orbital forcing during the mid-Pliocene. While GrIS volumes are lower than modern under even the most extreme (cold) mid-Pliocene orbit (losing at least 35% of its ice mass), the EAIS can both grow and shrink, losing up to 20% or gaining up to 10% of its present-day volume. The changes in ice sheet volume incurred by altering orbital forcing alone means that global sea level can vary by more than 25 m during the mid-Pliocene. However, we have also shown that the response of the ice sheets to mPWP orbital hemispheric forcing can be in anti-phase, whereby the greatest reductions in EAIS volume are concurrent with the smallest reductions of the GrIS. If this anti-phase relationship is in operation throughout the mPWP, then the total eustatic sea level response would be dampened compared to the ice sheet fluctuations that are theoretically possible. This suggests that maximum eustatic sea level rise does not correspond to orbital maxima, but occurs at times where the anti-phasing of Northern and Southern Hemisphere ice sheet retreat is minimised.

Glacial outburst flood sediments within Disenchantment Bay, Alaska: Implications of recognizing marine jökulhlaup deposits in the stratigraphic record

Glacial outburst floods have been observed or inferred in most glaciated regions of the world. Outburst flooding events along past ice-sheet margins have been discussed as possible causes for changes in ocean circulation while subglacial lake drainage under ice sheets may influence ice-sheet stability. The recognition of these geologically instantaneous events in the marine sediment or rock record is critical in order to characterize the magnitude and frequency of past outburst events as well as to predict the likelihood of future occurrences. In this paper, we characterize outburst lithofacies and infer depositional processes from two outburst floods within Disenchantment Bay, Alaska, and compare these deposits to other recognized outburst deposits within marine depositional settings. Here we show that the outburst deposits are lithologically similar and are composed of stratified mud, sand, and diamicton. We interpret these facies to have been formed by sediment gravity flows and hypopycnal plume sedimentation. These results show that the volume of sediment, lithofacies, and inferred depositional processes cannot be easily distinguished from normal sediment accumulation within the proximal temperate glacimarine depositional environment. The paucity of fluvial subglacial discharge in polar glacimarine environments suggests the potential for recognition of such deposits in the Antarctic.