The retreat of the West Antarctic Ice Sheet (WAIS) in the Ross Sea after the Last Glacial Maximum (LGM) was more significant than for any other Antarctic sector. Here we combined the available chronology of retreat with new mapping of seismically resolvable grounding zone wedges (GZWs). Mapping GZWs is important because they record the locations of former stillstands in the extent of grounded ice for individual ice streams during the overall retreat. Our analysis shows that the longest stillstands occurred early in the deglacial period and had millennial durations. Stillstands ended abruptly with retreat distances measured in the tens to hundreds of kilometers creating deep embayments in the extent of grounded ice across the Ross Sea. The location of embayments shifted through time. The available chronological data show that cessation of WAIS and East Antarctic Ice Sheet (EAIS) stillstands was highly asynchronous across at least 5000 years. There was a general shift to shorter stillstands throughout the deglacial period. The asynchronous collapse of individual catchments during the deglacial period suggests that the Ross Sea sector would have contributed to multiple episodes of relatively small-amplitude sea-level rise as the WAIS and EAIS retreated from the region. The high sinuosity of the modern grounding zone in the Ross Sea suggests that this style of retreat persists.

Abstract. The post-LGM retreat of the West Antarctic Ice Sheet (WAIS) in Ross Sea was greater than for any other Antarctic sector. Here we combined the available chronology of retreat with new mapping of seismically-resolvable grounding zone wedges (GZWs). Mapping GZWs is important because they record the locations and durations of former stillstands in the extent of grounded ice for individual ice streams during the overall retreat. Our analysis shows that the longest stillstands occurred early in the deglacial and had millennial durations. Stillstands ended abruptly with retreat distances measured in the tens to hundreds of kilometers creating deep embayments in the extent of grounded ice across Ross Sea. The location of embayments shifted through time. The available chronological data shows that cessation of WAIS stillstands was highly asynchronous across at least five thousand radiocarbon years. There was a general shift to shorter stillstands as the deglacial progressed. Asynchronous collapse of individual catchments over the course of the post-LGM suggests that the Ross Sea sector would have contributed to multiple episodes of relatively-small amplitude, sea-level rise. The high sinuosity of the modern ground zone in Ross Sea suggests that this style of retreat persists.

Geological records of ice sheet collapse can provide perspective on the ongoing retreat of grounded and floating ice. An abrupt retreat of the West Antarctic Ice Sheet (WAIS) that occurred during the early deglaciation is well recorded on the eastern Ross Sea continental shelf. There, an ice shelf breakup at 12.3 ± 0.6 cal. (calibrated) kyr BP caused accelerated ice-mass loss from the Bindschadler Ice Stream (BIS). The accelerated mass loss led to a significant negative mass balance that re-organized WAIS flow across the central and eastern Ross Sea. By ~ 11.5 ± 0.3 cal kyr BP, dynamic thinning of grounded ice triggered a retreat that opened a ~ 200-km grounding-line embayment on the Whales Deep Basin (WDB) middle continental shelf. Here, we reconstruct the pattern, duration and rate of retreat from a backstepping succession of small-scale grounding-zone ridges that formed on the embayment’s eastern flank. We used two end-member paleo-sediment fluxes, i.e., accumulation rates, to convert the cumulative sediment volumes of the ridge field to elapsed time for measured distances of grounding-line retreat. The end-members fluxes correspond to deposition rates for buttressed and unbuttressed ice stream flow. Both scenarios require sustained rapid retreat that exceeded several centuries. Grounding-line retreat is estimated to have averaged between ~ 100 ± 32 and ~ 700 ± 79 ma −1 . The evidence favors the latter scenario because iceberg furrows that cross cut the ridges in deep water require weakly buttressed flow as the embayment opened. In comparison with the modern grounding-zone dynamics, this paleo-perspective provides confidence in model projections that a large-scale sustained contraction of grounded ice is underway in several Pacific-Ocean sectors of the WAIS.

Marine geophysical data collected from the Ross Sea continental shelf during several oceanographic expeditions enabled evaluation of the Last Glacial Maximum (LGM) extent of the Antarctic Ice Sheet (AIS) through the presence of large Grounding Zone Wedges (GZWs), particularly evident in the outer reaches of the Drygalski and Joides basins to the north of Coulman Island. Seismo-stratigraphic obser-vations confirmed by geomorphological and stratigraphic data show a deep grounding line embayment dating back to the early deglacial transition, which preceded the last rapid sea-level and atmospheric CO2 rise. In this work, a new reconstruction based on the analysis of mor-pho-bathymetric and seismic reflection data from the middle reaches of the Drygalski Basin shows that the post-LGM retreat was followed by a short-lived re-advance of the grounding line during the Antarctic Cold Reversal (ACR). Evidences include GZWs that partly overprint mega-scale glacial lineations associated with the Coulman Island grounding line, followed by a Holocene retreat phase, which caused the final south-ward withdrawal of the grounded and floating ice. This late re-advance suggests a significant impact on the extent and thickness of the ground-ed ice from relatively small amplitude climate oscillations, able to exert a significant control on the AIS during the latest Pleistocene (i.e. the Last Glacial Termination). Given that the marine-based portion of the AIS in the Ross Sea was sensitive to millennial-scale climate oscillations, this evidence will contribute to clarify how the ice sheet may respond to ongoing and future climate change.

The present-day morpho-stratigraphy of the Ross Sea is the result of Cenozoic tectonic and cryospheric events, and constitutes a key record of Antarctica's cryospheric evolution. An enduring problem in interpreting this record in a broader regional context is that the correlation between eastern and western Ross Sea stratigraphy has remained uncertain due to the limited number of drill sites. We correlate the glacial-related features observed on a dense network of seismic reflection profiles in McMurdo Sound with those identified in the Nordenskjo??ld and Drygalski Basins, as well as the basins farther east in the central Ross Sea. We present an improved corre-lation of the regional patterns of early to middle Miocene ice-sheet variance across the Ross Sea constrained by new evaluation of seismic facies and age models from one site recovered by the Antarctic Drilling Project (ANDRILL) in the southwestern most part of McMurdo Sound. We also integrate this correlation with the recently published seismic framework in the central Ross Sea. The formation of U-shaped valleys during the early Miocene in McMurdo Sound, together with prograding sedimentary wedges in the western-most basins, and the central Ross Sea, suggest two major phases of overall advance of a marine-terminating ice sheet between-18 Ma and-17.4 Ma. Widespread formation of turbiditic channel-levee systems in McMurdo Sound and rapid sediment deposition in Nordernskjo??ld Basin point to subsequent ice-sheet retreat between-17.4 Ma and-15.8 Ma, coinciding with the onset of the Miocene Climate Optimum (MCO;-17???14.5 Ma). However, the carving of troughs and formation of irregular morphologic features suggest that an extensive ice sheet still remained along the western Ross margin at-17.4 Ma and a brief episode of ice-sheet advance occurred at-16.8 Ma in the earliest interval of the MCO. Subsequent marine-based ice sheet advance during the Middle Miocene Climate Transition (MMCT,-14.0???13.8 Ma) is indicated by widespread erosional features. Our results reconcile the semi-continouous seismic and drill core stratigraphy of the offshore Ross Sea continental shelf with inferences of ice sheet dynamics from continuous far-field deep sea and sea level records, as well as the highly discontinous (and heavily debated) onshore records of pre-MMCT glaciation and aridification of the Transantarctic Mountains at 14 Ma.

Additional information regarding methods (Reflection seismic processing, Drill-site measurements, Core-log-seismic correlations, Spatial Velocity calculations, and Reflection Tomography model) and regional stratigraphy descriptions, as well as detailed considerations regarding the opal distribution and depth.

For the period between 14.7 and 11.5 cal. (calibrated) kyr B.P, the sediment flux of Bindschadler Ice Stream (BIS; West Antarctica) averaged 1.7 × 108 m3 a−1. This implies that BIS velocity averaged 500 ± 120 m a-1. At a finer resolution, the data suggest two stages of ice stream flow. During the first 2400 ± 400 years of a grounding-zone stillstand, ice stream flow averaged 200 ± 90 m a-1. Following ice-shelf breakup at 12.3 ± 0.2 cal. kyr B.P., flow accelerated to 1350 ± 580 m a-1. The estimated ice volume discharge after breakup exceeds the balance velocity by a factor of two and implies ice mass imbalance of ~40 Gt a-1 just before the grounding zone retreated >200 km. We interpret that the paleo-BIS maintained sustainable discharge throughout the grounding-zone stillstand first due to the buttressing effect of its fringing ice shelf and then later (i.e., after ice-shelf breakup) due to the stabilizing effects of grounding-zone wedge aggradation. Major paleo–ice stream retreat, shortly after the ice-shelf breakup that triggered the inferred ice flow acceleration, substantiates the current concerns about rapid, near-future retreat of major glaciers in the Amundsen Sea sector where Pine Island and Thwaites Glaciers are already experiencing ice-shelf instability and grounding-zone retreat that have triggered upstream-propagating thinning and ice acceleration.

A robust collection of seismic and geomorphic data is used to examine the evolution of the Antarctic Ice Sheet within the Ross Sea Embayment. We use geomorphic data to reconstruct Last Glacial Maximum and post-Last Glacial Maximum ice sheet drainage and demonstrate retreat behaviours for the East Antarctic and West Antarctic sectors of the ice sheet. Using this framework, we then use seismic data and chronostratigraphic information from drill cores to reconstruct the long-term evolution of the ice sheet. Early ice sheet evolution during the Late Oligocene was characterized by isolated ice caps on bathymetric highs, followed by an interval of sediment infilling of rift basins and the development of more subdued relief in the eastern Ross Sea than in the western Ross Sea. Both ice sheets have experienced multiple episodes of expansion across the continental shelf since the Middle Miocene, with the frequency increasing during the Plio-Pleistocene. We conclude that seafloor bathymetry has been the principal control on ice sheet palaeodrainage and retreat behaviour since at least the middle Miocene, demonstrated by broad West Antarctic ice streams loosely guided by south to north cross-shelf troughs, whereas East Antarctic ice streams were funnelled through troughs that merge and converge around banks.

Habitats proximal to grounded ice and below ice shelves are rarely studied for microfossils. A recently described, well-resolved deglaciation record from the Whales Deep Basin of the eastern Ross Sea provided an opportunity to study sub-fossil foraminifera in such settings. Among other foraminiferal taxa, two forms with pustulose/spinose ornamentation were especially important as they were restricted to habitats associated with proximity to the calving front or presence of an ice-shelf. Based on gradation from strongly pustulose/spinose to typical morphologies and existing molecular data, these rarely reported forms are considered to be morphotypes of Globocassidulina biora (Crespin, 1960) and Trifarina earlandi (Parr, 1950). They seemed to flourish in polynya areas near grounding-line and in sub-ice-shelf environments with bottom currents. Their unusual morphologies may be a response to limited food resources. These foraminifera deserve special attention because they appear to be restricted to extreme Antarctic environments and hence are potentially very important for paleoenvironmental reconstructions.

After the Last Glacial Maximum (LGM), the Antarctic Ice Sheet retreated several hundred kilometers from the outer continental shelf. An overall backstepping stratal pattern of grounding zone wedges (GZWs) indicates that the post-LGM retreat occurred in a step-wise fashion. Each GZW records a prior interval of ice-stream sedimentation during which the location of paleo-grounding zone (i.e., the former seaward limit of flowing ice grounded to the seafloor) was relatively stationary. Here we investigate whether the locations of post-LGM grounding-zone stillstands in the Whales Deep Basin (WDB) of the eastern Ross Sea were controlled by the antecedent topography. The Bindschadler Ice Stream formerly occupied the WDB. We used similar to 7500 km of seismic reflection data to create a depth-converted map of the LGM unconformity that was sub-glacially eroded when the WAIS was grounded at the continental shelf edge (i.e., prior to the deposition of the post-LGM GZWs). Our mapping shows that the first three paleo-grounding zones were located slightly downstream of relatively low-relief obstacles on the LGM unconformity. The subsequent four grounding zones were located above a low-relief saddle on the LGM unconformity at a bottleneck constriction in the width of the paleo-ice-stream basin. These observations support the general hypotheses that post-LGM grounding-zone locations were at least partly controlled by the antecedent subglacial topography. We cannot exclude the possibility that other factors may have also been important. However, the results suggest that models seeking to accurately predict the changing extent of grounded ice in response to ongoing climate changes should incorporate highly resolved maps of the existing subglacial topography as well as considerations as to how subglacial topography evolves in response to contemporaneous ice-stream erosion and sedimentation.

Reductions in the thickness and extent of Antarctic ice shelves are triggering increased discharge of marine-terminating glaciers. While the impacts of recent changes are well documented, their role in modulating past ice-sheet dynamics remains poorly constrained. This reflects two persistent issues; first, the effective discrimination of sediments and landforms solely attributable to sub-ice-shelf deposition, and second, challenges in dating these records. Recent progress in deciphering the geological imprint of Antarctic ice shelves is summarised, including advances in dating methods and proxies to reconstruct drivers of change. Finally, we identify several challenges to overcome to fully exploit the paleo record.

Recent thinning and loss of Antarctic ice shelves has been followed by near synchronous acceleration of ice flow that may eventually lead to sustained deflation and significant contraction in the extent of grounded and floating ice. Here, we present radiocarbon dates from foraminifera that constrain the time elapsed between a previously described paleo-ice-shelf collapse and the subsequent major grounding-line retreat in the Whales Deep Basin (WDB) of eastern Ross Sea. The dates indicate that West Antarctic Ice Sheet (WAIS) grounding-line retreat from the continental shelf edge was underway prior to 14.7 ± 0.4 cal kyr BP. A paleo-ice-shelf collapse occurred at 12.3 ± 0.2 cal kyr BP. The grounding position was maintained on the outer-continental shelf until at least 11.5 ± 0.3 cal kyr BP before experiencing a 200-km retreat. Given the age uncertainties, the major grounding-line retreat lagged ice-shelf collapse by at least two centuries and by as much as fourteen centuries. In the WDB, the centuries-long delay in the retreat of grounded ice was partly due to rapid aggradational stacking of an unusually large volume of grounding-zone-wedge sediment as ice-stream discharge accelerated following ice-shelf collapse. This new deglacial reconstruction shows that ongoing changes to ice shelves may trigger complex dynamics whose consequences are realized only after a significant lag.

ABSTRACTContemporary ice stream flow is directly linked to conditions at the ice/bed interface, yet this environment is logistically difficult to access. Instead, we investigate subglacial processes important for ice stream flow by studying tills on the deglaciated Antarctic continental shelf. We test currently-accepted hypotheses surrounding subglacial processes and till properties with a Ross Sea dataset. Till shear strengths indicate a continuum of simultaneous processes acting at the bed, rather than discrete ‘deformation’ and ‘lodgement’ end-members. We identify a threshold water content representing saturated pore spaces, leading to basal sliding and meltwater channelization. Based on observations of till properties relative to glacial landforms, we challenge the assumption that low shear strength is linked to intense deformation. Spatial variability in landform morphology reflects variability in deforming processes at the sub-ice stream scale and suggests a maximum deforming bed thickness of 2 m at the grounding line. Regional till properties generally correlate with seafloor geology and deglacial history; the western Ross Sea is characterized by higher and more variable shear strengths and water contents, while lower-shear strength till was preserved in the Eastern Basin. These observations inform till interpretation and provide context for deforming beds beneath the modern ice sheet and on glaciated continental shelves.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Reconstructions of Ross Sea paleoenvironments based on foraminifera are hindered by the dearth of actualistic data, especially from below the Ross Ice Shelf. To fill this gap, we used a recently developed well-resolved deglaciation record from the Whales Deep Basin of the eastern Ross Sea to understand the ecological affinities of different foraminiferal assemblages. During open-water conditions similar to the present, two benthic foraminiferal communities are strongly dominated by agglutinated species. However, in the underlying sediments that represent sub-ice-shelf and grounding-zone proximal settings on the outer continental shelf, we identified five different assemblages of calcareous foraminifera. By Antarctic margin standards, these calcareous assemblages are periodically truly abundant. Along with assemblages dominated by benthic calcareous species that are well known from many Antarctic settings, i.e., Globocassidulina biora, Trifarina earlandi, and Astrononion echolsi, two important assemblages are dominated by a heretofore undescribed pustulose morphotype of G. biora and poorly known spinose morphotype of T. earlandi. Based on our correlations to the deglacial record, these two assemblages live near the grounding line or in environments with especially intense bottom water currents. As grounded and floating ice retreated south to the middle continental shelf, these later sub-ice-shelf and grounding-zone proximal settings were largely devoid of calcareous foraminifera and instead were inhabited by agglutinated taxa. This improved understanding of foraminiferal assemblage distributions from a variety of environments within the eastern Ross Sea may contribute to better use of foraminiferal data to investigate the evolution of environmental changes in grounding-line proximal environments.

The West Antarctic Ice Sheet (WAIS) retreated more than 1,000 km since last grounding at the Ross Sea outer continental shelf. Here we show an interpretation of former grounding line positions from a new large‐area multibeam survey and a regional grid of chirp cross‐sectional data from the Whales Deep Basin in eastern Ross Sea. The basin is a paleo‐glacial trough that was occupied by the Bindschadler Ice Stream when grounded ice advanced to the shelf edge during the Last Glacial Maximum. These new geophysical data provide unambiguous evidence that the WAIS occupied at least seven grounding line positions within 60 km of the shelf edge. Four of seven grounding zone wedges (GZWs) are partly exposed over large areas of the trough. The overlapping stratal arrangement created a large‐volume compound GZW. Some of the groundings involved local readvance of the grounding line. Subsequent to these seven outer continental shelf groundings, the ice sheet retreated more than 200 km towards Roosevelt Island on the middle continental shelf. The major retreat across the middle continental shelf is recorded by small‐scale moraine ridges that mantle the top of GZW7, and these are suggestive of relatively continuous grounding line recession. The results indicate that retreat was considerably more complex than was possible to reconstruct with reconnaissance‐level data. The added details are important to climate models, which must first be able to reproduce the recent retreat pattern in all of its complexities to improve confidence in model predictions of the system's future response.