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The East Antarctic ice sheet was larger than present during past cold periods. Seafloor geophysical data show that in the Ross Sea, the extended ice sheet was underlain by an active hydrologic system during the glacial termination.

Studying the history of ice-sheet behaviour in the Ross Sea, Antarctica's largest drainage basin can improve our understanding of patterns and controls on marine-based ice-sheet dynamics and provide constraints for numerical ice-sheet models. Newly collected high-resolution multibeam bathymetry data, combined with two decades of legacy multibeam and seismic data, are used to map glacial landforms and reconstruct palaeo ice-sheet drainage. During the Last Glacial Maximum, grounded ice reached the continental shelf edge in the eastern but not western Ross Sea. Recessional geomorphic features in the western Ross Sea indicate virtually continuous back-stepping of the ice-sheet grounding line. In the eastern Ross Sea, well-preserved linear features and a lack of small-scale recessional landforms signify rapid lift-off of grounded ice from the bed. Physiography exerted a first-order control on regional ice behaviour, while sea floor geology played an important subsidiary role. Previously published deglacial scenarios for Ross Sea are based on low-spatial-resolution marine data or terrestrial observations; however, this study uses high-resolution basin-wide geomorphology to constrain grounding-line retreat on the continental shelf. Our analysis of retreat patterns suggests that (1) retreat from the western Ross Sea was complex due to strong physiographic controls on ice-sheet drainage; (2) retreat was asynchronous across the Ross Sea and between troughs; (3) the eastern Ross Sea largely deglaciated prior to the western Ross Sea following the formation of a large grounding-line embayment over Whales Deep; and (4) our glacial geomorphic reconstruction converges with recent numerical models that call for significant and complex East Antarctic ice sheet and West Antarctic ice sheet contributions to the ice flow in the Ross Sea.

The Ross Sea Embayment is the largest drainage basin of the Antarctic Ice Sheet. Multiple ice streams sourced from both the East Antarctic and West Antarctic ice sheets merged in the Ross Sea during the Last Glacial Maximum (LGM). The continental shelf record is important for understanding processes that influence retreat, constraining deglacial patterns and assessing Antarctica's contribution to eustatic sea-level rise since the LGM. Newly acquired, high-resolution multibeam bathymetry provides a detailed record of the complex deglacial history of a sector in the western Ross Sea (Fig. 1a). Retreat of grounded ice is marked by small, ice-marginal retreat landforms that were poorly resolved prior to the use of an upgraded multibeam system. Dynamic grounding zone behaviour is influenced by physiography and the presence of subglacial meltwater. Fig. 1. ( a ) Multibeam survey south of Crary Bank between Franklin Island and the Central Basin, western Ross Sea shelf (basemap modified by L. Prothro from GeoMapApp). Acquisition system Kongsberg EM122. Frequency 12 kHz. Grid-cell size 20 m. ( b ) Ross Sea showing the location of the survey area ((a) in red box; map from IBCSO v. 1.0). LAB, Little American Basin; GCB, Glomar-Challenger Basin; RB, Ross Bank; PT, Pennell Trough; PB, Pennell Bank; JT, JOIDES Trough; MB, Mawson Bank; CB, Central Bank; CrB Crary Bank; FrIS, Franklin Island; DT, Drygalski Ice Tongue; VLB, Victoria Land Basin; McM, McMurdo Sound. ( c ) Two sets of linear features interpreted as glacial lineations separated by an abrupt boundary (grey dashed line). Inset shows seismic profile across glacial lineations. Acquisition system Knudsen CHIRP. Frequency 3.5 kHz. ( d ) Asymmetrical ridges, interpreted as GZWs, deposited along the banks of a channel and on top of a volcanic landform with lineated topsets. ( e ) Profile of 4 m GZW deposited on volcanic landform showing underlying surface reflection. ( f ) Example of channel cross-sectional profile. …

A lack of reliable chronology often leaves Antarctic ice sheet reconstructions incomplete. Despite being the most heavily investigated region of the Antarctic continental shelf, the post-Last Glacial Maximum (LGM) record of marine-based ice sheet dynamics in the Ross Sea has remained largely elusive and at odds with fringing terrestrial records of ice sheet decay. Issues with radiocarbon dating stem primarily from poorly preserved carbonate, contamination by glacially-recycled carbon, and insufficient consideration for glacial geomorphic context and sedimentary facies. We assess nearly 70 newly acquired and more than 200 published radiocarbon ages in the context of a consistent sedimentary facies and geomorphic framework to interpret the timing of the local glacial maximum and subsequent retreat of the East Antarctic Ice Sheet (EAIS) in the Ross Sea and potential causes of retreat. Compound-specific radiocarbon ages help to provide good constraints on open marine onset, and we demonstrate that, with proper corrections and considerations, acid insoluble organic (AIO) radiocarbon ages can be reliable in open marine sediments and carbonate ages can be reliable in grounding-zone proximal sediments. We find that the ice flowing through Pennell and JOIDES troughs remained at maximum extent for approximately 10,000 years before retreating at 15.1 and 13 cal ka BP, respectively. An ice shelf covered the outer continental shelf until ∼9.5 cal ka BP, after which it collapsed in JOIDES Trough, creating deep iceberg furrows. Retreat through Pennell Trough was more episodic, marked by grounding-zone wedges that backstep upstream and onto lateral banks creating deglacial emergent ice rises. As ice retreated from ∼9 to ∼4 cal ka BP, there was considerable reorganization of drainage and at least one episode of readvance through southern JOIDES Trough that began drawdown of terrestrial ice near Ross Island at ∼7.8 cal ka BP. The ice shelf appears to have retreated to a near-present configuration by 2 cal ka BP. However, due to inconsistencies in calendar ages, grounding line position must be inferred from relative chronology based on glacial landforms, which suggests grounded ice retreated nearly to Drygalski Ice Tongue and lingered atop Crary Bank where it was maintained by ice from David Glacier. Our reconstruction further suggests that the EAIS in the western Ross Sea was not a discernible contributor to Meltwater Pulse 1A, but likely contributed to eustatic sea level rise well through the middle—perhaps even late—Holocene.