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The influence of Antarctica and the Southern Ocean on Late Pliocene global climate reconstructions has remained ambiguous due to a lack of well-dated Antarctic-proximal, paleoenvironmental records. Here we present ice sheet, sea-surface temperature, and sea ice reconstructions from the ANDRILL AND-1B sediment core recovered from beneath the Ross Ice Shelf. We provide evidence for a major expansion of an ice sheet in the Ross Sea that began at ∼3.3 Ma, followed by a coastal sea surface temperature cooling of ∼2.5 °C, a stepwise expansion of sea ice, and polynya-style deep mixing in the Ross Sea between 3.3 and 2.5 Ma. The intensification of Antarctic cooling resulted in strengthened westerly winds and invigorated ocean circulation. The associated northward migration of Southern Ocean fronts has been linked with reduced Atlantic Meridional Overturning Circulation by restricting surface water connectivity between the ocean basins, with implications for heat transport to the high latitudes of the North Atlantic. While our results do not exclude low-latitude mechanisms as drivers for Pliocene cooling, they indicate an additional role played by southern high-latitude cooling during development of the bipolar world.

Grounding-line fans originate from subglacial and basal stream tunnels at grounding lines of glaciers terminating in a marine environment. During melt seasons discharge forms a turbulent jet beyond the efflux. Over the initial zone of flow establishment in the jet, it may remain in contact with the sea floor for up to 13.4D (D-tunnel diameter) along a runout distance, if velocity is constant. Plug flow in this zone deposits marine outwash sediment commonly having chaotic texture and graded and welded contacts. Texture may vary rapidly because of discharge pulses, and the jet may move laterally to produce sheet or cut-and-fill geometries. Imbricate gravel is deposited near the efflux, then farther out are sheet or weakly channelized finer-grained gravels and sands. In some instances a migrating barchanoid bar forms at the detachment zone and forms large-scale trough cross-beds. Beyond the detachment zone the turbulent jet becomes vertical and sediment cascades from it in a 'veil'. This sediment is added to by flows from continuous failures of bed load sediment in the detachment zone. Resulting deposits are interstratified sediment gravity flows and thick, coarse end-members of cyclopsams. The jet changes to a plume when inertial forces become less than buoyancy forces. Turbulent vorticies within the jet can re-entrain particles into a buoyant plume. The plume remains vertical, although continuously spreading, until reachin~ neutral buoyancy. Fan depocentres have been observed to accumulate at over 106 m 3 a-in temperate glacial areas. Eventually, at quasi-stable grounding lines, fans may aggrade to sea level to form ice-contact deltas. At incipient deltas when the delta plain is intertidal, coarse sediment is redistributed to the prodelta with each tidal cycle. Intense prodelta rhythmites mark the transition of a fan into a delta.

To predict the future contributions of the Antarctic ice sheets to sea-level rise, numerical models use reconstructions of past ice-sheet retreat after the Last Glacial Maximum to tune model parameters . Reconstructions of the West Antarctic Ice Sheet have assumed that it retreated progressively throughout the Holocene epoch (the past 11,500 years or so). Here we show, however, that over this period the grounding line of the West Antarctic Ice Sheet (which marks the point at which it is no longer in contact with the ground and becomes a floating ice shelf) retreated several hundred kilometres inland of today's grounding line, before isostatic rebound caused it to re-advance to its present position. Our evidence includes, first, radiocarbon dating of sediment cores recovered from beneath the ice streams of the Ross Sea sector, indicating widespread Holocene marine exposure; and second, ice-penetrating radar observations of englacial structure in the Weddell Sea sector, indicating ice-shelf grounding. We explore the implications of these findings with an ice-sheet model. Modelled re-advance of the grounding line in the Holocene requires ice-shelf grounding caused by isostatic rebound. Our findings overturn the assumption of progressive retreat of the grounding line during the Holocene in West Antarctica, and corroborate previous suggestions of ice-sheet re-advance . Rebound-driven stabilizing processes were apparently able to halt and reverse climate-initiated ice loss. Whether these processes can reverse present-day ice loss on millennial timescales will depend on bedrock topography and mantle viscosity-parameters that are difficult to measure and to incorporate into ice-sheet models.

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