Efforts to improve sea level forecasting on a warming planet have focused on determining the temperature, sea level and extent of polar ice sheets during Earth's past interglacial warm periods 1-3. At ~400 thousand years before present, during the interglacial period known as Marine Isotopic Stage 11 (MIS 11), the Earth, globally was 1-2°C warmer 2 and sea level was 6-13m 1,3 higher. Sea level estimates in excess of ~10m, however, have been discounted as these require contribution from the East Antarctic Ice Sheet 3 , which has been argued to have remained stable at MIS 11 and for millions of years prior 4,5. Here, we show how the evolution of 234 U enrichment within subglacial waters of East Antarctica records the ice sheet response to MIS 11 warming. Within the Wilkes Basin, subglacial chemical precipitates of opal and calcite record the accumulation of 234 U, the product of rock-water contact within an isolated subglacial reservoir, up to 20 times higher than marine waters. The timescales of 234 U enrichment place the inception of this reservoir to MIS 11. Informed by the observed 234 U cycling in the Laurentide ice sheet, where 234 U accumulated during periods of ice stability 6 and was purged in response to deglaciation 7 , we interpret our East Antarctic dataset to record ice loss within the Wilkes Basin at MIS 11. The 234 U ingrowth within the Wilkes Basin is shared by the McMurdo Dry Valley brines 8-10 , supporting 11 brine origination beneath the adjacent East Antarctic ice sheet. The requirement that brine salts 10 and bacteria 12 are from marine waters implies that MIS 11 ice loss was coupled with marine flooding. Collectively these data indicate that during one of the warmest Pleistocene interglacials, the ice sheet margin at the Wilkes Basin retreated to the proximity of the precipitate location, ~700km inland from the current position, which assuming current ice volumes, would contribute ~3-4m 13 to global seas.

Ice cores and offshore sedimentary records demonstrate enhanced ice loss along Antarctic coastal margins during millennial-scale warm intervals within the last glacial termination. However, the distal location and short temporal coverage of these records leads to uncertainty in both the spatial footprint of ice loss, and whether millennial-scale ice response occurs outside of glacial terminations. Here we present a >100kyr archive of periodic transitions in subglacial precipitate mineralogy that are synchronous with Late Pleistocene millennial-scale climate cycles. Geochemical and geochronologic data provide evidence for opal formation during cold periods via cryoconcentration of subglacial brine, and calcite formation during warm periods through the addition of subglacial meltwater originating from the ice sheet interior. These freeze-flush cycles represent cyclic changes in subglacial hydrologic-connectivity driven by ice sheet velocity fluctuations. Our findings imply that oscillating Southern Ocean temperatures drive a dynamic response in the Antarctic ice sheet on millennial timescales, regardless of the background climate state.

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