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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Throughout the Late Pleistocene, millennial-scale cycles in the rate of poleward heat transport resulted in repeated heating and cooling of the Southern Ocean1. Ice sheet models2 suggest that this variation in Southern Ocean temperature can force fluctuations in the mass of the Antarctic ice sheet that transiently impact sea level by up to 15 meters. However, current geologic evidence for Antarctic ice response to this ocean thermal forcing is unable to calibrate these models, leaving large uncertainty in how Antarctica contributes to sea level on millennial timescales. Here we present a >100kyr archive of East Antarctic Ice Sheet response to Late Pleistocene millennial-scale climate cycles recorded by transitions from opal to calcite in subglacial precipitates. 234U-230Th dates for two precipitates define a time series for 32 mineralogic transitions that match Antarctic climate fluctuations, with precipitation of opal during cold periods and calcite during warm periods. Geochemical evidence indicates opal precipitation via cryoconcentration of silica in subglacial water and calcite precipitation from the admixture of meltwater flushed from the ice sheet interior. These freeze-flush cycles represent changes in subglacial hydrologic-connectivity driven by ice sheet thickness response to Southern Ocean temperature oscillations around the Ross Embayment. Changes in Ross Embayment ice mass require high ocean-ice heat exchange2, and would occur only after retreat of the West Antarctic Ice Sheet3 and large portions of the East Antarctic Ice sheet margin4. These results point to high Antarctic ice sheet sensitivity to millennial-scale ocean thermal forcing throughout the Late Pleistocene, and when combined with modeling results2, predict that an Antarctic ice volume of at least 2–5 meters sea level equivalent is vulnerable to millennial-scale climate forcing.

Records of changing ice mass in offshore sediments and ice cores suggest that the West Antarctic ice sheet experienced millennial-scale ice loss during the last termination. However, the distal location and short temporal coverage of these records leads to uncertainty in both the spatial footprint of ice response, and whether this response persists outside of glacial terminations. Here we present a >100kyr archive of episodic Antarctic ice sheet basal melt events recorded by mineralogic variation in subglacial precipitates. 234U-230Th dates for two precipitates are used to build a time series of 32 opal-calcite transitions that correspond to Late Pleistocene millennial-scale climate cycles, with precipitation of opal during cold periods and calcite during warm periods. Geochemical data indicate that opal precipitation occurs via cryoconcentration of silica in brines beneath the ice sheet margin, while calcite precipitation is triggered by the addition of subglacial meltwater originating from the ice sheet interior. These freeze-flush cycles represent changes in subglacial hydrologic-connectivity driven by ice sheet velocity fluctuations, which likely occur in response to Southern Ocean thermal forcing acting on grounding lines within the Ross Embayment. Our results suggest that oscillating temperatures in the Southern Ocean affect the mass of the Antarctic ice sheet by regulating the delivery of heat to buttressing ice shelves and grounding lines on millennial timescales, regardless of the background climate state.