Calibrating Antarctic ice sheet mass loss due to millennial-scale ocean thermal forcing

Abstract

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.