![]() Millennial-scale Southern Ocean temperature oscillations are driven by a feedback between ocean-atmosphere teleconnections that is modulated by Atlantic Meridional Overturn Circulation (AMOC): 10 the mean state ocean circulation responsible for cross-equatorial heat transport from the Southern Hemisphere to the Northern Hemisphere. On millennial timescales this feedback could cause substantial velocity changes in these fast-flowing ice drainage pathways 8, ultimately affecting continent-wide ice sheet mass balance 9. Ice sheet models suggest that ice shelf decay can result in enhanced flow of grounded ice up to 1000 km upstream of the grounding lines of large outlet glaciers and ice streams 7. Ice sheet stability is regulated by ice shelves 5 and grounding line positions 6, which are vulnerable to thinning and retreat when contacted by warm ocean waters. The key link between this ocean-thermal forcing and ice sheet mass lies in the delivery of heat to the ice sheet margins, which affect ice shelves and grounding lines. Modern observations 2, 3 of retreating ice near marine-terminating ice sheet margins demonstrate the potential for rapid AIS mass fluctuations brought on by changing Southern Ocean temperature 4 (hereafter referred to as ocean-thermal forcing). ![]() One of the persistent challenges involved in both reconstructions and projections of global mean sea level is determining what sectors of the Antarctic Ice Sheet (AIS) are vulnerable to significant retreat, the timescales of such retreat, and the conditions that trigger ice loss events 1. 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. These freeze-flush cycles represent cyclic changes in subglacial hydrologic-connectivity driven by ice sheet velocity fluctuations. 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. Here we present a >100kyr archive of periodic transitions in subglacial precipitate mineralogy that are synchronous with Late Pleistocene millennial-scale climate cycles. ![]() 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. Herein we place the origin and recorded chemical history of these subglacial precipitates into a larger context of subglacial water history, both within the Wilkes Basin and in relation to the McMurdo Dry Valley brines.Ice cores and offshore sedimentary records demonstrate enhanced ice loss along Antarctic coastal margins during millennial-scale warm intervals within the last glacial termination. Extremely depleted δ 13C values record carbon sourcing from soils at the base of the ice sheet, and radiogenic 87Sr/ 86Sr compositions are the product of subglacial chemical weathering of the silicate crust by these fluids. For example, elevated 234U/ 238U values stem from long-term rock-water interaction, while extremely depleted δ 18O compositions require either distal sourcing or an additional fractionation by subglacial freezing which leaves the remaining fluids depleted in 18O. These compositional changes reflect changes in subglacial waters. Over the 100 ka recorded timescale we resolve fluctuations and trends in subglacial water chemistry including: 87Sr/ 86Sr, 207Pb/ 206Pb, 234U/ 238U, δ 18O and δ 13C. Opal and calcite precipitation is cyclical, occurring at ~10 ka. U-Th dates range from ~250 ka at the sample base to 150 ka at the sample top, pointing to precipitation rates on the order of 0.4 mm/ka. Our focus precipitate sample is strongly bedded, consisting of mostly discrete calcite and opal layers. ![]() Such a record is used here to examine the compositional, mineralogical, and isotopic changes recorded by a single 1.5 cm-thick subglacial precipitate from Elephant Moraine in the East Antarctic Ice Sheet. Because they can be dated by radioisotopic methods, these precipitates record the chemical and isotopic history of subglacial waters on geologic timescales. These water-lain chemical precipitates form as a byproduct of subglacial freezing, a process that consumes subglacial waters, concentrating solutes within the remaining waters to the point of, most often, calcite or amorphous silica/opal precipitation. Subglacial chemical precipitates record the chemical, isotopic, and elemental characteristics of waters from which they form.
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