Antarctic Subglacial Trace Metal Mobility Linked to Climate Change Across Termination III

Abstract. Antarctic meltwater is a significant source of iron that fertilizes present-day Southern Ocean ecosystems and may enhance marine carbon burial on geologic timescales. However, it remains uncertain how this nutrient flux changes through time, particularly in response to climate, due to an absence of geologic records detailing trace metal mobilization beneath ice sheets. In this study, we present a 25 kyr record of aqueous trace metal cycling beneath the East Antarctic Ice Sheet measured in a subglacial chemical precipitate that formed across glacial termination III (TIII). The deposition rate and texture of this sample describe a shift in basal meltwater flow following the termination. Alternating layers of opal and calcite deposited in the 10 kyr prior to TIII record centennial-scale subglacial flushing events, whereas reduced basal flushing resulted in slower deposition of a trace metal-rich (Fe, Mn, Mo, Cu) calcite in the 15 kyr after TIII. This sharp increase in calcite metal concentrations following TIII indicates that diminished subglacial meltwater flow restricted the influx of oxygen from basal ice melt to precipitate-forming waters, causing dissolution of redox-sensitive trace metals from the bedrock substrate. These results are consistent with a possible feedback between orbital climate cycles and Antarctic subglacial iron discharge to the Southern Ocean, whereby heightened basal meltwater flow during terminations supplies oxygen to subglacial waters along the ice sheet periphery, which reduces the solubility of redox sensitive elements. As the climate cools, thinner ice and slower ice flow reduce basal meltwater production rates, limiting oxygen delivery and promoting more efficient mobilization of subglacial trace metals. Using a simple model to calculate the concentration of Fe in Antarctic basal water through time, we show that the rate of Antarctic iron discharge to the Southern Ocean is highly sensitive to this heightened mobility, and may therefore, increase significantly during cold climate periods.