Gavin Piccione, Terrence Blackburn, Mathis Hain, Slawek Tulaczyk, Patrick Rafter, Jessica Gagliardi, Colin Carney

Ice sheets may influence the global carbon cycle by releasing chemical weathering products and carbon from basal environments. However, limited data describing subglacial biogeochemical cycles beneath Antarctic and Greenland ice leaves open fundamental questions regarding the feedbacks between climate, ice sheets, and the carbon cycle. Most notably: does subglacial chemical weathering beneath ice sheets act as a source or sink of atmospheric CO2? Here, we present constraints on biogeochemical reactions beneath the Antarctic ice sheet using geochemical and geochronologic measurements of carbonate precipitates, which formed from basal waters in ten locations along the ice sheet margins and Transantarctic Mountains (TAM), since the late Miocene. Precipitate carbon and oxygen isotopic compositions reveal a consistent geographic pattern in subglacial carbon cycling and chemical weathering, where parent waters in catchments draining through the TAM source CO2 through microbial metabolism of organic matter that drives silicate weathering, while waters beneath the ice sheet periphery acquire carbon mainly through the dissolution of carbonate minerals. Proxies for parent water pH (P/Ca) and solute source (87Sr/86Sr) show that bedrock composition and reactivity are the main factors determining intensity of silicate weathering in TAM waters. While the dominance of subglacial silicate versus carbonate weathering is determined by the presence of carbonate minerals and/or basal water residence time, which can fluctuate across global climate cycles. Because carbonate and silicate dissolution have quantitively different effects on the carbon cycle, the balance between them dictates whether Antarctica acts as a positive or negative climate-carbon cycle feedback.