SEDIMENTARY EVIDENCE FOR DRAINING SUBGLACIAL MELTWATER FROM THE LARSEN A EMBAYMENT, ANTARCTICA

Antarctic ice loss mostly occurs at the ice sheet’s margins, where grounded ice in contact with the underlying bed gives way to floating ice shelves. Basal melting and iceberg calving cause ice thinning and shrinking, which reduce the ability of ice shelves to impede the flow of upstream grounded ice, adding to sea-level rise. Understanding the fate of Antarctica’s ice shelves is vital to predicting future sea-level change. The role of subglacial meltwater, which flows from the ice sheet interior into ice shelf cavities and can melt the underside of the ice, is an understudied mechanism of ice shelf thinning. Since contemporary subglacial drainage is difficult to observe directly and few records of the pre-satellite era exist, sediments from beneath current and former ice shelves can be used to understand the history and styles of subglacial meltwater drainage. We present two legacy sediment cores (JGC20 and JGC23) collected from the Greenpeace Trough, Antarctica during the research cruise NBP0003. Cores were retrieved from an open marine environment following the disintegration of the Larsen A Ice Self (LAIS) in 1995. We combine high-resolution computed tomography imaging with particle size analysis, organic carbon composition, and mineral phase identification to reveal a laminated mud unit deposited as subglacial meltwater drained into a portion of the Greenpeace Trough occupied by a subglacial lake. Sediment grain size modes, clast counts, organic carbon isotopic composition, and clay mineral abundance distinguish this sedimentary unit from the overlying diamict deposited by grounded ice. Laminated bedforms developed as drainage phases oscillated between quiescent conditions and higher flows, while deformed laminae by dropstones archive periods of basal melting. The sedimentary sequence described here shows that subglacial drainage was a persistent process in the Holocene before the LAIS formed 10.7 ± 0.5 ka and sub-ice meltwater flow may induce basal melting. These results help improve our understanding of Antarctic subglacial hydrology before the rise of contemporary geophysical observations.