Dynamic subglacial meltwater history archived in Antarctic subglacial lake sediments

There is a paucity of records on decadal to millennial scales archiving the evolution of the Antarctic subglacial hydrologic system, leaving a gap in our knowledge of the links between meltwater drainage and ice behavior. We employed a suite of sedimentological and geochemical methods to assess the stratigraphic distribution of lithofacies and evaluate sedimentary processes from sediment cores collected from Mercer Subglacial Lake, located beneath the Mercer Ice Stream in West Antarctica. The composite 2.06 m sediment record consisted of massive-to-stratified diamict, massive muds, and laminated muds. Chloride concentrations indicate sediment porewater was primarily derived from glacial melt with sediment deposition in freshwater conditions. Whereas diamicts are associated with basal ice contact during ice stream grounding events (tills), sorted mud beds buried below diamict lack coarse-grained detritus (>2 mm) indicating deposition from suspension settling in slowly flowing or ponded meltwater. Rhythmically laminated sediments capping the sedimentary sequence capture modern subglacial lake conditions and are likely influenced by a continuum of processes controlled by suspended sediment delivered into the lake, water column velocity changes associated with fill-drain cycles, and sediment fallout from basal ice melt. These sedimentary facies characterize a complex subglacial hydrologic system providing evidence that basal conditions alternated from grounded ice to water-filled cavities fed and drained by subglacial meltwater. Our data provide new information on subglacial sediments beneath an Antarctic ice stream that can be used to refine our knowledge of subglacial hydrology, its coupling with ice dynamics, and as an analog for studying ancient glacial deposits.