Reconstructing physical and hydrologic processes beneath the Mercer Ice Stream, Antarctica using subglacial sediments

Abstract

The West Antarctic Ice Sheet's (WAIS) response to warming ocean and atmospheric temperatures is of scientific interest because of its potential contributions to global sea level rise. Beneath the ice sheet is a dynamic hydrologic network of channels, lakes, and groundwater reservoirs that in part lubricate the base, supporting fast ice flow, and have a profound effect on ice sheet dynamics. Expanding our records of the temporal and spatial variability of subglacial hydrology and basal conditions will improve our understanding to the future behavior of the ice sheet. I employed a suite of sedimentological, geochemical, and image processing analyses to reconstruct past basal conditions and meltwater activity at Mercer Subglacial Lake (SLM), West Antarctica. A composite 2.06 m sediment record comprised massive-to-stratified diamicts, massive muds, and laminated muds with drop stones. I interpret the lithostratigraphic variability to reflect the emplacement of glacial tills interbedded with meltwater drainage deposits and capped by rhythmically laminated subglacial lake sediments. The meltwater sediments were deposited by suspension settling in a slowly flowing or ponded setting. The rhymically laminated lake sediments were produced by changes in the sedimentation rate and sorting of suspended sediment transported into the lake and fallout of material from melting basal ice. Borehole camera imagery revealed a ~5 m thick accreted basal ice sequence directly above the ice-lake water interface. Image analysis of the borehole wall imagery demonstrated that the basal ice layer was composed of two main ice sequences and were differentiated by sediment debris content. The lower sequence contained up to 25 times more sediment than the upper unit. The observed stratigraphic variability in basal ice sequence represents contrasting periods of meltwater availability that range from meltwater abundant to restrictive. Meltwater availability also determines the style and volume of sediment incorporation into basal ice, forming two ice types. Collectively, the two geologic records presented here provide a complex archive of subglacial hydrologic conditions and thermal histories that are linked to past ice stream variability within the late Holocene.