Investigating Antarctic subglacial hydrologic processes from marine sediment cores

Understanding processes operating beneath glacial ice is imperative for holistic knowledge of glaciated regions and for accurately projecting future behavior of and sea-level contributions from the Antarctic Ice Sheet. Evolution of subglacial hydrologic networks is one such process where knowledge gaps persist, in large part because of the challenges in observing the subglacial environment and the subsequent paucity of empirical data spanning decadal to millennial temporal scales. Sediment cores from deglaciated continental shelves preserve evidence of subglacial drainage events into the ocean, providing a link between the marine geologic record and the subglacial hydrologic network through time. Yet, several analytical methods with the potential to reveal details of subglacial sedimentary processes and paleo-subglacial hydrological networks from meltwater plume deposits were heretofore unexplored. Five sediment cores from the calving margin of Thwaites Glacier, the most rapidly changing glacial system in West Antarctica today, are used to reconstruct a local meltwater drainage history through the Holocene (Chapter 2, Lepp et al., 2022). Trace elemental ratios for thick meltwater plume deposits reveal persistence, and transience, of subglacial drainage pathways over millennia, contextualizing modern observations. Core stratigraphy revealed by computed tomography scanning reflects distinctive styles of meltwater evacuation beneath eastern and western Thwaites and suggest that higher-magnitude drainage events have occurred in recent centuries compared to past millennia. Such events likely enhance ocean-driven melting, and imply existing models underestimate both ice-shelf melt rates and volume of freshwater input to the ocean in this rapidly changing region of the cryosphere. In Chapter 3, knowledge gaps around production of meltwater plume deposits and subglacial sedimentary processes are addressed by assessing grain shape and microtexture (Lepp et al., preprint). Silt-sized sediments from meltwater plume deposits and glacial diamictons from six relict and extant glacial systems in both hemispheres are quantified with metrics describing grain regularity and form, and the most significant grain-shape alteration between populations is observed in grains from systems with widespread melting of the ice surface. Grain surface textures provide comprehensive evidence that meltwater silts are produced widely through subglacial, rather than hydrological, transport. Grain micromorphology is thus a valuable addition to glaciomarine sediment analyses and can support experimental efforts to quantify sedimentary signatures of subglacial stress and deformation. Ice shelves are sensitive to subglacial drainage, but reconstructing plume activity through the Holocene is challenging. Isotopic evidence of subglacial meltwater discharge into the ocean is investigated using stable water isotopes from sediment porewater and the ratio of meteoric to lithogenic beryllium in sediments in Chapter 4 (Lepp et al., in prep.). Isotopic diffusion is unable to explain the observed downcore variation in porewater composition. The most depleted porewater, indicating a glacial source, are collected from ice-proximal, clay-rich diamictons, while elevated 10Be/9Be ratios are seen in meltwater plume deposits in ice-proximal and more distal settings. These results suggest that an isotopic approach could be appropriate towards quantifying fluxes sediment into the ocean and mixing of water masses beneath ice shelves. The findings herein advance our ability to link glaciomarine sediment records to subglacial processes, hydrology, and ice-marginal response to subglacial discharge. While this work focuses on late Pleistocene and Holocene-age records, the sedimentological and geochemical methods we employ can be used to examine drill core records from earlier glacial periods, moving towards a more holistic understanding of Antarctic subglacial hydrologic evolution and behavior that are necessary for improving accurate projections of mass loss from the Antarctic Ice Sheet.