Seasonal icings reveal subsurface water drainage of a Greenland Ice Sheet outlet glacier
Rebecca McCerery, Joseph A. Graly, Christopher L. Hansen, Kate Winter, William P. Gilhooly III, Trinity L. Hamilton, Moses Jatta, Jeff R. Havig, Felipe Napoleoni, Alicia M. Rutledge, Matthew Terrell, Tanya M. Trott, Kayla Woodie, Bernd Kulessa, Sevi Modestou, Hannah Bahrani, Tori Kennedy, Alexandra Messerli, Kathy LichtIn many glacial settings, winter outflows of proglacial water create stratified domed ice structures on glacial forefields. These structures, called naledi, provide an opportunity to characterise solute concentrations in hydrological systems during winter, which are poorly constrained compared with summer outflows dominated by supraglacial meltwater. To characterise the different hydrological systems feeding naledi and provide conceptual models of their formation, we sampled 12 overwinter naledi and 4 summer ice accumulations (accreted ice) from the forefield of Isunnguata Sermia, a western outlet glacier of the Greenland Ice Sheet, during four field campaigns. Major ions and stable water isotopes reveal complex geochemical signatures, where the composition of successive naled layers indicates fluctuations in water source and transport throughout winter. In comparison, summer accreted ice is sediment laden and shows uniform geochemistry throughout, suggesting freeze-on of water from a single source in quick, discrete events. These findings are supported by Electrical Resistivity Tomography (ERT) geophysical surveys showing shallow hydrological pathways (1–5 m below the surface) in the glacial foreland (surveyed 400 m from the glacier terminus). These results inform the first conceptual models of summer vs. wintertime subglacial water routing and naled formation in the proglacial zone of the Greenland Ice Sheet. We show that wintertime naledi form from a combination of top-down (unconfined) and bottom-up (confined) freezing processes incorporating subglacial discharge, overland flow and precipitation. In contrast, accreted ice forms from upwellings of high-velocity, highly pressurised water which freezes upon release. These seasonal changes drive different geochemical and solute outputs, which are critical for evaluating glacier meltwater contributions to proglacial groundwater systems and the impact of glacial discharge on downstream ecosystems.