The Greenland Ice Sheet harbours a wealth of microbial life, yet the total biomass stored or exported from its surface to downstream environments is unconstrained. Here, we quantify microbial abundance and cellular biomass flux within the near-surface weathering crust photic zone of the western sector of the ice sheet. Using groundwater techniques, we demonstrate that interstitial water flow is slow (~10-2 m d-1), while flow cytometry enumeration reveals this pathway delivers 5 × 108 cells m-2 d-1 to supraglacial streams, equivalent to a carbon flux up to 250 g km-2 d-1. We infer that cellular carbon accumulation in the weathering crust exceeds fluvial export, promoting biomass sequestration, enhanced carbon cycling, and biological albedo reduction. We estimate that up to 37 kg km-2 of cellular carbon is flushed from the weathering crust environment of the western Greenland Ice Sheet each summer, providing an appreciable flux to support heterotrophs and methanogenesis at the bed.

Bristol Glaciology Centre School of Geographical Sciences University of Bristol Bristol UK

Centre for Gas Hydrate Environment and Climate Department of Geosciences UiT The Arctic University of Norway Tromsø Norway

Department of Ecology Faculty of Science Charles University Prague Czechia

Department of Environmental Science Aarhus University Frederiksborgvej Roskilde Denmark

Department of Geography and Earth Sciences Aberystwyth University Aberystwyth UK

Department of Geography University of Sheffield Sheffield UK

Department of Glaciology and Climate Geological Survey of Denmark and Greenland Copenhagen Denmark

Geography Research Unit University of Oulu Oulu Finland

Institute at Brown for Environment and Society Brown University Providence RI USA

Institute of Biological Environmental and Rural Sciences Aberystwyth University Aberystwyth UK

Institute of Geography and Oeschger Center for Climate Change Research University of Bern Bern Switzerland

School of Geographical and Earth Sciences University of Glasgow Glasgow UK

School of Geography Politics and Sociology Newcastle University Newcastle upon Tyne UK

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