Albedo-a primary control on surface melt-varies considerably across the Greenland Ice Sheet yet the specific surface types that comprise its dark zone remain unquantified. Here we use UAV imagery to attribute seven distinct surface types to observed albedo along a 25 km transect dissecting the western, ablating sector of the ice sheet. Our results demonstrate that distributed surface impurities-an admixture of dust, black carbon and pigmented algae-explain 73% of the observed spatial variability in albedo and are responsible for the dark zone itself. Crevassing and supraglacial water also drive albedo reduction but due to their limited extent, explain just 12 and 15% of the observed variability respectively. Cryoconite, concentrated in large holes or fluvial deposits, is the darkest surface type but accounts for <1% of the area and has minimal impact. We propose that the ongoing emergence and dispersal of distributed impurities, amplified by enhanced ablation and biological activity, will drive future expansion of Greenland's dark zone.

Centre for Arctic Gas Hydrate Environment and Climate Department of Geology University of Tromsø 9037 Tromsø Norway

Centre for Glaciology Department of Geography and Earth Sciences Aberystwyth University Aberystwyth SY23 3DB UK

Department of Ecology Faculty of Science Charles University 12844 Prague Czech Republic

Department of Geochemistry Geological Survey of Denmark and Greenland 1350 Copenhagen Denmark

Department of Geography University of California Los Angeles Los Angeles CA 90095 USA

Department of Geography University of Sheffield Sheffield S10 2TN UK

Department of Glaciology and Climate Geological Survey of Denmark and Greenland 1350 Copenhagen Denmark

Institute at Brown for Environment and Society Brown University Providence RI 02906 USA

Institute of Biological Environmental and Rural Sciences Aberystwyth University Aberystwyth SY23 3DB UK

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Storage and export of microbial biomass across the western Greenland Ice Sheet

Glacier Algae: A Dark Past and a Darker Future