Greenland Ice Sheet surface melt captured by high frequency water vapor isotopes measurements at Thule

Recent studies of the isotopic composition of precipitation and water vapor have revealed a significant flux of water vapor off the Greenland Ice Sheet (GIS). This is coupled, in part, with the ever-increasing extent of surface melt of the ice sheet, marked by the extensive and early onset of the 2019 melt season. To fully comprehend the mass balance of the GIS, this flux of water vapor away from the ice sheet merits investigation and quantification. Measuring the deuterium excess of water vapor offers an effective approach to study evaporation and sublimation, as this parameter is controlled by changes in moisture sources. Here, we present measurements of hydrogen (δD) and oxygen (δ18O) isotopic ratios and deuterium excess (d- excess or d; d = δD - 8δ18O) of water vapor from Thule, NW Greenland. We focus on how the extent and timing of surface melt in the summers of 2015, 2017, 2018 and 2019 affect the isotopic composition. Water vapor sourced from sublimation of snow and ice and/or evaporation of meltwater has relatively high d-excess, especially compared to moisture evaporating from cold marine sources, like neighboring Baffin Bay. A dominant pattern in the data is the isotopic difference of water vapor blowing off the Greenland Ice Sheet (GIS) to the east and from northern Baffin Bay to the west, where easterly winds off the GIS bring water vapor with significantly higher d-excess than westerly winds. We further investigate water vapor fluxes from the ice sheet by examining only winds blowing off the ice sheet (i.e., winds with an easterly component). We find that increases in GIS surface melt extent produce significantly higher d- excess at Thule as more moisture is sourced from the GIS itself. The magnitude of the melt extent effect on d-excess is comparable to, if not greater than, other factors that also influence sublimation and evaporation, including temperature, relative humidity, and wind speed. We attempt to partition moisture sourced from sublimation of snow/ice and meltwater evaporation using a range of end members based on observation- and modeling-based studies. Finally, combining measurements of d-excess and humidity, we quantify the flux of water vapor sourced from the ice sheet as a result of increased melt extent. Our results demonstrate significant water content is lost from the GIS by atmospheric transport.