Seasonal Deuterium Excess Variations of Precipitation at Summit, Greenland, and their Climatological Significance

The hydrogen and oxygen isotopic composition of ice cores from Summit, Greenland, has provided invaluable information about variations in past climate. However, interpretations of these isotopic data have been made despite a paucity of direct isotopic studies of Summit precipitation. We provide insight to such interpretations by examining the annual cycle of deuterium excess (d-excess) in precipitation samples from Summit and by considering the climatic controls on the annual cycle. Precipitation was collected daily from July 2011 to September 2014 at heights of 1, 2, and 4m. The isotopic composition of precipitation sampled at 4m above the snow surface is free of contamination from blowing snow. Precipitation d-excess is high in the summer and low in the winter, a pattern opposite to that found at most high-latitude locations, where summer d-excess is low relative to winter. Low winter d-excess values at Summit can be explained by varying degrees of Rayleigh distillation of moisture sourced from isotopically similar marine sources. However, the observed summer d-excess maximum at Summit is anomalously high compared with other Arctic locations, and we propose that this is due to high d-excess moisture contributed by sublimation of surface snow on the Greenland Ice Sheet. We demonstrate the plausibility of this hypothesis through simple isotopic mass balance calculations, analyses of cloud heights, and back trajectories to identify moisture sources. We show that Rayleigh distillation, sublimation, and the phase of the d-excess annual cycle are all important factors that should be considered in ice core d-excess interpretations.