Snowpack and Firn Densification Gradients in the Percolation Zone of the Greenland Ice Sheet: Implications for Estimates of ice Sheet Mass Balance

The proportion of surface generated meltwater that subsequently refreezes in the snowpack and firn plays a critical role in controlling the mass balance of polythermal ice masses. In Greenland, changes in the volumes of meltwater that refreeze in the superimposed and percolation zones are likely in response to any future climate change. However, processes of meltwater percolation and refreezing may cause a re-distribution of mass through densification, resulting in surface elevation change but with no associated mass loss. Seasonal densification gradients caused by variations in altitude (and thus temperature) must therefore be accounted for to reduce errors associated with mass balance estimates derived from satellite and airborne altimetry platforms. Here, we investigate the spatial variability in seasonal densification in the snowpack and firn along a 57 km transect of the EGIG line (~67.5N, 47W) in the percolation zone of the Greenland Ice Sheet. Snowpack and firn densities (derived from shallow cores) were obtained from seven sites along the transect at approximately 10 km intervals and spanning a 350 m elevation range between 1680 and 2050 m. As expected, there was no significant variation in spring snowpack density (prior to melt) but following the summer processes of melt, percolation and refreezing, density decreased by 36 kg m-3 for every 100 m increase in elevation. Measurements over three melt-seasons at three sites spanning an 155 m elevation range reveal substantial variability in the seasonal densification gradient. The implications of the observed densification gradients for ice sheet elevation and thus mass balance estimates in a warming climate are discussed.