Measurement and Modeling of Firn Densification in the Percolation Zone of the Greenland Ice Sheet

We present a test of the hypothesis that firn densification rates in the upper percolation zone have strongly increased in recent decades due to infiltrating meltwater. During 2007-2009 we collected detailed firn stratigraphy and mass measurements along a 90 km transect of the percolation zone in western Greenland, extending from 2000-1400 m elevation. Thirty-three, 10 m firn cores were taken approximately 5 km apart along the transect. From ~1780-1600 m elevation, we observed average ice layer thickness increased from 9 cm to 26 cm. Average ice density was 843 kg/m3 with a standard deviation of 36.25 kg/m3. To quantify firn densification and melt water infiltration we sum each core’s water equivalent (density), producing load-depth profiles. Between 2000-1625 m elevation, load-depth profiles indicate minimal melt water infiltration, showing smoothly increasing density of ~20 kg/m3 per m depth, similar to a compaction/diffusion profile. From 1625-1330 m the profiles are ‘stair-stepped’, with sections of slowly increasing density and sections of ice layer density. Below ~1330 m elevation pore space was nearly filled with refrozen infiltrated melt water. Field measurements were compared to a numerical firn densification model driven by measured and scenario-based surface temperature fluctuations. A 1D heat equation is coupled to a model for grain-growth and compaction using empirically derived, temperature-dependent formulas and an Arrehnius-type relation. We also compare real and simulated firn density profiles to melt-day data inferred from passive microwave data. Our results have implications for determining relationships between surface melt days and melt water infiltration, the runoff limit, and for interpreting surface elevation changes of the Greenland Ice Sheet.