Constraints on meltwater infiltration in cold firn from measurements and modeling through the full thickness of western Greenland’s percolation zone

Abstract:

The fate of meltwater generated in the Greenland Ice Sheet’s percolation zone has important implications for the ice sheet’s mass balance. As little as 50% of annual melt may run off, thereby contributing to ice sheet mass loss. But partitioning of melt in to runoff/refreezing components is highly uncertain, depending on processes controlling meltwater infiltration and firn capacity that require improved understanding. Measurements of firn structure and meltwater infiltration are rare, and commonly restricted to shallow depths that capture only a fraction of the total pore space. Here, we summarize findings from ongoing field and modeling activities conducted through the full firn package in Greenland’s percolation zone. Our modeling and interpretation is guided by measurements of temperature, ice content, and density in a transect of deep (>30m) cores and hot water boreholes drilled through the full firn package at six sites in 2018-2019 extending >35 km along the EGIG line. Hot water drilling also created perched aquifer conditions, permitting novel experimentation of water infiltration limits in dense firn.

We find: (A) firn capacity at depth can be a sensitive function of horizontal ice flow interactions with topography and climate gradients; (B) grain-scale water infiltration and heat flow constraints in dense firn result in a close-off density for water that is less than the close-off density for air; (C) meltwater infiltration from heterogeneous fingering can penetrate through many years of accumulated firn, refreezing in amounts that are comparable to a uniform wetting front. These results reveal the importance of piping events in distributing meltwater to depth, but also identifies physical constraints on the limit of infiltration in dense firn.