Extreme Melt Season Ice Layers Reduce Firn Permeability in Greenland's Interior

Seasonal surface melting significantly affects the mass balance of the Greenland Ice Sheet. Meltwater runoff both decreases surface mass balance and, where it reaches the glacier bed, impacts ice flow. In the accumulation zone, meltwater largely percolates into porous firn where it can be stored in firn aquifers or refreeze as ice lenses or ice slabs. In this way, the firn potentially delays the onset of runoff by decades. However, if perched impermeable horizons develop, such as the ice slabs observed near the equilibrium zone, runoff can initiate much more rapidly. While these ice slabs develop over multiple years, the extreme surface melt event in 2010 has been linked to extensive ice layers observed on a single transect in southwest Greenland at much higher elevations. This raises the possibility that individual extreme melt seasons might have long-lasting effects on interior firn storage capacity.

We present Greenland-wide ice-penetrating radar evidence of an extensive refrozen ice layer that formed in the near-surface firn of the shallow percolation zone following the extreme melt season in 2012. We characterize the spatial extent, lateral connectivity, and density of this melt layer and investigate their correlation with climate variables simulated by MAR. Repeated flights between 2013 and 2017 show this layer continued to densify and accrete new ice, suggesting it is sufficiently impermeable to concentrate new refreezing at or above its horizon. In northwest and southeast Greenland, we find evidence of down-glacier connections between this melt layer and ice slabs and a perched firn aquifer. Our results demonstrate that perched, low-permeability horizons can develop in the Greenland interior in a single extreme melt season. The dominant mechanism forming these layers varies regionally according to the surface melt magnitude, melt variability, firn cold content, and firn microstructure. Overall, interior ice layers reduce the meltwater storage capacity of the firn and likely aid the inland expansion of both ice slabs and firn aquifers, ultimately promoting lateral water flow over local storage. Extreme melt season ice layers could, therefore, be a key determinant of Greenland's future sea-level contribution, impacting both surface mass balance and hydrological controls on ice dynamics.