Enhanced Firn Densification in High-Accumulation Shear Margins of the NE Greenland Ice Stream

Firn thickness across the NE Greenland Ice Stream is a function of accumulated strain, with thinner firn in the high-strain margins of the ice stream. We present a novel technique for extracting firn density from previously collected seismic reflection profiles and apply this technique across both shear margins of NE Greenland Ice Stream. Firn is up to 30m thinner in the vicinity of the ice stream shear margins. Snow accumulation rates across the ice stream were calculated from airborne ice-penetrating radar data, calibrated with ground-based firn density measurements from a shallow core. We find that accumulation is 20% higher in the shear margins compared to the surroundings. The higher density firn adjacent to shear margins is due to high along-flow stresses that accelerate firn densification and develops despite the higher accumulation rate favoring lower density. These firn density variations influence subglacial hydropotential by changing the ice surface slope and overburden pressure and may influence subglacial water flow. These results demonstrate the importance of high-resolution firn surveys in studies of shear-margin dynamics. Plain Language Summary Snow that falls on ice sheets is squeezed to ice under its own weight. Old snow during its transformation to ice is called firn. Ice sheets in Greenland and Antarctica are covered in 10-100m of firn, and many studies on ice sheets require knowledge of firn properties. Observations of the firn, however, are logistically difficult and expensive to make. We present a new technique for estimating firn properties from previously collected seismic surveys. We then use this technique to show that firn is unexpectedly thin along the edges of the fast-moving Northeast Greenland Ice Stream. Ice and firn stretch as they move into the ice stream and speed up. This causes the firn to change to ice more rapidly, contributing to formation of subtle troughs in the surface along the edge of the ice stream, which trap some extra drifting snow. Properly accounting for this extra snow and the thinner firn allows us to more accurately calculate the weight of the ice. Water under the ice moves to where there is the least weight above it. Ice moves more rapidly where more water lubricates the bed, so this new knowledge contributes to better understanding of the motion of the ice stream.