Shallow Fracture Buffers High Elevation Runoff in Northwest Greenland

The growth of shallow, low-permeability ice slabs in Greenland’s firn is known to increase surface meltwater runoff by hindering vertical percolation. However, the partitioning of meltwater between local impoundment, downslope runoff, and drainage to the ice sheet bed is still poorly constrained. Northwest Greenland is a particularly interesting study area for understanding the role of englacial hydrology because ice-penetrating radar surveys have identified coexisting ice slabs and firn aquifers in this region. These results suggest that ice slabs may not necessarily preclude local firn water storage. However, the mechanism that would allow these two distinct facies to develop together is unclear.

               To examine the relationship between firn aquifers and ice slabs in Northwest Greenland, we analyzed six-years of NASA Operation IceBridge radar data between 2011 and 2017. These observations show that isolated, short-lived water pockets frequently develop beneath the ice slabs and over time refreeze to form kilometer-scale ellipsoidal buried ice masses. These ice blobs covered ~14% of the 1176 radar line-kms flown in 2017 and analysis of Landsat imagery between 2000 and 2016 shows they are spatially correlated with visible runoff in supraglacial lakes, streams, or massive slush swamps. High-resolution optical satellite imagery also shows that surface crevassing is widespread in this region and that many of the ice blobs are associated with moulins or lake drainage events in the 2012 melt season. This suggests that ice blobs form where fractures create high permeability pathways through the ice slab through which surface meltwater can drain into the relict firn. In the short term, this process impounds water and heat in the upper 30 meters of the ice column, reducing surface runoff and limiting the immediate impact of surface melt on local ice dynamics. However, on longer timescales, this efficient filling of pore space beneath supraglacial flow paths may lead to more efficient surface hydrology and full ice thickness hydrofracture at high elevations.