Rapid Thinning at the Grounding Line of Northeast Greenland's N79 Glacier Revealed by New Elevation Reconstruction

Abstract:

Nioghalvfjerdsfjorden (N79) Glacier is one of the two outlets of the Northeast Greenland Ice Stream (NEGIS), the only large ice stream of the Greenland Ice Sheet, which drains approximately 12% of its surface area and containing a sea-level rise equivalent of 1.1 m. Until recently it was believed that N79's floating ice shelf would be stable for more than a century, but thinning was detected at altimetry flightline crossovers near its grounding line using the Surface Elevation Reconstruction and Change Detection (SERAC) method. To further explore this, we use the novel Mosaic Utility and Large Dataset Integration for SERAC (MOULINS) algorithm with high resolution digital elevation models (DEMs) from the Worldview satellites and altimetry from the Airborne Topographic Mapper (ATM) and ICESat to complete a high-accuracy annual reconstruction of the NEGIS outlet region between 2012 and 2017. Lagrangian elevation differences reveal a zone of thinning near the grounding line, especially concentrated in a channel extending from the grounding line and curving to meet a pair of lateral crevasses, which appeared between 2012 and 2014 and has basal thinning rates inferred from buoyancy that can exceed 50 meters per year. This basal reconstruction is consistent with available ice-penetrating radar profiles. The onset corresponds to N79's primary point of subglacial drainage, based on inferred basal hydropotential, suggesting turbulent entrainment of infiltrating warm Atlantic water as a probable cause of thinning; and the timing suggests anomalously high melt rates of 2012 as a likely trigger. Basal channels are thought to have contributed to the collapse of several Antarctic ice shelves, and thus we highlight this channel as an area of concern.