Greenland ice sheet hydrology: insights from an isotope mixing model during the 2011 and 2012 melt seasons

The Greenland Ice Sheet (GrIS) is the Northern Hemisphere's largest terrestrial permanent ice mass. Each summer, subglacial drainage systems are altered by sustained inputs of meltwater from the ice sheet surface (Bartholomew et al., 2010). Prior to the development of an efficient subglacial channel system, constrained and inefficient meltwater flow paths at the glacier-bed interface cause hydrological forcing that results in the fastest annual rates of acceleration. However, most land terminating glaciers in Greenland slow down appreciably after the development of an efficient subglacial hydrological system. We used naturally-occurring radioactive and stable isotopes (7Be, 222Rn, 18O, D) as passive flow tracers to construct a high-resolution time series model of meltwater sourced from recent surface snow, glacial ice and delayed-flow basal meltwater during 2011 and 2012. Our field site is Leverett Glacier, a large outlet glacier (600 km2) on the western Greenland margin that discharges through a single, large proglacial river. Our isotope mixing model is complemented by concurrent stream discharge measurements that constrain the magnitude, timing, and routing of meltwater discharge. Furthermore, global positioning systems (GPS) were deployed within the catchment area to continuously measure changes in ice velocity. Our model results show diurnal changes in each meltwater component. Furthermore, we observe episodic outburst events of delayed-flow basal meltwater, snowmelt, and ice melt. The volume of delayed-flow basal meltwater co-varies with ice velocity during both diurnal cycles and episodic speed up and uplift events. For example in July 2011, a large ice uplift and acceleration event was preceded by a decrease in delayed-flow basal meltwater, followed by ice acceleration and a release of an abnormally large volume of delayed-flow basal meltwater. Similar analyses will be presented for the 2012 melt season, which included extreme ice melting conditions in July as recorded by satellite measurements and stream discharge measurements at our field site. These results demonstrate the intimate relationship between subglacial hydrology and glacial dynamics in the GrIS. Bartholomew, I., Nienow, P., Mair, D., Hubbard, A., King, M.A., Sole, A., 2010. Seasonal evolution of subglacial drainage and acceleration in a Greenland outlet glacier. Nat Geosci 3, 408-411.