14 October 2014

Direct Observations of Evolving Subglacial Drainage Beneath the Greenland Ice Sheet

Summary


The Greenland Ice Sheet speeds up every summer as melt from the surface penetrates km-thick ice through moulins, vertical shafts melted through the ice. Water reaching the bed of the ice sheet lubricates the ice-bed interface, causing the ice sheet to accelerate each summer. Greater melt is predicted for Greenland in the future, but its impact on ice sheet flux and associated sea level rise is uncertain; direct observations of the subglacial drainage system are lacking and its evolution over the melt season is poorly understood.

We use the first simultaneous measurements in Greenland of moulin and borehole hydraulic head and ice velocity to investigate the ice-sheet response to water-pressure changes within the subglacial hydrologic system. Ice velocity is well correlated with moulin hydraulic head but out of phase with that of nearby boreholes. Moulins connect to an efficient, channelized component of the subglacial drainage system, which exerts the primary control on diurnal and short-term multi-day changes in ice velocity. Boreholes in our study monitor a hydraulically-isolated component of the subglacial drainage system that responds primarily to changes in the channelized system via ice flow coupling. The ability of moulin-connected channels to convey supraglacial melt does not increase during the latter part of the melt season, failing to explain decreasing trends in ice velocity. Instead, decreased velocity is likely caused by changes in connectivity in hydraulically-isolated regions of the subglacial drainage system.

Our observations identify a previously unrecognized role of changes in hydraulically-isolated regions of the bed in controlling evolution of subglacial drainage over summer. Understanding this process will be crucial for predicting the effect of increasing melt on summer speed-up and associated autumn slowdown of the ice sheet into the future.

 

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Acknowledgments

This project was supported by United States National Science Foundation grants OPP-0908156, OPP-0909454 and ANT-0424589 (to CReSIS), Swiss National Science Foundation grant 200021_127197, and National Geographic Society grant 9067-12. L.C.A. was also supported by UTIG Ewing-Worzel and Gale White Graduate Student Fellowships. M.J.H. was also supported by NASA Cryospheric Sciences and Climate Modeling Programs within the US Department of Energy, Office of Science. J.D.G. was also supported by an NSF Postdoctoral Fellowship (EAR-0946767). Logistical support was provided by CH2MHill Polar Services. The GPS base station and several on-ice GPS units were provided by the UNAVCO facility with support from the NSF and NASA under cooperative agreement EAR-0735156. The University of Minnesota Polar Geospatial Center, funded under NSF OPP collaborative agreement ANT-1043681, provided WorldView imagery. We thank K. M. Schild,
J. A. MacGregor, J. D. Nowinski, B. F. Morriss and others for assistance in the field.