Atop ice sheets, vertical shafts called moulins funnel water melting on the surface to drain to the base. The draining water has lubricating effects that may cause the ice sheet to flow faster. By examining the Greenland Ice Sheet, researchers found that most of the moulins they investigated formed from large ice stresses. The lakes on the ice sheet surface draining through the half-mile-thick ice sheet to the bed of the ice sheet stresses caused the stresses.
The research shows that the sudden drainage of lakes atop glaciers and ice sheets can trigger moulin formation miles away. Moulins control where the majority of summer meltwater enters the ice sheet and the underlying bed, where it may speed ice flow. Scientists may need to consider lake drainage and moulin formation in detail in future models.
Moulins are holes that funnel water on the surface of a glacier or ice sheet to its base. There, the water acts like a lubricant and may cause the ice flow to speed up in the summer when melting occurs. Scientists have struggled to model moulins. They knew that forming a moulin in the Greenland Ice Sheet requires a crack on the surface to fill with enough water to drive the crack all the way through the half-mile-thick ice. Researchers expected moulins to form near the crevasse fields atop the sheet. However, a large fraction of moulins in Greenland forms away from the ice sheet’s crevasse fields. To understand why moulins formed where they did, researchers compared moulins mapped from images taken by space satellites to computer simulations of stresses in the ice. They constrained the model with hourly, on-site ice velocity measurements from local GPS stations. They found that the calculated stresses were not usually large enough to lead to a large drainage formation. However, fractures associated with large lakes on the ice surface were prone to rare but large drainage events, with the lake draining from the surface to the bedrock below in just a few hours. These infrequent but large events are more likely to cause the ice sheet flows over the bedrock to speed up. The large drainage events are also capable of triggering the formation of new moulins far from the lake site and the crevasse fields. This long-distance triggering mechanism could make new regions of the ice sheet vulnerable to faster motion, even at higher elevations.
This work was supported by a grant to M.J.H. from the Laboratory Directed Research and Development Early Career Research Program at Los Alamos National Laboratory. Support for M.P. and S.F.P. was provided by the Scientific Discovery through Advanced Computing program funded by the Department of Energy (DOE), Office of Science, Advanced Scientific Computing Research, and Biological and Environmental Research offices. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for DOE’s National Nuclear Security Administration. L.C.A. was supported by an appointment to the National Aeronautics and Space Administration (NASA) Postdoctoral Program at the Goddard Space Flight Center, administered by Universities Space Research Association under contract with NASA, and UTIG Ewing‐Worzel and Gale White Graduate Student Fellowships. J.J. was supported by National Science Foundation Office of Polar Programs. M.L. was supported by Swiss National Science Foundation. Satellite data access for this work provided by the Polar Geospatial Center under National Science Foundation Office of Polar Programs awards.
M.J. Hoffman, M. Perego, L.C. Andrews, S.F. Price, T.A. Neumann, J.V. Johnson, G. Catania, and M.P. Lüthi, “Widespread moulin formation during supraglacial lake drainages in Greenland.” Geophysical Research Letters 45, 778(2018). [DOI: 10.1002/2017GL075659]