title: Direct observations of evolving subglacial drainage beneath the Greenland Ice Sheet.
abstract: Seasonal acceleration of the Greenland Ice Sheet is influenced by the dynamic response of the subglacial hydrologic system to variability in meltwater delivery to the bed via crevasses and moulins (vertical conduits connecting supraglacial water to the bed of the ice sheet). As the melt season progresses, the subglacial hydrologic system drains supraglacial meltwater more efficiently, decreasing basal water pressure and moderating the ice velocity response to surface melting. However, limited direct observations of subglacial water pressure mean that the spatiotemporal evolution of the subglacial hydrologic system remains poorly understood. Here we show that ice velocity is well correlated with moulin hydraulic head but is out of phase with that of nearby (0.3-2 kilometres away) boreholes, indicating that moulins connect to an efficient, channelized component of the subglacial hydrologic system, which exerts the primary control on diurnal and multi-day changes in ice velocity. Our simultaneous measurements of moulin and borehole hydraulic head and ice velocity in the Paakitsoq region of western Greenland show that decreasing trends in ice velocity during the latter part of the melt season cannot be explained by changes in the ability of moulin-connected channels to convey supraglacial melt. Instead, these observations suggest that decreasing late-season ice velocity may be caused by changes in connectivity in unchannelized regions of the subglacial hydrologic system. Understanding this spatiotemporal variability in subglacial pressures is increasingly important because melt-season dynamics affect ice velocity beyond the conclusion of the melt season.
authors.affiliation: 1] Institute for Geophysics, Jackson School of Geosciences, The University of Texas at Austin, Austin, Texas 78758, USA [2] Department of Geological Sciences, Jackson School of Geosciences, The University of Texas at Austin, Austin, Texas 78712, USA.
authors.forename: Lauren C
authors.initials: LC
authors.lastname: Andrews
authors.affiliation: 1] Institute for Geophysics, Jackson School of Geosciences, The University of Texas at Austin, Austin, Texas 78758, USA [2] Department of Geological Sciences, Jackson School of Geosciences, The University of Texas at Austin, Austin, Texas 78712, USA.
authors.forename: Ginny A
authors.initials: GA
authors.lastname: Catania
authors.affiliation: 1] Fluid Dynamics and Solid Mechanics Group, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA [2] NASA Goddard Space Flight Center, Greenbelt, Maryland 20771, USA.
authors.forename: Matthew J
authors.initials: MJ
authors.lastname: Hoffman
authors.affiliation: 1] Institute for Geophysics, Jackson School of Geosciences, The University of Texas at Austin, Austin, Texas 78758, USA [2] Department of Geological and Mining Engineering and Sciences, Michigan Technological University, Houghton, Michigan 49931, USA.
authors.forename: Jason D
authors.initials: JD
authors.lastname: Gulley
authors.affiliation: 1] Glaciology and Geomorphodynamics Group, Physical Geography Division, Department of Geography, University of Zürich, 8057 Zürich, Switzerland [2] Laboratory of Hydraulics, Hydrology and Glaciology, Swiss Federal Institute of Technology (ETH) Zürich, 8093 Zürich, Switzerland.
authors.forename: Martin P
authors.initials: MP
authors.lastname: Lüthi
authors.affiliation: Laboratory of Hydraulics, Hydrology and Glaciology, Swiss Federal Institute of Technology (ETH) Zürich, 8093 Zürich, Switzerland.
authors.forename: Claudia
authors.initials: C
authors.lastname: Ryser
authors.affiliation: Department of Earth Sciences, Dartmouth College, Hanover, New Hampshire 03755, USA.
authors.forename: Robert L
authors.initials: RL
authors.lastname: Hawley
authors.affiliation: NASA Goddard Space Flight Center, Greenbelt, Maryland 20771, USA.
authors.forename: Thomas A
authors.initials: TA
authors.lastname: Neumann