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Direct observations of evolving subglacial drainage beneath the Greenland Ice Sheet

Nature volume 514, pages 80–83 (2014)Cite this article

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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 bed1,2 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 efficiently1,2,3,4, decreasing basal water pressure4 and moderating the ice velocity response to surface melting1,2. However, limited direct observations of subglacial water pressure4,5,6,7 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 season8,9,10.

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Figure 1: Study area in the ablation zone of the western Greenland Ice Sheet.
Figure 2: Borehole and moulin hydraulic head and ice-surface velocity during 2011 and 2012.
Figure 3: Relationships between hydraulic head and ice-surface velocity.

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Acknowledgements

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.

Author information

Authors and Affiliations

  1. Institute for Geophysics, Jackson School of Geosciences, The University of Texas at Austin, Austin, 78758, Texas, USA

    Lauren C. Andrews, Ginny A. Catania & Jason D. Gulley

  2. Department of Geological Sciences, Jackson School of Geosciences, The University of Texas at Austin, Austin, 78712, Texas, USA

    Lauren C. Andrews & Ginny A. Catania

  3. Fluid Dynamics and Solid Mechanics Group, Los Alamos National Laboratory, Los Alamos, 87545, New Mexico, USA

    Matthew J. Hoffman

  4. NASA Goddard Space Flight Center, Greenbelt, 20771, Maryland, USA

    Matthew J. Hoffman & Thomas A. Neumann

  5. Department of Geological and Mining Engineering and Sciences, Michigan Technological University, Houghton, 49931, Michigan, USA

    Jason D. Gulley

  6. Physical Geography Division, Department of Geography, Glaciology and Geomorphodynamics Group, University of Zürich, 8057 Zürich, Switzerland,

    Martin P. Lüthi

  7. Laboratory of Hydraulics, Hydrology and Glaciology, Swiss Federal Institute of Technology (ETH) Zürich, 8093 Zürich, Switzerland,

    Martin P. Lüthi & Claudia Ryser

  8. Department of Earth Sciences, Dartmouth College, Hanover, 03755, New Hampshire, USA

    Robert L. Hawley

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Contributions

G.A.C., J.D.G., M.P.L., R.L.H. and T.A.N. designed this study. L.C.A., R.L.H., M.J.H., M.P.L., C.R. and J.D.G. performed the fieldwork. L.C.A. analysed the results and wrote the manuscript. All authors discussed the results and edited the manuscript.

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Correspondence to Lauren C. Andrews.

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Extended data figures and tables

Extended Data Figure 1 Borehole and moulin head and ice-surface velocity over two melt seasons.

a, 2011 measurements from FOXX moulin (blue), borehole 7 (dark red), borehole 6 (pink) and borehole 4 (red). b, 2012 measurements from moulin 3 (navy) and moulin 4 (light blue). c, 2011 ice velocity (black) and bed separation (green) for FOXX. Peak velocity on day 182 is 402.3 m yr−1 (exceeding the y-axis limit). d, 2012 ice velocity and bed separation for FOXX and 25N1 (grey, light green). Peak velocity for FOXX and 25N1 (312 m yr−1 and 337 m yr−1) occurred on day 173. e, f, 6-h averaged air temperature for 2011 and 2012. Grey bars are melt events.

Extended Data Figure 2 Components of vertical motion 2011 and 2012.

a, Components of vertical motion for 2011 at FOXX. Bed parallel motion (; black), strain thickening and thinning (; blue), elevation with measured winter elevation removed (red), and calculated bed separation (green). b, Components of vertical motion for 2012. Colours as in a for FOXX. Lighter colours correspond to components of vertical motion from 25N1.

Extended Data Figure 3 Modelled hydraulic head and conduit geometry.

a, Moulin 3 head (blue) and supraglacial input (red) are model inputs. Predicted downstream head is calculated from equation (8) (green). Subglacial discharge (black) is calculated as a function of head change and supraglacial inputs. It does not vary significantly from supraglacial input even when a large (∼20 m2) moulin geometry is used. b, Modelled subglacial channel cross-sectional area (black) changes rapidly (grey) during and shortly after expected melt events (grey bars).

Extended Data Figure 4 Seasonal relationship between moulin head and ice velocity for 2012 and 2011.

Moulin hydraulic head and associated ice velocity data plotted every 15 min over the course of the measurement periods for 2011 (a) and 2012 (b). 2011 data are truncated below 543 m by the high elevation of the moulin sensor.

Extended Data Figure 5 Borehole hydraulic heads and ice velocity at GULL during 2011.

Three hydraulic head records (red, yellow, blue) from boreholes located ∼0.5 km from a moulin. Ice velocity from GULL GPS (black), ∼0.75 km south of GULL boreholes.

Extended Data Table 1 Site coordinates and characteristics
Full size table
Extended Data Table 2 Cross-correlation analysis
Full size table
Extended Data Table 3 Parameters used in conduit-geometry calculations
Full size table

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Andrews, L., Catania, G., Hoffman, M. et al. Direct observations of evolving subglacial drainage beneath the Greenland Ice Sheet. Nature 514, 80–83 (2014). https://doi.org/10.1038/nature13796

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