Behn
Mark D.
Behn
Mark D.
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ArticleInfluence of ice-sheet geometry and supraglacial lakes on seasonal ice-flow variability(Copernicus Publications on behalf of the European Geosciences Union, 2013-07-26) Joughin, Ian ; Das, Sarah B. ; Flowers, G. E. ; Behn, Mark D. ; Alley, Richard B. ; King, Matt A. ; Smith, B. E. ; Bamber, Jonathan L. ; van den Broeke, Michiel R. ; van Angelen, J. H.Supraglacial lakes play an important role in establishing hydrological connections that allow lubricating seasonal meltwater to reach the base of the Greenland Ice Sheet. Here we use new surface velocity observations to examine the influence of supraglacial lake drainages and surface melt rate on ice flow. We find large, spatially extensive speedups concurrent with times of lake drainage, showing that lakes play a key role in modulating regional ice flow. While surface meltwater is supplied to the bed via a geographically sparse network of moulins, the observed ice-flow enhancement suggests that this meltwater spreads widely over the ice-sheet bed. We also find that the complex spatial pattern of speedup is strongly determined by the combined influence of bed and surface topography on subglacial water flow. Thus, modeling of ice-sheet basal hydrology likely will require knowledge of bed topography resolved at scales (sub-kilometer) far finer than existing data (several km).
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PreprintFracture propagation to the base of the Greenland Ice Sheet during supraglacial lake drainage( 2008-02-20) Das, Sarah B. ; Joughin, Ian ; Behn, Mark D. ; Howat, Ian M. ; King, Matt A. ; Lizarralde, Daniel ; Bhatia, Maya P.Surface meltwater that reaches the base of an ice sheet creates a mechanism for the rapid response of ice flow to climate change. The process whereby such a pathway is created through thick, cold ice has not, however, been previously observed. We describe the rapid (<2 hours) drainage of a large supraglacial lake down 980 m through to the bed of the Greenland Ice Sheet initiated by water-driven fracture propagation evolving into moulin flow. Drainage coincided with increased seismicity, transient acceleration, ice sheet uplift and horizontal displacement. Subsidence and deceleration occurred over the following 24 hours. The short-lived dynamic response suggests an efficient drainage system dispersed the meltwater subglacially. The integrated effect of multiple lake drainages could explain the observed net regional summer ice speedup.
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ArticleSeismicity on the western Greenland Ice Sheet : surface fracture in the vicinity of active moulins(John Wiley & Sons, 2015-06-25) Carmichael, Joshua D. ; Joughin, Ian ; Behn, Mark D. ; Das, Sarah B. ; King, Matt A. ; Stevens, Laura A. ; Lizarralde, DanielWe analyzed geophone and GPS measurements collected within the ablation zone of the western Greenland Ice Sheet during a ~35 day period of the 2011 melt season to study changes in ice deformation before, during, and after a supraglacial lake drainage event. During rapid lake drainage, ice flow speeds increased to ~400% of winter values, and icequake activity peaked. At times >7 days after drainage, this seismicity developed variability over both diurnal and longer periods (~10 days), while coincident ice speeds fell to ~150% of winter values and showed nightly peaks in spatial variability. Approximately 95% of all detected seismicity in the lake basin and its immediate vicinity was triggered by fracture propagation within near-surface ice (<330 m deep) that generated Rayleigh waves. Icequakes occurring before and during drainage frequently were collocated with the down flow (west) end of the primary hydrofracture through which the lake drained but shifted farther west and outside the lake basin after the drainage. We interpret these results to reveal vertical hydrofracture opening and local uplift during the drainage, followed by enhanced seismicity and ice flow on the downstream side of the lake basin. This region collocates with interferometric synthetic aperture radar-measured speedup in previous years and could reflect the migration path of the meltwater supplied to the bed by the lake. The diurnal seismic signal can be associated with nightly reductions in surface melt input that increase effective basal pressure and traction, thereby promoting elevated strain in the surficial ice.
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ArticleGreenland Ice Sheet flow response to runoff variability(John Wiley & Sons, 2016-11-12) Stevens, Laura A. ; Behn, Mark D. ; Das, Sarah B. ; Joughin, Ian ; Noel, Brice P. Y. ; van den Broeke, Michiel R. ; Herring, ThomasWe use observations of ice sheet surface motion from a Global Positioning System network operating from 2006 to 2014 around North Lake in west Greenland to investigate the dynamical response of the Greenland Ice Sheet's ablation area to interannual variability in surface melting. We find no statistically significant relationship between runoff season characteristics and ice flow velocities within a given year or season. Over the 7 year time series, annual velocities at North Lake decrease at an average rate of −0.9 ± 1.1 m yr−2, consistent with the negative trend in annual velocities observed in neighboring regions over recent decades. We find that net runoff integrated over several preceding years has a negative correlation with annual velocities, similar to findings from the two other available decadal records of ice velocity in western Greenland. However, we argue that this correlation is not necessarily evidence for a direct hydrologic mechanism acting on the timescale of multiple years but could be a statistical construct. Finally, we stress that neither the decadal slowdown trend nor the negative correlation between velocity and integrated runoff is predicted by current ice-sheet models, underscoring that these models do not yet capture all the relevant feedbacks between runoff and ice dynamics needed to predict long-term trends in ice sheet flow.
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ArticleConstraints on the lake volume required for hydro-fracture through ice sheets(American Geophysical Union, 2009-05-16) Krawczynski, Michael J. ; Behn, Mark D. ; Das, Sarah B. ; Joughin, IanWater-filled cracks are an effective mechanism to drive hydro-fractures through thick ice sheets. Crack geometry is therefore critical in assessing whether a supraglacial lake contains a sufficient volume of water to keep a crack water-filled until it reaches the bed. In this study, we investigate fracture propagation using a linear elastic fracture mechanics model to calculate the dimensions of water-filled cracks beneath supraglacial lakes. We find that the cross-sectional area of water-filled cracks increases non-linearly with ice sheet thickness. Using these results, we place volumetric constraints on the amount of water necessary to drive cracks through ∼1 km of sub-freezing ice. For ice sheet regions under little tension, lakes larger than 0.25–0.80 km in diameter contain sufficient water to rapidly drive hydro-fractures through 1–1.5 km of subfreezing ice. This represents ∼98% of the meltwater volume held in supraglacial lakes in the central western margin of the Greenland Ice Sheet.
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ArticleLimits to future expansion of surface-melt-enhanced ice flow into the interior of western Greenland(John Wiley & Sons, 2015-03-24) Poinar, Kristin ; Joughin, Ian ; Das, Sarah B. ; Behn, Mark D. ; Lenaerts, Jan T. M. ; van den Broeke, Michiel R.Moulins are important conduits for surface meltwater to reach the bed of the Greenland Ice Sheet. It has been proposed that in a warming climate, newly formed moulins associated with the inland migration of supraglacial lakes could introduce surface melt to new regions of the bed, introducing or enhancing sliding there. By examining surface strain rates, we found that the upper limit to where crevasses, and therefore moulins, are likely to form is ~1600 m. This is also roughly the elevation above which lakes do not drain completely. Thus, meltwater above this elevation will largely flow tens of kilometers through surface streams into existing moulins downstream. Furthermore, results from a thermal ice sheet model indicate that the ~1600 m crevassing limit is well below the wet-frozen basal transition (~2000 m). Together, these data sets suggest that new supraglacial lakes will have a limited effect on the inland expansion of melt-induced seasonal acceleration.