Seasonal speedup of a Greenland marine-terminating outlet glacier forced by surface melt–induced changes in subglacial hydrology

Abstract

[1] We present subdaily ice flow measurements at four GPS sites between 36 and 72 km from the margin of a marine-terminating Greenland outlet glacier spanning the 2009 melt season. Our data show that >35 km from the margin, seasonal and shorter-time scale ice flow variations are controlled by surface melt-induced changes in subglacial hydrology. Following the onset of melting at each site, ice motion increased above background for up to 2 months with resultant up-glacier migration of both the onset and peak of acceleration. Later in our survey, ice flow at all sites decreased to below background. Multiple 1 to 15 day speedups increased ice motion by up to 40% above background. These events were typically accompanied by uplift and coincided with enhanced surface melt or lake drainage. Our results indicate that the subglacial drainage system evolved through the season with efficient drainage extending to at least 48 km inland during the melt season. While we can explain our observations with reference to evolution of the glacier drainage system, the net effect of the summer speed variations on annual motion is small (∼1%). This, in part, is because the speedups are compensated for by slowdowns beneath background associated with the establishment of an efficient subglacial drainage system. In addition, the speedups are less pronounced in comparison to land-terminating systems. Our results reveal similarities between the inland ice flow response of Greenland marine-and land-terminating outlet glaciers.

Recent Advances in Our Understanding of the Role of Meltwater in the Greenland Ice Sheet System

“…Sole et al [110] found transient seasonal acceleration and subsequent deceleration of ice flow in response to surface meltwater inputs to the bed at sites > 35 km inland from a tidewater glacier terminus in south-west Greenland, indicating parallels between land-and marine-terminating subglacial hydrology. For a number of stable tidewater glaciers in west Greenland, Howat et al [111] reached similar conclusions based on remote sensing of ice velocity 4-6 km from their termini.…”

Abrupt drainage cycles of the Fennoscandian Ice Sheet

“…3). Additionally, the ice-margin advance flushed out pressurized subglacial water from the glacier base, a phenomenon that occurs in modern settings (36) and that can be inferred from modeling (41). Through the reduction of subglacial water availability, both phenomena diminish subglacial buoyancy, leading to increased basal friction.…”

“…This triggered ice-margin thinning and acceleration (Fig. 3) and the attendant increased calving rate (34)(35)(36)(37) (Fig. 3).…”

“…3 shows a schematic view of the proposed mechanisms). Indeed, changes in the basal effective pressure (ice overburden minus subglacial water pressures) lead either to enhanced basal buoyancy and subsequent ice-sheet gravitational collapse when subglacial pressure approaches ice overburden pressure or, conversely, to ice-sheet stabilization (34)(35)(36)(37).…”

Sensitive response of the Greenland Ice Sheet to surface melt drainage over a soft bed

“…V ariations in the Greenland Ice Sheet (GrIS) flow have been observed on timescales varying from hours to years [1][2][3][4][5][6][7] . In particular, the sudden delivery of surface meltwater to the bed during supraglacial lake (SGL) drainage events drive pronounced although short-lived, accelerations in flow 1,2 .…”