Matthew J. Burke

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

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.

Applications of Ground-Penetrating Radar to Glacial and Frozen Materials

Ground-penetrating radar is widely applied to the study of glacial and frozen materials. Significant areas of investigation include: 1) the water content of glaciers and frozen ground, often indicating the state of the thermal regime of glacier ice or permafrost, 2) route-ways for subglacial/subsurface drainage, 3) the internal structure of glacial and frozen materials, sometimes indicating flow properties of ice or permafrost, 4) the fracturing and deformation of frozen ground and glacier ice, and 5) inclusions in the sediment/ice matrix, such as salt water in sea ice, sediment in glacier ice and the ice content of frozen ground. This review of the application of GPR to glacial and frozen materials highlights recent scientific advances, assesses the limitations of the GPR technique for these environments and indicates possible future developments in the field.

The sedimentary architecture of outburst flood eskers: A comparison of ground-penetrating radar data from Bering Glacier, Alaska and Skeiðarárjökull, Iceland

We present ground-penetrating radar (GPR) profiles that reveal the sedimentary architecture of an esker deposited during a surge-associated outburst flood at the Bering Glacier, Alaska. The wide, up-flow end of the esker contains a transition from large backset beds to large foreset beds interpreted to reflect composite macroform development in an enlarged part of the conduit. By contrast, the narrow, down-flow portion of the esker is dominated by plane beds interpreted to have been deposited where the conduit was constricted and the flow was faster. A previously studied outburst esker at Skeidararjokull, Iceland, has a similar morphology and stratigraphic architecture. This suggests that outburst floods generate distinct depositional signatures in eskers, both in terms of morphology and sedimentary architecture. Identification of these distinct signatures in ancient eskers will help assess the paleohydraulic conditions under which ancient eskers formed and, by extension, the nature of meltwater drainage systems beneath the Laurentide and Eurasian ice sheets.

Structural controls on englacial esker sedimentation: Skeiðarárjökull, Iceland

Abstract We have used ground-penetrating radar (GPR) to observe englacial structural control upon the development of an esker formed during a high-magnitude outburst flood (jökulhlaup). The surge-type Skeiðarárjökull, an outlet glacier of the Vatnajökull ice cap, Iceland, is a frequent source of jökulhlaups. The rising-stage waters of the November 1996 jökulhlaup travelled through a dense network of interconnected fractures that perforated the margin of the glacier. Subsequent discharge focused upon a small number of conduit outlets. Recent ice-marginal retreat has exposed a large englacial esker associated with one of these outlets. We investigated structural controls on esker genesis in April 2006, by collecting >2.5km of GPR profiles on the glacier surface up-glacier of where the esker ridge has been exposed by meltout. In lines closest to the exposed esker ridge, we interpret areas of englacial horizons up to ~30m wide and ~10–15m high as an up-glacier continuation of the esker sediments. High-amplitude, dipping horizons define the base of esker materials across many lines. Similar dipping surfaces deeper in the profiles suggest that: (1) the dipping surfaces beneath the esker are englacial tephera bands; (2) floodwaters were initially discharged along structurally controlled englacial surfaces (tephra bands); (3) the rapid increase in discharge resulted in hydrofracturing; (4) establishment of preferential flow paths resulted in conduit development along the tephra bands due to localized excavation of surrounding glacier ice; and (5) sedimentation took place within the new accommodation space to form the englacial structure melting out to produce the esker.