Abstract
Abstract. The grounding zone of Whillans Ice Stream, West Antarctica, exhibits an abrupt transition in basal properties from the grounded ice to the ocean cavity over distances of less than 0.5–1 km. Active-source seismic methods reveal the downglacier-most grounded portion of the ice stream is underlain by a relatively stiff substrate (relatively high shear wave velocities of 1100±430 m s−1) compared to the deformable till found elsewhere beneath the ice stream. Changes in basal reflectivity in our study area cannot be explained by the stage of the tide. Several kilometres upstream of the grounding zone, layers of subglacial water are detected, as are regions that appear to be water layers but are less than the thickness resolvable by our technique. The presence of stiff subglacial sediment and thin water layers upstream of the grounding zone supports previous studies that have proposed the dewatering of sediment within the grounding zone and the trapping of subglacial water upstream of the ocean cavity. The setting enables calibration of our methodology using returns from the floating ice shelf. This allows a comparison of different techniques used to estimate the sizes of the seismic sources, a constraint essential for the accurate recovery of subglacial properties. We find a strong correlation (coefficient of determination=0.46) between our calibrated method and a commonly used multiple-bounce method, but our results also highlight the incomplete knowledge of other factors affecting the amplitude of seismic sources and reflections in the cryosphere.
Highlights
Grounding zones mark the transition from grounded to floating ice, standing sentinel over much of the contribution of glaciers and ice sheets to sea level
Subsequent studies have attributed Rutford Ice Stream’s spring–neap velocity variability to changes in subglacial pore water pressure (Rosier et al, 2015), while on Rutford and elsewhere others have pointed to contact with ice shelf pinning points and at the grounding zone as the causes of observed velocity changes (Robel et al, 2017; Minchew et al, 2017; Rosier and Gudmundsson, 2020)
Vs, and ρ values retrieved from Zoeppritz fitting exhibit a abrupt change at the grounding zone
Summary
Grounding zones mark the transition from grounded to floating ice, standing sentinel over much of the contribution of glaciers and ice sheets to sea level. In the Antarctic, tidally induced migration of the grounding line within the grounding zone varies from near zero in the case of abrupt changes in bed elevation and/or ice thickness to up to 10 km in the case of gently sloping ice plains (Brunt et al, 2011; Dawson and Bamber, 2020). Subsequent studies have attributed Rutford Ice Stream’s spring–neap velocity variability to changes in subglacial pore water pressure (Rosier et al, 2015), while on Rutford and elsewhere others have pointed to contact with ice shelf pinning points and at the grounding zone as the causes of observed velocity changes (Robel et al, 2017; Minchew et al, 2017; Rosier and Gudmundsson, 2020)
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