External forcing modulates Pine Island Glacier flow
Monday, 14 December 2015: 09:00
3007 (Moscone West)
Nearly 50 years ago, Mercer first suggested the Eemian sea-level high stand was a result of a collapse of the marine portions of the West Antarctic ice sheet. Recently, special attention has been paid to West Antarctica’s Amundsen Sea Embayment due to its steeply sloping retrograde beds that are well below sea level, and observations of rapid grounding-line retreat, high ice-shelf basal-melt rates, and basin-wide glacier thinning and acceleration. Despite this focus, accurate assessments of the past and future behavior of this embayment remain elusive due to a lack of understanding of calving processes and ice–ocean interactions. Here we present a continuous two-year (2012-2014) time series of oceanographic, borehole, glaciological, and seismological observations of Pine Island Glacier ice shelf, its sub-ice ocean cavity, and the adjacent Amundsen Sea. With these data, we captured the ice shelf’s response to a large fluctuation in the temperature of the water (~1 °C) entering the sub-ice-ocean cavity. Initially, the ice shelf slowed by 5%, but, by the end of 2014, it had nearly recovered its earlier speed. The generally smooth changes in ice flow were punctuated by rapid (2-3 week), high-amplitude (~2.5% of the background speed) speedups and slowdowns. Satellite and seismological observations indicate that rapid speedups are caused by reduction of lateral drag along the ice stream’s shear margins as a large iceberg calves and that rapid slowdowns may be due to periodic regrounding on bed highs at low tide. Coupled ice-stream/ice-shelf/ocean-plume flowband modeling informed by these new data indicates that the more-gradual changes in speed are related to ocean temperature, ice-front position, and past ice-flow history. Our observations highlight an ice shelf’s rapid response to external forcings and that past ice-flow behavior affects subsequent ice response to external forcing. Thus, long-term, multifaceted investigations are necessary to determine whether a glacier/ice-shelf system is responding to external forcing or undergoing a terminal collapse due to the marine ice-sheet instability.