Our overarching goal is to assess the potential contribution of East Antarctica to sea-level rise.  Large inland basins beneath the East Antarctica Ice Sheet that are below sea level are connected to outlet glaciers (Figure 1) that have potential to drain inland ice when the ice shelves surrounding Antarctica disintegrate. This is important because the Antarctic ice sheet has potential to contribute tens of meters of sea-level rise. This would have major societal impacts, especially since much smaller changes in sea level are already impacting low-lying areas in Florida, Southeast Asia, and islands in the tropical Pacific.

In this project, the primary focus was Beardmore Glacier, one of ten large outlet glaciers that drain East Antarctic ice through the Transantarctic Mountains into the southern Ross Sea Embayment (Figure 2). Today the glacier drains a catchment of ~90,000 km². At the grounding line (the location where inland ice that is grounded on bedrock starts to float), it is 25km wide, mean surface velocities are 360 m/year, and the modern flux of ice is 7 Gt/year. Glacial geologic data from Mt Kyffin (Figure 2) indicate that ice in the vicinity of the modern grounding line 17,000 years ago during the last ice age was ~950 m thicker than present.

To achieve the primary goal of the project our field team of five people spent two field seasons (austral summers of 2012-13 and 2013-14) on Beardmore Glacier conducting airborne and ground-based radar surveys, GPS and passive seismic surveys, and active seismic experiments (Figure 3).

Airborne and ground-based radar measurements across and along Beardmore Glacier show (i) bed elevation at the grounding line is about 1100m below present-day sea level, (ii) bed elevation 70km up glacier from the grounding line (near Cloudmaker) is more than 2200m below sea level, (iii) bed elevation in the upper reaches of the glacier (~230km up from the grounding line) rises above sea level (Figure 4). Furthermore, our new bed topography differs significantly from the earlier Antarctic Bedmap2 compilation, which iss data limited (Figure 5). Importantly, the inland sloping bed raises the possibility that the lower 70km of the glacier is susceptible to runaway collapse either if the glacier at the grounding line thins slightly, and/or if buttressing of the lower glacier was removed through disintegration of the Ross Ice Shelf. However, the high bed threshold (above sea level) in the upper reaches of the glacier limits the amount of East Antarctic ice that can be drawn down through this outlet.

Our ground-based radar profiles across the Beardmore grounding zone reveal a melt-water channel incised into the bottom of the floating ice shelf. The channel originates from beneath the glacier above the grounding line and enlarges rapidly downstream, likely because of convection-driven melting as the fresh melt-water entrains relatively warm ocean water as it rises along the bottom of the ice shelf. Beneath the glacier, the channel is 200m wide and incised 60m into the base of the glacier; 8.2km down-glacier from the grounding line, the melt-water channel is 750m wide and incised 260m into the bottom of the shelf (Figure 6). Our direct evidence for the existence of liquid water beneath Beardmore glacier is surprising; it implies a relatively high geothermal flux (~75 mW/m²), and it enables basal sliding, fast flow and rapid evacuation of inland ice.

Seismic emissions in the grounding zone are high, with up to 40 events/hour recorded using a network of passive seismometers. Our GPS and seismic data show tidal modulation of both flow speed and seismicity; seismic events are split evenly between falling and rising tide. Beamforming analyses reveal that most events originate from distinct two locations. Events at both locations occur during falling tides, but most of the rising-tide events occur at one location. The timing and location of events is likely a result of changes in strain rates as ice flows across the grounding line around Mt. Hope with extensional flow on the outside and compressional flow on the inside. Seismic events that occur on falling tides arise from both the increase in horizontal velocity along the ice shelf and also vertical stretching as ice shelf elevation decreases. Flow speeds indicate similar horizontal velocity increases in a specific region of the glacier during rising tide. Our results show how coupled geodetic and seismic observations in grounding zones provide important insight into mechanical interactions and processes when outlet glaciers discharge into ice shelves.

The project provided research experience, training and support for four graduate students from the University of Washington and one from Central Washington University. Research results have been disseminated through manuscripts and presentations at national and international meetings. Seismic and radar data and metadata are publically available through the Global Change Master directory: https://gcmd.gsfc.nasa.gov/search/Metadata.do?entry=USAP-1141866&subset=GCMD#metadata and https://gcmd.gsfc.nasa.gov/search/Metadata.do?entry=USAP-1141889

Last Modified: 09/27/2018
Modified by: Howard B Conway