Transition of flow regime along a marine-terminating outlet glacier in East Antarctica

Abstract

We present results of a~multi-methodological approach to characterize the flow regime of West Ragnhild Glacier, the widest glacier in Dronning Maud Land, Antarctica. A new airborne radar survey points to substantially thicker ice (> 2000 m) than previously thought. According to the new data, West Ragnhild Glacier discharges 13–14 Gt yr^&minus;1. Therefore, it is one of the three major outlet glaciers in Dronning Maud Land. Glacier-bed topography is distinct between the upstream and downstream section. In the downstream section (< 65 km upstream of the grounding line), the glacier overlies a wide and flat basin well below the sea level while the upstream region is more mountainous. Spectrum analysis of the bed topography reveals a clear contrast between these two regions, suggesting that the downstream area is sediment covered. The bed returned power varies by 30 dB within 20 km near the bed flatness transition, which suggests that water content at bed/ice interface increases over a short distance downstream, hence pointing to water-rich sediment. Ice flow speed observed in the downstream part of the glacier (~ 250 m yr^&minus;1) can only be explained if basal motion accounts for ~ 60% of the surface motion. All above lines of evidence (sediment bed, wetness and basal motion) and the relative flat grounding zone give the potential for West Ragnhild Glacier to be more sensitive to external forcing compared to other major outlet glaciers in this region which are more stable due to their bed geometry (e.g. Shirase Glacier)

The control of an uncharted pinning point on the flow of an Antarctic ice shelf

“…Callens and others (2014) showed that the downstream part of the West Ragnhild Glacier bed is wide, flat and covered by wet sediments, which induces basal sliding, thus a low coefficient C. At that place, the High-resolution scenario indicates a more slippery bed because of slightly higher (+50 m a −1 ) surface velocities between our flow field and that of Rignot and others (2011b). The sticky bed on Derwael Ice Rise and the western promontory concurs with low velocities and the negligible basal sliding in those areas.…”

“…Compared with ice shelves of other major outlet glaciers (e.g. Jutulstraulmen and Shirase Glacier), the RBIS flows more slowly (maximum speed ≈345 m a ) and ice is discharged through a comparatively wide gate at the grounding line (Callens and others, 2014).…”

“…This pinning point is absent in the bathymetry, ice thickness and surface elevation of Bedmap2 (Fretwell and others, 2013). The West Ragnhild Glacier drains ∼10% of the ice in Dronning Maud Land (13-14 Gt a −1 ; Callens and others, 2014) and is therefore among the major contributors to the ice discharge of Dronning Maud Land. Compared with ice shelves of other major outlet glaciers (e.g.…”

Topographic Shelf Waves Control Seasonal Melting Near Antarctic Ice Shelf Grounding Lines

“…The ApRES (Brennan et al, ; Nicholls et al, ) was deployed from January to December in 2016 on the RBIS (Figure ) about 90 km from the ice shelf front and 5 km seaward from the grounded ice on the fast‐flowing portion of the West Ragnhild glacier, which is the third largest outlet glacier along the Dronning Maud Land Coast (Callens et al, ). The ice thickness at the site was ∼300 m in the trough of a channel (Drews, ) but increases up to 600 m in the grounding zone upstream, and ice flow velocities in this region range between 250 and 300 m/a (Rignot et al, ).…”

“…Here, we present new and highly resolving time series of melt rates observed on the Roi Baudouin Ice Shelf (RBIS), which is part of the belt of slowly melting (Rignot et al, ) smaller ice shelves situated over the narrow continental shelf along the coast of Dronning Maud Land, East Antarctica (Figure ). The observations are taken near the grounding line where the bed lowers toward the interior of the ice sheet (Callens et al, ; Fretwell et al, ) and our analysis explores the oceanographic mechanisms that control basal melting in this region that is potentially susceptible to marine ice sheet instability (Favier et al, ).…”