Influence of subglacial geology on the onset of a West Antarctic ice stream from aerogeophysical observations

Abstract

Marine ice-sheet collapse can contribute to rapid sea-level rise 1 . Today, the West Antarctic Ice Sheet contains an amount of ice equivalent to approximately six metres of sea-level rise, but most of the ice is in the slowly moving interior reservoir. A relatively small fraction of the ice sheet comprises several rapidly flowing ice streams which drain the ice to the sea. The evolution of this drainage system almost certainly governs the process of ice-sheet collapse 2-5 . The thick and slow-moving interior ice reservoir is generally fixed to the underlying bedrock while the ice streams glide over lubricated beds at velocities of up to several hundred metres per year. The source of the basal lubricant-a watersaturated till 6,7 overlain by a water system 8 -may be linked to the underlying geology. The West Antarctic Ice Sheet rests over a geologically complex region characterized by thin crust, high heat flows, active volcanism and sedimentary basins 9-16 . Here we use aerogeophysical measurements to constrain the geological setting of the onset of an active West Antarctic ice stream. The onset coincides with a sediment-filled basin incised by a steep-sided valley. This observation supports the suggestion 5,17 that ice-stream dynamics-and therefore the response of the West Antarctice Ice Sheet to changes in climate-are strongly modulated by the underlying geology.We use satellite imagery to identify streaming ice in central West Antarctica 18 (Fig. 1). The 20-km-wide region, with distinct margins bordering featureless ice (Fig. 2a), is dominated by flow-parallel banding and surface undulations, characteristic of streaming ice 19,20 . Streaming has been confirmed by GPS (Global Positioning System) measurements of surface ice velocity along a seismic line crossing the southern margin (Fig. 2c) 21 . New aerogeophysical data support the ice-streaming identification. Airborne ice-penetrating radar profiles show disrupted internal layering between the identified margins (Fig. 3). In the satellite image, the flow-parallel bands begin downslope (west) of a prominent, dark 'S'-shaped feature. Airborne laser altimetry (Fig. 2b) shows that this 'S' is caused by a 100-m-high escarpment in the ice surface which marks a significant break in ice surface slope. Upstream (east) of the 'S', the ice surface has a mean slope over 25 km of 7 m km −1 which decreases to 3 m km −1 downstream. The corresponding driving stress for ice-sheet flow decreases from over 100 kPa upstream of the 'S' to 60 kPa downstream. We interpret the 'S' as the onset of streaming, and will refer to the portion of the ice stream downstream of this 'S' as the onset region.Ice-penetrating radar data (Fig. 2d) shows that the onset of streaming rests over low-lying southwest-dipping subglacial topography. The onset 'S' is bounded by a plateau to the north and a prominent ridge to the south. Two kilometres west (downstream) of the 'S', the ice stream flows in a subglacial valley bounded by steep

Initial results from geophysical surveys and shallow coring of the Northeast Greenland Ice Stream (NEGIS)

“…A 1200 yr record of sulfate, calcium, and sodium at the B20 site(78 • 50 N, 36 • 30 W; 2150 m a.s.l.) in northeast Greenland also demonstrated regular annual layer thicknesses of approximately 10 cm water equivalent (Bigler et al, 2002). The B20 core chronology was established primarily by the identification of volcanic deposition strata, although annual cycles in sulfate, calcium, and sodium were also detected and counted with a chronological uncertainty of ±5 yr between volcanic strata.…”

“…The ice core chronology obtained from NGRIP (Wolff et al, 2010) (Fischer et al, 1998b). The NGT results demonstrate consistently lower snow accumulation rates to the east than to the west of the Greenland ice divide, and also highlight anthropogenic sulfate and nitrate production during the 20th century (Fischer et al, 1998a;Bigler et al, 2002). A 1200 yr record of sulfate, calcium, and sodium at the B20 site(78 • 50 N, 36 • 30 W; 2150 m a.s.l.)…”

Stress balances of ice streams in a vertically integrated, higher-order formulation

“…For the steady-state stream solution, there is an oscillation between which side of the domain 'feeds' the stream, an instability also seen by Sayag and Tziperman (2011) when they use a very similar model setup. Benefits to this set-up include the fact that it is possibly more representative of the natural system, since pure ice streams often have prominent bedrock topography in their onset regions (Anandakrishnan and others, 1998;Bell and others, 1998).…”

Weak bed control of the eastern shear margin of Thwaites Glacier, West Antarctica

“…Subglacial geology (e.g. the presence of deformable till) also influences the position of ice-stream margins (Anandakrishnan and others, 1998;Bell and others, 1998; Blankenship and others, 2001; Studinger and others, 2001). Most of Thwaites Glacier's tributaries clearly originate within bedrock troughs (Holt and others, 2006; Fig.…”