Robert A. Bindschadler

Subglacial melt channels and fracture in the floating part of Pine Island Glacier, Antarctica

[1]xa0A dense grid of ice-penetrating radar sections acquired over Pine Island Glacier, West Antarctica has revealed a network of sinuous subglacial channels, typically 500xa0m to 3xa0km wide, and up to 200xa0m high, in the ice-shelf base. These subglacial channels develop while the ice is floating and result from melting at the base of the ice shelf. Above the apex of most channels, the radar shows isolated reflections from within the ice shelf. Comparison of the radar data with acoustic data obtained using an autonomous submersible, confirms that these echoes arise from open basal crevasses 50–100xa0m wide aligned with the subglacial channels and penetrating up to 1/3 of the ice thickness. Analogous sets of surface crevasses appear on the ridges between the basal channels. We suggest that both sets of crevasses were formed during the melting of the subglacial channels as a response to vertical flexing of the ice shelf toward the hydrostatic condition. Finite element modeling of stresses produced after the formation of idealized basal channels indicates that the stresses generated have the correct pattern and, if the channels were formed sufficiently rapidly, would have sufficient magnitude to explain the formation of the observed basal and surface crevasse sets. We conclude that ice-shelf basal melting plays a role in determining patterns of surface and basal crevassing. Increased delivery of warm ocean water into the sub-ice shelf cavity may therefore cause not only thinning but also structural weakening of the ice shelf, perhaps, as a prelude to eventual collapse.

Post-Stagnation Behavior in the Upstream Regions of Ice Stream C, West Antarctica

The region where two active tributaries feed into the now stagnant Ice Stream C (ISC), West Antarctica, is thickening. In this region, we observe a correlation between faster ice flow (the tributaries) and elevated topography. We conclude that stagnation of ISC resulted in compression and thickening along the tributaries, eventually forming a bulge on the ice-sheet surface. Modern hydraulic potential gradients would divert basal meltwater from ISC to Ice Stream B (ISB). These gradients are primarily controlled by the bulge topography, and so likely formed subsequent to trunk stagnation. As such, we argue against water piracy as being the cause for ISCs stagnation. Kinematic-wave theory suggests that thickness perturbations propagate downstream over time, but that kinematic-wave speed decreases near the stagnant trunk. This and modest diffusion rates combine to trap most of the tributary-fed ice in the bulge region. Using interferometric synthetic aperture radar velocity measurements, we observe that half of the ice within ISCs southern tributary flows into ISB. That flow pattern and other observations of non-steady flow in the region likely result from stagnation-induced thickening along upper ISC combined with a longer period of thinning on upper ISB. If current trends in thickness change continue, more ice from upper ISC will be diverted to ISB.

Sub-ice-shelf sediments record history of twentieth-century retreat of Pine Island Glacier

The West Antarctic Ice Sheet is one of the largest potential sources of rising sea levels. Over the past 40 years, glaciers flowing into the Amundsen Sea sector of the ice sheet have thinned at an accelerating rate, and several numerical models suggest that unstable and irreversible retreat of the grounding line—which marks the boundary between grounded ice and floating ice shelf—is underway. Understanding this recent retreat requires a detailed knowledge of grounding-line history, but the locations of the grounding line before the advent of satellite monitoring in the 1990s are poorly dated. In particular, a history of grounding-line retreat is required to understand the relative roles of contemporaneous ocean-forced change and of ongoing glacier response to an earlier perturbation in driving ice-sheet loss. Here we show that the present thinning and retreat of Pine Island Glacier in West Antarctica is part of a climatically forced trend that was triggered in the 1940s. Our conclusions arise from analysis of sediment cores recovered beneath the floating Pine Island Glacier ice shelf, and constrain the date at which the grounding line retreated from a prominent seafloor ridge. We find that incursion of marine water beyond the crest of this ridge, forming an ocean cavity beneath the ice shelf, occurred in 1945 (±12 years); final ungrounding of the ice shelf from the ridge occurred in 1970 (±4 years). The initial opening of this ocean cavity followed a period of strong warming of West Antarctica, associated with El Niño activity. Thus our results suggest that, even when climate forcing weakened, ice-sheet retreat continued.

Corrigendum: Sub-ice-shelf sediments record history of twentieth-century retreat of Pine Island Glacier

This corrects the article DOI: 10.1038/nature20136