Seabed corrugations beneath an Antarctic ice shelf revealed by autonomous underwater vehicle survey: Origin and implications for the history of Pine Island Glacier

Abstract

Ice shelves are critical features in the debate about West Antarctic ice sheet change and sea level rise, both because they limit ice discharge and because they are sensitive to change in the surrounding ocean. The Pine Island Glacier ice shelf has been thinning rapidly since at least the early 1990s, which has caused its trunk to accelerate and retreat. Although the ice shelf front has remained stable for the past six decades, past periods of ice shelf collapse have been inferred from relict seabed “corrugations” (corrugated ridges), preserved 340 km from the glacier in Pine Island Trough. Here we present high‐resolution bathymetry gathered by an autonomous underwater vehicle operating beneath an Antarctic ice shelf, which provides evidence of long‐term change in Pine Island Glacier. Corrugations and ploughmarks on a sub‐ice shelf ridge that was a former grounding line closely resemble those observed offshore, interpreted previously as the result of iceberg grounding. The same interpretation here would indicate a significantly reduced ice shelf extent within the last 11 kyr, implying Holocene glacier retreat beyond present limits, or a past tidewater glacier regime different from today. The alternative, that corrugations were not formed in open water, would question ice shelf collapse events interpreted from the geological record, revealing detail of another bed‐shaping process occurring at glacier margins. We assess hypotheses for corrugation formation and suggest periodic grounding of ice shelf keels during glacier unpinning as a viable origin. This interpretation requires neither loss of the ice shelf nor glacier retreat and is consistent with a “stable” grounding‐line configuration throughout the Holocene.

Submarine glacial-landform distribution across the West Antarctic margin, from grounding line to slope: the Pine Island–Thwaites ice-stream system

“…2c) and elsewhere in Pine Island Trough has been debated . The rhythmic pattern in height and spacing in both sets of corrugations likely represents a tidal modulation , while their subtle form and transverse expression fits an ice-keelgrounding interpretation (alongside several other plausible mechanisms; see discussion in Graham et al 2013). For the particular set of features under Pine Island Glacier ice shelf, the periodic grounding of sub-ice-shelf keels during the final phases of glacier unpinning has been proposed as the most likely formation mechanism.…”

Past ice-sheet behaviour: retreat scenarios and changing controls in the Ross Sea, Antarctica

“…2h, k;Anderson, 1999;Jakobsson et al, 2011;Klages et al, 2015). Although the exact mechanism for their formation remains somewhat controversial, corrugation ridges are thought to form as icebergs move vertically with tides, causing iceberg keels to intermittently contact the bed (Jakobsson et al, 2011;Graham et al, 2013). Their association with vertical tidal movement is based on the occurrence of identical features in proglacial arcuate iceberg furrows (Anderson, 1999) and comparison of corrugation amplitude and spacing with tidal modelling results (Jakobsson et al, 2011).…”

Mapping submarine glacial landforms using acoustic methods

“…AUVs are also able to map areas unreachable by surface vessels -for example, underneath floating ice shelves. Corrugation ridges were mapped beneath the ice shelf of the Pine Island Glacier using a Kongsberg EM2000 (200 kHz) mounted on the AUV Autosub3 (Graham et al 2013).…”

“…ROVs, flying close to the seafloor, can provide highresolution multibeam imagery of, for example, sediments, landforms and the accompanying surface biota, and some also have a shallow sub-bottom profiling capability (e.g. Graham et al 2013).…”

“…Today AUVs also deploy multibeam systems in the upwardlooking mode to map the underside of floating sea ice and ice shelves (Dutrieux et al 2016;Wilkinson & Wadhams 2016) as well in the more conventional downward-looking configuration to map the seafloor and its component submarine landforms (e.g. Graham et al 2013). …”