Past water flow beneath Pine Island and Thwaites glaciers, West Antarctica

Abstract

Abstract. Outburst floods from subglacial lakes beneath the Antarctic Ice Sheet modulate ice flow velocities over periods of months to years. Although subglacial lake drainage events have been observed from satellite altimetric data, little is known about their role in the long term evolution of ice sheet basal hydrology. Here, we systematically map and model past water flow through an extensive area containing over 1000 subglacial channels and 19 former lake basins exposed on over 19,000 km2 of seafloor by the retreat of Pine Island and Thwaites glaciers, West Antarctica. At 560 m wide and 50 m deep on average, the channels offshore of present day Pine Island and Thwaites glaciers are approximately twice as deep, three times as wide, and cover an area over 400 times larger than the terrestrial meltwater channels comprising the Labyrinth in the Antarctic Dry Valleys. The channels incised into bedrock offshore of contemporary Pine Island and Thwaites glaciers would have been capable of accommodating discharges of up to 8.8 × 106 m3 s−1. We suggest that the channels were formed by episodic, high magnitude discharges from subglacial lakes trapped during ice sheet advance and retreat over multiple glacial periods. Our results document the widespread influence of episodic subglacial drainage events during past glacial periods, in particular beneath large ice streams similar to those that continue to dominate contemporary ice-sheet discharge.

Revealing the former bed of Thwaites Glacier using sea-floor bathymetry

“…3a). Troughs and channels in the adjacent eastern part of Pine Island Bay have been described comprehensively by Nitsche et al (2013) and Kirkham et al (2019). The larger troughs in our study area, which have also been identified on gravity-derived regional bathymetry maps (Millan et al, 2017;Jordan et al, 2020.…”

“…These areas with low surface slopes are atypical when compared with other bathymetric highs in the area, which have rugged surface morphologies characterised by bedrock grooves and channels (Fig. 2, 4d) (Nitsche et al, 2013;Arndt et al, 2018;Kirkham et al, 2019). Instead, the low slope values are similar to those derived for the base of the troughs in front of Thwaites Ice Shelf (Fig.…”

“…3a) or the moderate-relief areas of lineated terrain with fewer bathymetric highs in eastern Pine Island Bay (Fig. 2a) (Nitsche et al, 2013;Arndt et al, 2018;Kirkham et al, 2019). The continuity and orientation of the trough and ridge relates to the structure of basement rocks on the inner shelf.…”

“…Final grids were visualised and analysed in QPS Fledermaus and ArcGIS 10.6. Sea-floor trough and channel metrics (including widths, depths, symmetry, form ratio, u-/v-shape characterisation) were derived using the methods described in Kirkham et al (2019).…”

“…S3) with continuous, parallel sided-flanks, undulating thalwegs, and incise into rugged sea-floor areas interpreted as bedrock. It is widely accepted that channels of the latter type were eroded by pressurised subglacial water flow (Lowe and Anderson, 2003;Nitsche et al, 2013;Kirkham et al, 2019), whereas the troughs likely relate, at least in part, to underlying geological structures such as mafic dykes and tectonic deformations (cf. Gohl et al, 2013) that have been variously modified by ice.…”