Bed conditions of Pine Island Glacier, West Antarctica

Abstract

Although 90% of Antarctica's discharge occurs via its fast‐flowing ice streams, our ability to project future ice sheet response has been limited by poor observational constraints on the ice‐bed conditions used in numerical models to determine basal slip. We have helped address this observational deficit by acquiring and analyzing a series of seismic reflection profiles to determine basal conditions beneath the main trunk and tributaries of Pine Island Glacier (PIG), West Antarctica. Seismic profiles indicate large‐scale sedimentary deposits. Combined with seismic reflection images, measured acoustic impedance values indicate relatively uniform bed conditions directly beneath the main trunk and tributaries, comprising a widespread reworked sediment layer with a dilated sediment lid of minimum thickness 1.5 ± 0.4 m. Beneath a slow‐moving intertributary region, a discrete low‐porosity sediment layer of 7 ± 3 m thickness is imaged. Despite considerable basal topography, seismic observations indicate that a till layer at the ice base is ubiquitous beneath PIG, which requires a highly mobile sediment body to maintain an abundant supply. These results are compatible with existing ice sheet models used to invert for basal shear stress: existing basal conditions upstream will not inhibit further rapid retreat of PIG if the high‐friction region currently restraining flow, directly upstream of the grounding line, is breached. However, small changes in the pressure regime at the bed, as a result of stress reorganization following retreat, may result in a less‐readily deformable bed and conditions which are less likely to maintain high ice‐flow rates.

High‐resolution sub‐ice‐shelf seafloor records of twentieth century ungrounding and retreat of Pine Island Glacier, West Antarctica

“…These surveys inferred a thick sedimentary basin extending upstream of the grounding line that would provide an abundant source for sediments deposited as mass flows, MSGL, and GZWs in Zones 2–4. These observations indicate that Jenkins Ridge marks a transition between hard, resistant crystalline bedrock to more erodible, soft sedimentary bed upstream of the present‐day grounding line [ Brisbourne et al , ]. Such transitions have been observed farther seaward on the continental shelf and associated with contrasts in the distribution of sediment and character of geomorphic features [ Lowe and Anderson , ; Wellner et al , , ; Graham et al , ].…”

How dynamic are ice-stream beds?

“…4a) The physical properties of the bed between PIG and RIS also differ. Seismic reflection surveying of several sites along PIG's trunk, including upstream and downstream of our repeat survey sites, has confirmed that the bed immediately below the ice pervasively consists of dilated sediments, which are at least several metres thick Brisbourne et al, 2017). Along seismic profile S1 2007 potentialfield data indicate that a transition from sedimentary to crystalline bedrock lies beneath the cap of deformable sediments (at position 9 km on Fig.…”

“…2a, 3a and 3f, black lines are repeat radar surveys. S1 2007 is a seismic survey presented in Smith et al (2013) and S1 2014 is a section of this profile resurveyed in 2014 (presented in Brisbourne et al, 2017). EHT31 is a single point repeat survey presented in Smith et al (2012).…”

“…Therefore we compare relative bed profiles by applying a static correction to a common bed datum (0 m) for both surveys. The seismic surveys S1 2007 and S1 2014 in the vicinity of R1 were processed and analysed by Smith et al (2013; profile only) and Brisbourne et al (2017;both profiles). Brisbourne et al (2017) primarily report on the calculation of acoustic impedance at the bed from both profiles, which we will consider in our discussion below.…”

“…The seismic surveys S1 2007 and S1 2014 in the vicinity of R1 were processed and analysed by Smith et al (2013; profile only) and Brisbourne et al (2017;both profiles). Brisbourne et al (2017) primarily report on the calculation of acoustic impedance at the bed from both profiles, which we will consider in our discussion below. For this paper, we also investigated the possibility of directly picking the bed for the repeated 5 km section in an analogous manner to the radar picking described above.…”