How dynamic are ice-stream beds?

Abstract

10Projections of sea-level rise contributions from West Antarctica's dynamically thinning ice streams contain high uncertainty because some of the key processes involved are extremely challenging to observe. An especially poorly observed parameter is sub-decadal stability of ice-stream beds. Only two previous studies have made repeated geophysical measurements of icestream beds at the same locations in different years, but both studies were limited in spatial extent. Here, we present the results from repeat radar measurements of the bed of Pine Island Glacier, West Antarctica, conducted 3-6 years apart, along a 15 cumulative ~60 km of profiles. Analysis of the correlation of bed picks between repeat surveys show that 90% of the icestream bed displays no significant change despite the glacier increasing in speed by up to 40% over the last decade. We attribute the negligible detection of morphological change at the bed of Pine Island Glacier to the ubiquitous presence of a deforming till layer, wherein sediment transport is in steady state, such that sediment is transported along the basal interface without inducing morphological change to the radar-sounded bed. Significant change was only detected in one 500 m section of the 20 bed where a change in bed morphology occurs with a difference in vertical amplitude of 3-5 m. Given the precision of our measurements, the maximum possible erosion rate that could go undetected along our profiles is 500 mm a -1 , far exceeding erosion rates reported for glacial settings from proglacial sediment yields, but substantially below subglacial erosion rates of 1000 mm a -1 previously reported from repeat geophysical surveys in West Antarctica.

Bedforms of Thwaites Glacier, West Antarctica: Character and Origin

“…Analogy to Rutford Ice Stream suggests that sediment transport may be notably nonsteady, further complicating modeling (Smith et al., 2007; cf. Davies et al., 2018). The existence of seismically thick water in some places beneath Thwaites (Clyne et al., 2020; Muto, Anandakrishnan, et al., 2019) suggests nonsteadiness; persistent thick water likely would have been filled by sediment over the lifespan of the ice sheet.…”

The precision of radar-derived subglacial bed topography: a case study from Pine Island Glacier, Antarctica

“…Variations in firn thickness or wave-speed on the scale of 100-200 m of horizontal distance (the scale required to explain the bed topography observed) are therefore unlikely. Such an assessment of accuracy becomes relevant when comparing the results of two surveys of the same area to determine changes in ice thickness over time (Davies and others, 2018) but is not relevant to geomorphological studies of, for example, bedform shape and size (Spagnolo and others, 2017). This is because the thickness of ice above the bedforms is of secondary importance compared to the peak-to-trough height of individual drumlins or megascale glacial lineations.…”

Bed properties and three-dimensional topography from radar at Rutford Ice Stream, West Antarctica

“…How mobile or stable is the bed beneath RIS, and what are possible implications for the stability of the ice flow? Previous studies on RIS have found substantial temporal changes of the bed conditions as well as the bed topography, whereas observation of surface flow velocity has given no indications for changes in flow dynamics (Davies et al, 2018;Smith et al, 2007). However, so far, the analysis of temporal changes at the bed has been limited to a small area (∼10×5 km Smith & Murray, 2009) on the bed of RIS.…”

“…et al (2004) argued that the stiffness of sediments can vary over time, triggered by temporal variation in water pressure. Temporal changes in sediment stiffness have been observed to occur rapidly (within a few years; Murray et al, 2008;Smith, 2006;Vaughan et al, 2003), whereas in other places, the basal conditions are stable over time (Davies et al, 2018;Smith et al, 2007). Basal water pressure is dependent on the influx and outflux of water, properties of the subglacial material, efficiency (therefore geometry) of the subglacial drainage system and the thermal state at the ice-bed interface (Benn & Evans, 2011).…”

“…Repeated seismic and radar surveys show that, on the one hand, in some areas the bed topography has changed within 5-10 years, even though ice flow has remained relatively stable and negligible surface lowering has been observed (Davies et al, 2018). On the other hand, in other areas the bed topography seems to be constant (Smith et al, 2007).…”