Diagnosing ice sheet grounding line stability from landform morphology

Abstract

Abstract. The resilience of a marine-based ice sheet is strongly governed by the stability of its grounding lines, which are in turn sensitive to ocean-induced melting, calving, and flotation of the ice margin. Since the grounding line is also a sedimentary environment, the constructional landforms that are built here may reflect elements of the processes governing this dynamic and potentially vulnerable environment. Here we analyse a large dataset (n = 6275) of grounding line landforms mapped on the western Ross Sea continental shelf from high-resolution geophysical data. The population is divided into two distinct morphotypes by their morphological properties: recessional moraines (consistently narrow, closely spaced, low amplitude, symmetric, and straight) and grounding zone wedges (broad, widely spaced, higher amplitude, asymmetric, sinuous, and highly variable). Landform morphotypes cluster with alike forms that transition abruptly between morphotypes both spatially and within a retreat sequence. Their form and distribution are largely independent of water depth, bed slope, and position relative to glacial troughs. Similarly, we find no conclusive evidence for morphology being determined by the presence or absence of an ice shelf. Instead, grounding zone wedge construction is favoured by a higher sediment flux and a longer-held grounding position. We propose two endmember modes of grounding line retreat: (1) an irregular mode, characterised by grounding zone wedges with longer standstills and accompanied by larger-magnitude retreat events, and (2) a steady mode, characterised by moraine sequences that instead represent more frequent but smaller-magnitude retreat events. We suggest that while sediment accumulation and progradation may prolong the stability of a grounding line position, progressive development of sinuosity in the grounding line due to spatially variable sediment delivery likely destabilises the grounding position by enhanced ablation, triggering large-magnitude retreat events. Here, the concept of stability is multifaceted and paradoxical, and neither mode can be characterised as marking fast or slow retreat. Diagnosing grounding line stability based on landform products should be considered for a wider geographic range, yet this large dataset of landforms prompts the need to better understand the sensitivity of marine-based grounding lines to processes and feedbacks governing retreat and what stability means in the context of future grounding line behaviour.

The marine geological imprint of Antarctic ice shelves

“…Push moraines with similar dimensions to those observed beneath Mackay Glacier (~2 m) have also been imaged on deglaciated parts of the western Ross Sea shelf (e.g., JOIDES Trough). Their formation has been associated with push and squeeze in basal crevasses as well as delivery and deposition from sub- or englacial material 26,86 although no clear association between these features and the presence of an ice canopy was found 86 . As such it remains unclear, if such moraines are diagnostic of ice shelf presence or simply indicate a specific GL configuration and/or retreat pattern.…”

“…GZWs form when the GL remains near-stationary allowing the accumulation and forward progradation of sediment to build a wedge geometry (Figs. 5 and 6a) 86–88 . Their vertical restriction and asymmetry implies limited accommodation for sediments above the grounding zone, due to the low seaward slope of an extending, floating ice tongue 88 .…”

Characteristics of the deforming bed: till properties on the deglaciated Antarctic continental shelf

“…9.Neither lineation morphology nor the distribution of shear strength correlates with paleo-ice stream velocities, inferred to be fastest at the center of paleo-ice stream troughs. (a) Lineation peaks were auto-detected (following Simkins and others, 2018) along a transect across Glomar Challenger Basin; transect location is shown in Figure 2. Lineation width and height does not display a clear trend across this trough.…”

“…Conversely, the eastern Ross Sea has a more subdued topography, which imposed less topographic influence on ice flow. Large, isolated grounding zone wedges record rapid, large-scale grounding line retreat events that left behind well-preserved linear features (Mosola and Anderson, 2006; Bart and Cone, 2012; Bart and others, 2017a, b; Simkins and others, 2018).…”

“…These two regions experienced different styles of deglaciation (Halberstadt and others, 2016). Landward of the LGM grounding zone wedges, the seafloor in the western Ross Sea is imprinted with relatively small but numerous grounding line landforms (Simkins and others, 2018) indicative of complex recession patterns, suggesting that a steep-profiled ice sheet retreated rapidly in troughs and remained grounded on banks (Anderson and others, 2014; Halberstadt and others, 2016). Conversely, the eastern Ross Sea is dominated by extensive glacial lineations and a few large grounding zone wedges that formed as paleo-ice streams thinned and decoupled from the bed in large, discrete back-stepping events (Mosola and Anderson, 2006; Bart and Cone, 2012; Bart and others, 2017a).…”