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.
Highlights
Marine-based ice sheet stability is strongly influenced by perturbations near the grounding line; the most downstream location ice is in contact with the underlying bed (e.g. Schoof, 2011; Robel et al, 2014)
While ice grounding is fundamentally dictated by water depth, at a regional scale grounding line landform morphology does not appear to be strongly governed by properties of the bed topography such as water depth and bed slope
Of the potential controls on landform morphology that we have explored here, we find inconclusive evidence that a distinct set of controls/processes can wholly explain the formation of either morphotype
Summary
Marine-based ice sheet stability is strongly influenced by perturbations near the grounding line; the most downstream location ice is in contact with the underlying bed (e.g. Schoof, 2011; Robel et al, 2014). Described as till tongues (King et al, 1991), till deltas (Alley et al, 1987, 1989), and diamict aprons (Hambrey et al, 1991; Eittreim et al, 1995), grounding zone wedges are composed of prograding strata of dilatant deforming till (King, 1993; Powell and Alley, 1997; Anderson, 1999; Dowdeswell and Fugelli, 2012; Batchelor and Dowdeswell, 2015; Simkins et al, 2017a) Both types of grounding line landforms have been observed to contain features described as grounding line fans: lobate or bulbous deposits building from a point source and linked to both glaciofluvial deposition at the mouth of a subglacial channel and re-mobilisation of grounding line sediments by gravity flows (Powell and Alley, 1997; Bjarnadóttir et al, 2013). We characterise morphological traits and the spatial distribution of 6275 grounding line landforms from the western Ross Sea continental shelf, formerly occupied by a marine-based sector of the East Antarctic Ice Sheet, to characterise landform morphology, examine those factors that control landform morphology and distribution, and explore drivers of grounding line stability and instability
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