Diagnosing ice sheet grounding line stability from landform morphology

• Published in: The Cryosphere
• Main Authors: L. M. Simkins, S. L. Greenwood, J. B. Anderson
• Format: Article in Journal/Newspaper
• Language: English
• Published: Copernicus Publications 2018
• Subjects: envir; geo; Ross Sea; Ice Sheet; Ice Shelf; The Cryosphere
• Online Access: https://doi.org/10.5194/tc-12-2707-2018; https://www.the-cryosphere.net/12/2707/2018/tc-12-2707-2018.pdf; https://doaj.org/article/df6a11c5ffa043c2a572b75339aabeea

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 ...