Evidence and implications of rapid early-Holocene thinning of Scott Glacier, East Antarctica

The East Antarctic Ice Sheet is showing evidence of mass loss, particularly near its coastal margin in the Wilkes and Aurora Subglacial Basins, and at Denman Glacier. Quantitative satellite observations indicate significant grounding line retreat at these sites, suggesting potential vulnerability to Marine Ice Sheet Instability. Reconstructions of past ice sheet behaviour using cosmogenic exposure dating can provide robust geological constraints on prior ice sheet thinning. This helps us assess whether contemporary thinning and retreat is unprecedented, while also establishing a geologically constrained precedent for ice-sheet behaviour and sensitivity to which projections can be compared.Here we present the first reconstruction of Holocene thinning of Scott Glacier, East Antarctica, located 56 km east of Denman Glacier. Scott Glacier is currently stable, pinned by subglacial topography, though is projected to retreat into the Denman trough, and ultimately contribute to the instability of the Denman-Scott System. Together, the Denman and Scott Glacier could contribute up to 1.5 m to global sea level rise if fully deglaciated. To examine its past behaviour, we collected 11 bedrock and erratic samples over an elevation transect at Grace Rocks (-66.421S, 100.508E), an ice-free nunatak adjacent to the modern-day grounding line. Exposure ages were then derived from measured cosmogenic Berilyum-10 and in-situ Carbon-14 concentrations. Together, they provide consistent evidence for rapid thinning during the early-Holocene, with estimates suggesting a maximum thinning rate of ~1 m/yr, comparable to thinning observed in parts of the ice sheet today. This thinning history provides robust geological constraints on the past behaviour and sensitivity of Scott Glacier, and a baseline from which to assess its contemporary and projected retreat and vulnerability.We also derive projections of ice elevation change at Scott Glacier from simulations with the Ice-sheet and Sea-level System Model (ISSM). Projections show thinning rates exceeding 2 m/yr over the next two centuries across the Denman-Scott region, with an average thinning rate of 0.65 m/yr at Grace Rocks projected until 2085. While there are uncertainties associated with these models, the rates and sensitivity we established from the early-Holocene geological record suggest that modelled changes of this magnitude are plausible, and that despite contemporary stability, Scott Glacier is at risk of contributing significantly to regional icesheet instability and sea level rise in coming decades to centuries.