Sub-ice-shelf sediments record history of twentieth-century retreat of Pine Island Glacier

Abstract

The West Antarctic Ice Sheet is one of the largest potential sources of rising sea levels. Over the past 40 years, glaciers flowing into the Amundsen Sea sector of the ice sheet have thinned at an accelerating rate, and several numerical models suggest that unstable and irreversible retreat of the grounding line-which marks the boundary between grounded ice and floating ice shelf-is underway. Understanding this recent retreat requires a detailed knowledge of grounding-line history, but the locations of the grounding line before the advent of satellite monitoring in the 1990s are poorly dated. In particular, a history of grounding-line retreat is required to understand the relative roles of contemporaneous ocean-forced change and of ongoing glacier response to an earlier perturbation in driving ice-sheet loss. Here we show that the present thinning and retreat of Pine Island Glacier in West Antarctica is part of a climatically forced trend that was triggered in the 1940s. Our conclusions arise from analysis of sediment cores recovered beneath the floating Pine Island Glacier ice shelf, and constrain the date at which the grounding line retreated from a prominent seafloor ridge. We find that incursion of marine water beyond the crest of this ridge, forming an ocean cavity beneath the ice shelf, occurred in 1945 (±12 years); final ungrounding of the ice shelf from the ridge occurred in 1970 (±4 years). The initial opening of this ocean cavity followed a period of strong warming of West Antarctica, associated with El Niño activity. Thus our results suggest that, even when climate forcing weakened, ice-sheet retreat continued.

Increased West Antarctic and unchanged East Antarctic ice discharge over the last 7 years

“…Over the last decade, it is evident that larger-scale changes in discharge are relatively modest (< 7 % for all basins) compared to the fractional imbalance between discharge and surface mass budget (up to several tens of percent). This suggests that the recent pattern of mass loss in Antarctica, dominated by the Amundsen Sea sector, is likely a part of a longerterm phase of enhanced glacier flow initiated in the 1990s as indicated by satellite records (Konrad et al, 2017;Mouginot et al, 2014) or as early as the 1940s as proposed from subice-shelf sediment records (Smith et al, 2017).…”

The sensitivity of West Antarctica to the submarine melting feedback

“…Retreat rates may prove to be sensitive to whether oceanic melting can thin the ice upstream of discrete pinning points that support basal drag, allowing them to go afloat. Sediment cores collected beneath Pine Island ice shelf indicate that this is a viable mechanism for loss of basal drag and acceleration of retreat [Smith et al, 2017]. For now, the degree to which this mechanism applies at the subgrid scale remains uncertain, so it is not clear how much melting, if any, should be applied to the partially grounded cells.…”

Increased West Antarctic ice discharge and East Antarctic stability over the last seven years

“…of enhanced glacier flow initiated in the 1990s as indicated by satellite records (Konrad et al, 2017;Mouginot et al, 2014) or as early as the 1940s as proposed from sub-ice-shelf sediment records (Smith et al, 2017). Going forward, the ability to frequently measure ice flow at continental scales will give new insights into the dynamic variability and interconnection of these fast-flowing glaciers, as well as the apparent stability of the East Antarctic ice sheet.…”