Constraining the timing and spatial extent of an early Holocene ungrounding in the Weddell Sea using ice cores: how fast and how far-reaching?

During the Last Glacial Maximum, the Antarctic ice sheet was significantly larger than today, holding an additional 6 to 14 meters sea level equivalent. Although less than 10% of the total glacial-interglacial range in eustatic sea level change, understanding the when and where of how Antarctica reconfigures during a deglaciation is crucial to understanding how the ice sheet will behave in the future. Some models show that many areas of the Antarctic ice sheet are inherently unstable during the Last Glacial Maximum and, when forced by increasing temperatures and rising sea level during the last deglaciation, undergo a rapid retreat to their present-day grounding line configurations (if not beyond). In particular, the Ross and Weddell Sea regions, which are now largely covered by floating ice shelves, were susceptible to this tipping point behaviour. Recently, evidence from the Skytrain Ice Rise ice core (~79S, 078W, 784m altitude) using water isotopes and total air content (a proxy for elevation) provided strong evidence that the Weddell Sea underwent such a transition about 8,000 years before present (BP) (Grieman et al., in press). Here we present new total air content data from the Fletcher Promontory ice core (~78S, 082W, 873m altitude) which also lies in the Weddell Sea region about 220 km from Skytrain Ice Rise site, with the fast-flowing Rutford Ice Stream situated in between. The data were measured with a novel, high-accuracy total air content system and span approximately 11,000 to 6,000 years BP with an average resolution of 150 years. The most notable feature is an 8.8 mmol/kg (+/-2.0) increase between 8,000 and 7,000 years BP. This confirms the shift observed in the Skytrain Ice Rise ice core (~6.6 mmol/kg) that has been attributed to a 430 110 m drop in elevation. Using both Skytrain Ice Rise and Fletcher Promontory as the two independently derived elevation histories we will discuss the reliability of total air content as an elevation proxy as well as provide crucial constraints on state-the-art ice sheet model predictions of past tipping point behaviour. Grieman, M., Nehrbass-Ahles, C., Hoffmann, H., Bauska, T. K., King, A. C. F., Mulvaney, R., Rhodes, R. H., Rowell, I. F., Thomas, E. R., and Wolff, E. W.: Abrupt Holocene ice loss due to thinning and ungrounding in the Weddell Sea Embayment, Nature Geoscience, In Press.