Ice core nitrogen isotopes archive dramatic changes in West Antarctic Ice Sheet thinning

The behaviour of ice sheets during ice mass loss is currently not well constrained and is a major limiting factor in accurate predictions of ice sheet behaviour in our warming climate. Proxies from ice cores can record the history of ice mass loss at exceptional temporal resolution and unrivalled chronological accuracy. A recent record of Total Air Content (TAC) and ice core chemistry from Skytrain Ice Rise resolved a 450 m drop in ice sheet elevation at the site in the Weddell Sea Sector of the Antarctic Ice Sheet 8000 years ago, an event which occurred over just 200 years. The event is thought to represent an ungrounding and removal of the buttressing effect on the ice sheet in the region. However, proxy records for ice elevation, TAC, can show unexpected signals which indicates an imperfect understanding of how such gas records are captured in ice cores during rapid changes in ice sheet conditions, inhibiting expansion of such studies to other sites. Here we use ice core nitrogen isotope measurements to elucidate the dynamic evolution of the firn column, where such gas records are gradually trapped, during the 8 ka rapid ice mass loss. The horizontal divergence imparted on the ice rise during the event dramatically thinned the firn column to the extent that dynamic thinning of the firn is the dominating factor in how nitrogen isotopes are captured. As a result, the recorded signal of nitrogen isotopes directly opposes the signal predicted by current firn models which do not include such ice dynamics, suggesting that it is a critical factor to include in firn modelling studies of sites susceptible to rapid ice mass changes. Our findings allow us to tightly constrain where reliable elevation signals, not disrupted by changing ice dynamics, are available in ice core records. Moreover, our study demonstrates that the combination of TAC and nitrogen isotopes can be a powerful tool in constraining ice sheet dynamics at a site, thus helping to inform the physics of ice sheet models.