Firn processes and N-15 of N2: potential for a gas-phase climate proxy

Ice cores provide unique records of past atmospheric composition, but these must be corrected for the age difference between ice and gas at a given depth in order to compare gas concentration changes with climate changes recorded in the ice. This gas age-ice age difference depends on the age of the ice at the bottom of the firn layer, where the bubbles are closed-off. Firn densification models are used to calculate this age difference in the past, but have an uncertainty on the order of 1000 years for Central Antarctic sites. A proxy for climate in the gas phase would remove the need for this correction. δ 15N of N2 records physical fractionation processes in the firn column. Here we present the δ 15N record from the EPICA Dome C (EDC) ice core covering the last three glacial terminations and 5 glacial-interglacial cycles between 300 and 800 ka. We show that δ 15N is positively correlated with the ice deuterium content, a proxy for temperature, over the entire available EDC record. Thermal fractionation is an unlikely explanation for this correlation; instead, we propose an accumulation-permeability-convection mechanism. Comparison of inferred convection zone depth and accumulation rate over individual glacial-interglacial cycles reveals hysteresis-like behavior. The tightest correlation is observed over glacial terminations, supporting the idea that δ 15N is a property in the gas phase that records deglacial warming.