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

Abstract

In order to quantify the sequence of events between changes in atmospheric composition and climate changes recorded in ice cores, we must accurately account for the age difference between ice and gas at a given depth. 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 how this age difference varied in the past, but have an uncertainty on the order of 1000 years for central Antarctic sites. Here we explore the possibility that δ 15N of N 2 is a gas phase proxy of climate, which can be used to synchronize gas and ice records. We present the δ 15N record from the EPICA Dome C (EDC) ice core covering the last three glacial terminations and five glacial‐interglacial cycles between 300 and 800 ka. Previous studies have shown that gravitational settling enriches δ 15N as a function of the diffusive column height in the firn. If densification models’ prediction of deeper firn close-off under glacial conditions is correct, then we would expect heavier δ 15N during glacial periods, and a negative correlation with temperature. Instead, EDC δ 15N is positively correlated with the ice deuterium content, a proxy for temperature, as previously reported at Vostok, Dome Fuji, and EPICA Dronning Maud Land. We propose a mechanism that links accumulation rate, firn permeability, and convective mixing in the top meters of the firn to explain this correlation between δ 15N and ice deuterium content. The tightest correlation is observed over glacial terminations, supporting the idea that δ 15N is a property in the gas phase that records changes in surface conditions linked to deglacial warming.