Inferring past temperature from δ15N measurements in air bubbles trapped in Antarctic ice

Ice cores are unique archives capturing records of past temperature (through the ice isotopic composition, e.g. δD) and past atmosphere composition over the last 800 kyr. In particular, their analysis revealed that glacial-interglacial transitions, altering the Earth's climate since the beginning of the Quaternary, are associated with significant variations in the atmospheric levels of CO₂ and CH₄. However, comparison of past temperatures imprinted in ice-phase and atmospheric composition records imprinted in the air-phase is difficult. Indeed, the air is trapped at a depth of 50-100 m, at the bottom of the firn, where snow transforms into ice. Therefore, at a given depth, the air is always younger than the ice and firn densification modeling is needed to estimate the age difference between the air and the ice at each level. Firn densification modeling is associated with large uncertainties when it is applied to low accumulation and low temperature drilling sites of the East Antarctic plateau.

An alternative approach to reconstruct air temperature directly in the air bubbles involves analyzing the isotopic composition of N₂ (δ¹⁵N). Indeed, local temperature and accumulation rate evolutions affect firn thickness and hence modulated the δ¹⁵N in air bubbles trapped at the bottom of the firn via gravitational enrichment of δ¹⁵N over large glacial-interglacial transition on the East Antarctic plateau. The observation of a robust correlation between ice core records of δ¹⁵N and δD (Dreyfus et al., 2010) confirms the strong influence of local climate on the δ¹⁵N. δ¹⁵N measurements have already been applied to determine the phasing between CO₂ and temperature increases over Antarctic temperature increase associated with glacial terminations. However, this strong relationship between δ¹⁵N and δD is not necessarily valid outside of glacial terminations. Here, we address the question to what extent the δ¹⁵N can be used to infer past temperatures and to study the CO₂-temperature relationship, hence circumventing age uncertainties that arise when comparing ice and gas phase measurements.

We first examine the δ¹⁵N record from EPICA Dome C with respect to East Antarctic climate over the last eight glacial-interglacial cycles. We use the good agreement between δD and δ¹⁵N over Termination II as a satisfactory criterion to discern when the δ¹⁵N is a reliable proxy of past temperature. Using this criterion, we assert that the correlation between δ¹⁵N and δD is robust over the past eight terminations. Focusing on the 100-300 ka BP period, we note also three intervals characterized by a weak correlation: the glacial inceptions from MIS 7e to 7d and 7a to 6e, and the MIS 6 glacial period. To explain why δ¹⁵N and δD evolutions contrast over these periods, we connect water stable isotopes with new δ¹⁵N measurements from EDC ice core and explore various snow densification scenarios yielded by a firn model under different climate conditions at the ice sheet surface. Our study permits to identify a criterion to safely use δ¹⁵N as an indicator of the past temperature in the air bubbles of the EDC ice core to study the CO₂-local temperature relationship.