How Well Do Nitrate Isotopes in Alpine Ice Cores Preserve Atmospheric Signals?

In the face of natural and anthropogenic emissions, the habitability of Earth’s atmosphere is maintained thanks to atmospheric oxidants – photochemically produced reactive species which destroy toxic pollutants, remove greenhouse gases and maintain chemical stability. Understanding the chemical dynamics of the atmosphere is hence crucial for accurate predictions of air quality and radiative forcing in a changing climate, as identified in IPCC AR6. Because the species involved are often highly reactive, studying historic atmopsheric chemistry is challenging: consequently, there is no consensus on the magnitude or the sign of the relationship between atmospheric oxidative capacity and global climate.

One avenue for past atmospheric chemistry reconstructions is isotopic analysis of nitrate – an oxidation product of atmospheric NO_x – archived in non-polar ice cores. The N and O isotope compositions of ice core nitrate depend on past oxidation reactions and past NO_x sources, while newly-accessible nitrate clumped isotopes may provide complementary information on nitrate formation pathways. However, useful signals can be obscured by isotopic fractionation during nitrate transport, deposition and burial, while seasonal variations in atmospheric chemistry or snow accumulation can bias ice core records. These difficulties often make ice core nitrate isotope interpretations non-unique, limiting their utility as investigative tools for past atmospheric chemistry.

To investigate the processes controlling nitrate isotopes archived in non-polar ice cores, we collected firn cores and weekly high-volume atmospheric samples at high altitude sites in the Mont-Blanc massif (France/Italy) and the Cordillera Oriental (Bolivia). Using the newly-adapted Electrospray-Orbitrap mass spectrometer, we investigated the seasonality of atmospheric nitrate isotope ratios δ¹⁵N, δ¹⁸O and Δ¹⁷O, and clumped isotopes Δ¹⁵N¹⁸O and Δ¹⁸O¹⁸O, and compared atmospheric isotopic signals to those in contemporaneously-deposited firn over an accumulation season. We find substantial seasonal isotopic variability in atmospheric nitrate, which is partially preserved in firn core records. However, several isotopic disagreements could reflect syn- or post-depositional isotopic fractionation processes, and the isotopic seasonality should be carefully considered when intepreting ice core records where accumulation is seasonal.