Investigation of post-depositional processing of nitrate in East Antarctic snow: Isotopic constraints on photolytic loss, re-oxidation, and source inputs

Abstract

Snowpits along a traverse from coastal East Antarctica to the summit of the ice sheet (Dome Argus) are used to investigate the post-depositional processing of nitrate (NO3-) in snow. Seven snowpits from sites with accumulation rates between 24 and 172 kg m-2 a-1 were sampled to depths of 150 to 300 cm. At sites from the continental interior (low accumulation, < 55 kg m-2 a-1), nitrate mass fraction is generally > 200 ng g-1 in surface snow and decreases quickly with depth to < 50 ng g-1. Considerably increasing values of δ15N of nitrate are also observed (16-461‰ vs. air N2), particularly in the top 20 cm, which is consistent with predicted fractionation constants for the photolysis of nitrate. The δ18O of nitrate (17-84‰ vs. VSMOW (Vienna Standard Mean Ocean Water)), on the other hand, decreases with increasing δ15N, suggestive of secondary formation of nitrate in situ (following photolysis) with a low δ18O source. Previous studies have suggested that δ15N and δ18O of nitrate at deeper snow depths should be predictable based upon an exponential change derived near the surface. At deeper depths sampled in this study, however, the relationship between nitrate mass fraction and δ18O changes, with increasing δ18O of nitrate observed between 100 and 200 cm. Predicting the impact of post-depositional loss, and therefore changes in the isotopes with depth, is highly sensitive to the depth interval over which an exponential change is assumed. In the snowpits collected closer to the coast (accumulation > 91 kg m-2 a-1), there are no obvious trends detected with depth and instead seasonality in nitrate mass fraction and isotopic composition is found. In comparison to the interior sites, the coastal pits are lower in δ15N (-15-71 ‰ vs. air N2) and higher in δ18O of nitrate (53-111 ‰ vs. VSMOW). The relationships found amongst mass fraction, δ15N, δ18O and δ17O (Δ17O= Δ17O-0.52 × δ18O) of nitrate cannot be explained by local post-depositional processes alone, and are instead interpreted in the context of a primary atmospheric signal. Consistent with other Antarctic observational and modeling studies, the isotopic results are suggestive of an important influence of stratospheric ozone chemistry on nitrate formation during the cold season and a mix of tropospheric sources and chemistry during the warm season. Overall, the findings in this study speak to the sensitivity of nitrate isotopic composition to post-depositional processing and highlight the strength of combined use of the nitrogen and oxygen isotopes for a mechanistic understanding of this processing.