Impacts of post-depositional processing on nitrate isotopes in the snow and the overlying atmosphere at Summit, Greenland

Abstract

The effect of post-depositional processing on the preservation of snow nitrate isotopes at Summit, Greenland, remains a subject of debate and is relevant to the quantitative interpretation of ice-core nitrate (isotopic) records at high snow accumulation sites. Here we present the first year-round observations of atmospheric nitrate and its isotopic compositions at Summit and compare them with published surface snow and snowpack observations. The atmospheric δ15N(NO3-) remained negative throughout the year, ranging from −3.1 ‰ to −47.9 ‰ with a mean of (−14.8 ± 7.3) ‰ (n=54), and displayed minima in spring which are distinct from the observed spring δ15N(NO3-) maxima in snowpack. The spring average atmospheric δ15N(NO3-) was (−17.9 ± 8.3) ‰ (n=21), significantly depleted compared to the snowpack spring average of (4.6 ± 2.1) ‰, while the surface snow δ15N(NO3-) of (−6.8 ± 0.5) ‰ was in between the atmosphere and the snowpack. The differences in atmospheric, surface snow and snowpack δ15N(NO3-) are best explained by the photo-driven post-depositional processing of snow nitrate, with potential contributions from fractionation during nitrate deposition. In contrast to δ15N(NO3-), the atmospheric Δ17O(NO3-) was of a similar seasonal pattern and magnitude of change to that in the snowpack, suggesting little to no changes in Δ17O(NO3-) from photolysis, consistent with previous modeling results. The atmospheric δ18O(NO3-) varied similarly to atmospheric Δ17O(NO3-), with summer low and winter high values. However, the difference between atmospheric and snow δ18O(NO3-) was larger than that of Δ17O(NO3-). We found a strong correlation between atmospheric δ18O(NO3-) and Δ17O(NO3-) that is very similar to previous measurements for surface snow at Summit, suggesting that atmospheric δ18O(NO3-) versus Δ17O(NO3-) relationships were conserved during deposition. However, we found the linear relationships between δ18O and Δ17O(NO3-) were significantly different for snowpack compared to atmospheric samples. This likely suggests the oxygen isotopes are also affected before preservation in the snow at Summit, but the degree of change for δ18O(NO3-) should be larger than that of Δ17O(NO3-). This is because photolysis is a mass-dependent process that would directly affect δ18O(NO3-) in snow but not Δ17O(NO3-) as the latter is a mass-independent signal. Although there were uncertainties associated with the complied dataset, the results suggested that post-depositional processing at Summit can induce changes in nitrate isotopes, especially δ15N(NO3-), consistent with a previous modeling study. This reinforces the importance of understanding the effects of post-depositional processing before ice-core nitrate isotope interpretation, even for sites with relatively high snow accumulation rates.