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

• Published in: The Cryosphere
• Main Authors: Jiang, Zhuang, Savarino, Joel, Alexander, Becky, Erbland, Joseph, Jaffrezo, Jean-Luc, Geng, Lei
• Format: Article in Journal/Newspaper
• Language: English
• Published: Copernicus Publications 2022
• Subjects: article; Verlagsveröffentlichung; Greenland; ice core; The Cryosphere
• Online Access: https://doi.org/10.5194/tc-16-2709-2022; https://noa.gwlb.de/receive/cop_mods_00061835; https://noa.gwlb.de/servlets/MCRFileNodeServlet/cop_derivate_00061217/tc-16-2709-2022.pdf; https://tc.copernicus.org/articles/16/2709/2022/tc-16-2709-2022.pdf

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(NO 3-) 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(NO 3-) maxima in snowpack. The spring average atmospheric δ15N(NO 3-) 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(NO 3-) of (−6.8 ± 0.5) ‰ was in between the atmosphere and the snowpack. The differences in atmospheric, surface snow and snowpack δ15N(NO 3-) 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(NO 3-), the atmospheric Δ17O(NO 3-) was of a similar seasonal pattern and magnitude of change to that in the snowpack, suggesting little to no changes in Δ17O(NO 3-) from photolysis, consistent with previous modeling results. The atmospheric δ18O(NO 3-) varied similarly to atmospheric Δ17O(NO 3-), with summer low and winter high values. However, the difference between atmospheric and snow δ18O(NO 3-) was larger than that of Δ17O(NO 3-). We found a strong correlation between atmospheric δ18O(NO 3-) and Δ17O(NO 3-) that is very similar to previous measurements for surface snow at Summit, suggesting that atmospheric δ18O(NO 3-) versus Δ17O(NO 3-) relationships were conserved during deposition. However, we found the linear relationships between δ18O and Δ17O(NO 3-) were significantly different for snowpack compared to atmospheric samples. This ...