부산대학교 도서관

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 δ15 N(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 δ15 N(NO 3-) maxima in snowpack. The spring average atmospheric δ15 N(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 δ15 N(NO 3-) of (- 6.8 ± 0.5) ‰ was in between the atmosphere and the snowpack. The differences in atmospheric, surface snow and snowpack δ15 N(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 δ15 N(NO 3-), the atmospheric Δ17 O(NO 3-) was of a similar seasonal pattern and magnitude of change to that in the snowpack, suggesting little to no changes in Δ17 O(NO 3-) from photolysis, consistent with previous modeling results. The atmospheric δ18 O(NO 3-) varied similarly to atmospheric Δ17 O(NO 3-), with summer low and winter high values. However, the difference between atmospheric and snow δ18 O(NO 3-) was larger than that of Δ17 O(NO 3-). We found a strong correlation between atmospheric δ18 O(NO 3-) and Δ17 O(NO 3-) that is very similar to previous measurements for surface snow at Summit, suggesting that atmospheric δ18 O(NO 3-) versus Δ17 O(NO 3-) relationships were conserved during deposition. However, we found the linear relationships between δ18 O and Δ17 O(NO 3-) 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 δ18 O(NO 3-) should be larger than that of Δ17 O(NO 3-). This is because photolysis is a mass-dependent process that would directly affect δ18 O(NO 3-) in snow but not Δ17 O(NO 3-) 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 δ15 N(NO 3-), 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. [ABSTRACT FROM AUTHOR]