Impacts of post-depositional processing on nitrate isotopes in the snow and the overlying atmosphere at Summit, Greenland
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Published:2022-07-12
Issue:7
Volume:16
Page:2709-2724
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ISSN:1994-0424
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Container-title:The Cryosphere
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language:en
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Short-container-title:The Cryosphere
Author:
Jiang ZhuangORCID, Savarino JoelORCID, Alexander BeckyORCID, Erbland JosephORCID, Jaffrezo Jean-Luc, Geng Lei
Abstract
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.
Funder
National Science Foundation Agence Nationale de la Recherche National Key Research and Development Program of China National Natural Science Foundation of China Fundamental Research Funds for the Central Universities
Publisher
Copernicus GmbH
Subject
Earth-Surface Processes,Water Science and Technology
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