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| Titel |
Isotopic effects of nitrate photochemistry in snow: a field study at Dome C, Antarctica |
| VerfasserIn |
T. A. Berhanu, J. Savarino, J. Erbland, W. C. Vicars, S. Preunkert, J. F. Martins, M. S. Johnson |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1680-7316
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 15, no. 19 ; Nr. 15, no. 19 (2015-10-09), S.11243-11256 |
| Datensatznummer |
250120087
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| Publikation (Nr.) |
 copernicus.org/acp-15-11243-2015.pdf |
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| Zusammenfassung |
| Stable isotope ratios of nitrate preserved in deep ice cores are expected to
provide unique and valuable information regarding paleoatmospheric
processes. However, due to the post-depositional loss of nitrate in snow,
this information may be erased or significantly modified by physical or
photochemical processes before preservation in ice. We investigated the
role of solar UV photolysis in the post-depositional modification of nitrate
mass and stable isotope ratios at Dome C, Antarctica, during the austral
summer of 2011/2012. Two 30 cm snow pits were filled with homogenized
drifted snow from the vicinity of the base. One of these pits was covered
with a plexiglass plate that transmits solar UV radiation, while the other
was covered with a different plexiglass plate having a low UV transmittance.
Samples were then collected from each pit at a 2–5 cm depth resolution and
a 10-day frequency. At the end of the season, a comparable nitrate mass loss
was observed in both pits for the top-level samples (0–7 cm) attributed to
mixing with the surrounding snow. After excluding samples impacted by the
mixing process, we derived an average apparent nitrogen isotopic
fractionation (15ϵapp) of
−67.8 ± 12 ‰ for the snow nitrate exposed to solar UV
using the nitrate stable isotope ratios and concentration measurements. For
the control samples in which solar UV was blocked, an apparent average
15ϵapp value of −12.0 ± 1.7 ‰ was
derived. This difference strongly suggests that solar UV photolysis plays a
dominant role in driving the isotopic fractionation of nitrate in snow. We
have estimated a purely photolytic nitrogen isotopic fractionation
(15ϵphoto) of −55.8 ± 12.0 ‰ from the difference
in the derived apparent isotopic fractionations of the two experimental
fields, as both pits were exposed to similar physical processes except
exposure to solar UV. This value is in close agreement with the
15ϵphoto value of −47.9 ± 6.8 ‰
derived in a laboratory experiment simulated for Dome C conditions (Berhanu
et al., 2014). We have also observed an insensitivity of 15ϵ
with depth in the snowpack under the given experimental setup. This is due to
the uniform attenuation of incoming solar UV by snow, as 15ϵ
is strongly dependent on the spectral distribution of the incoming light
flux. Together with earlier work, the results presented here represent a
strong body of evidence that solar UV photolysis is the most relevant
post-depositional process modifying the stable isotope ratios of snow nitrate
at low-accumulation sites, where many deep ice cores are drilled.
Nevertheless, modeling the loss of nitrate in snow is still required before a
robust interpretation of ice core records can be provided. |
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