Isotopic effects of nitrate photochemistry in snow: a field study at Dome C, Antarctica
-
Published:2015-10-09
Issue:19
Volume:15
Page:11243-11256
-
ISSN:1680-7324
-
Container-title:Atmospheric Chemistry and Physics
-
language:en
-
Short-container-title:Atmos. Chem. Phys.
Author:
Berhanu T. A., Savarino J.ORCID, Erbland J., Vicars W. C., Preunkert S.ORCID, Martins J. F.ORCID, Johnson M. S.ORCID
Abstract
Abstract. 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.
Funder
Seventh Framework Programme Agence Nationale de la Recherche
Publisher
Copernicus GmbH
Subject
Atmospheric Science
Reference40 articles.
1. Aristidi, E., Agabi, K., Azouit, M., Fossat, E., Vernin, J., Travouillon, T., Lawrence, J. S., Meyer, C., Storey, J. W. V., Halter, B., Roth, W. L., and Walden, V.: An analysis of temperatures and wind speeds above Dome C, Antarctica, Astron. Astrophys., 430, 739–746, https://doi.org/10.1051/0004-6361:20041876, 2005. 2. Berhanu, T. A., Meusinger, C., Erbland, J., Jost, R., Bhattacharya, S. K., Johnson, M. S., and Savarino, J.: Laboratory study of nitrate photolysis in Antarctic snow. II. Isotopic effects and wavelength dependence, J. Chem. Phys., 140, 244306, https://doi.org/10.1063/1.4882899, 2014. 3. Blunier, T., Floch, G. L., Jacobi, H. W., and Quansah, E.: Isotopic view on nitrate loss in Antarctic surface snow, Geophys. Res. Lett., 32, L13501, https://doi.org/10.1029/2005gl023011, 2005. 4. Bohlke, J. K., Mroczkowski, S. J., and Coplen, T. B.: Oxygen isotopes in nitrate: new reference materials for O-18 : O-17 : O-16 measurements and observations on nitrate-water equilibration, Rapid Commun. Mass Sp., 17, 1835–1846, https://doi.org/10.1002/Rcm.1123, 2003. 5. Casciotti, K. L., Sigman, D. M., Hastings, M. G., Bohlke, J. K., and Hilkert, A.: Measurement of the oxygen isotopic composition of nitrate in seawater and freshwater using the denitrifier method, Anal. Chem., 74, 4905–4912, https://doi.org/10.1021/Ac020113w, 2002.
Cited by
30 articles.
订阅此论文施引文献
订阅此论文施引文献,注册后可以免费订阅5篇论文的施引文献,订阅后可以查看论文全部施引文献
|
|