Isotopic effects of nitrate photochemistry in snow: a field study at Dome C, Antarctica
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 photochemi...
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ftcopernicus:oai:publications.copernicus.org:acp27767 2023-05-15T13:43:09+02:00 Isotopic effects of nitrate photochemistry in snow: a field study at Dome C, Antarctica Berhanu, T. A. Savarino, J. Erbland, J. Vicars, W. C. Preunkert, S. Martins, J. F. Johnson, M. S. 2018-09-19 application/pdf https://doi.org/10.5194/acp-15-11243-2015 https://www.atmos-chem-phys.net/15/11243/2015/ eng eng doi:10.5194/acp-15-11243-2015 https://www.atmos-chem-phys.net/15/11243/2015/ eISSN: 1680-7324 Text 2018 ftcopernicus https://doi.org/10.5194/acp-15-11243-2015 2019-12-24T09:53:03Z 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 ( 15 ε app ) 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 15 ε app 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 ( 15 ε photo ) 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 15 ε photo 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 15 ε with depth in the snowpack under the given experimental setup. This is due to the uniform attenuation of incoming solar UV by snow, as 15 ε 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. Text Antarc* Antarctica ice core Copernicus Publications: E-Journals Austral Atmospheric Chemistry and Physics 15 19 11243 11256 |
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Copernicus Publications: E-Journals |
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ftcopernicus |
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English |
description |
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 ( 15 ε app ) 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 15 ε app 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 ( 15 ε photo ) 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 15 ε photo 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 15 ε with depth in the snowpack under the given experimental setup. This is due to the uniform attenuation of incoming solar UV by snow, as 15 ε 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. |
format |
Text |
author |
Berhanu, T. A. Savarino, J. Erbland, J. Vicars, W. C. Preunkert, S. Martins, J. F. Johnson, M. S. |
spellingShingle |
Berhanu, T. A. Savarino, J. Erbland, J. Vicars, W. C. Preunkert, S. Martins, J. F. Johnson, M. S. Isotopic effects of nitrate photochemistry in snow: a field study at Dome C, Antarctica |
author_facet |
Berhanu, T. A. Savarino, J. Erbland, J. Vicars, W. C. Preunkert, S. Martins, J. F. Johnson, M. S. |
author_sort |
Berhanu, T. A. |
title |
Isotopic effects of nitrate photochemistry in snow: a field study at Dome C, Antarctica |
title_short |
Isotopic effects of nitrate photochemistry in snow: a field study at Dome C, Antarctica |
title_full |
Isotopic effects of nitrate photochemistry in snow: a field study at Dome C, Antarctica |
title_fullStr |
Isotopic effects of nitrate photochemistry in snow: a field study at Dome C, Antarctica |
title_full_unstemmed |
Isotopic effects of nitrate photochemistry in snow: a field study at Dome C, Antarctica |
title_sort |
isotopic effects of nitrate photochemistry in snow: a field study at dome c, antarctica |
publishDate |
2018 |
url |
https://doi.org/10.5194/acp-15-11243-2015 https://www.atmos-chem-phys.net/15/11243/2015/ |
geographic |
Austral |
geographic_facet |
Austral |
genre |
Antarc* Antarctica ice core |
genre_facet |
Antarc* Antarctica ice core |
op_source |
eISSN: 1680-7324 |
op_relation |
doi:10.5194/acp-15-11243-2015 https://www.atmos-chem-phys.net/15/11243/2015/ |
op_doi |
https://doi.org/10.5194/acp-15-11243-2015 |
container_title |
Atmospheric Chemistry and Physics |
container_volume |
15 |
container_issue |
19 |
container_start_page |
11243 |
op_container_end_page |
11256 |
_version_ |
1766185260311117824 |