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  The isotope effect of nitrate assimilation in the Antarctic Zone: Improved estimates and paleoceanographic implications

Fripiat, F., Martinez-Garcia, A., Fawcett, S. E., Kemeny, P. C., Studer, A. S., Smart, S. M., et al. (2019). The isotope effect of nitrate assimilation in the Antarctic Zone: Improved estimates and paleoceanographic implications. Geochimica et Cosmochimica Acta, 247, 261-279. doi:10.1016/j.gca.2018.12.003.

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 Creators:
Fripiat, Francois1, Author              
Martinez-Garcia, Alfredo1, Author              
Fawcett, Sarah E.2, Author
Kemeny, Preston C.2, Author
Studer, Anja S.1, Author              
Smart, Sandi M.2, Author
Rubach, Florian3, Author              
Oleynik, Sergey2, Author
Sigman, Daniel M.2, Author
Haug, Gerald H.1, Author              
Affiliations:
1Climate Geochemistry, Max Planck Institute for Chemistry, Max Planck Society, ou_2237635              
2external, ou_persistent22              
3Particle Chemistry, Max Planck Institute for Chemistry, Max Planck Society, ou_1826291              

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 Abstract: Both the nitrogen (N) isotopic composition (δ15N) of the nitrate source and the magnitude of isotope discrimination associated with nitrate assimilation are required to estimate the degree of past nitrate consumption from the δ15N of organic matter in Southern Ocean sediments (e.g., preserved within diatom microfossils). It has been suggested that the amplitude of isotope discrimination (i.e. the isotope effect) correlates with mixed layer depth, driven by a physiological response of phytoplankton to light availability, which introduces complexity to the interpretation of sedimentary records. However, most of the isotope effect estimates that underpin this hypothesis derive from acid-preserved water samples, from which nitrite would have been volatilized and lost during storage. Nitrite δ15N in Antarctic Zone surface waters is extremely low (−61 ± 20‰), consistent with the expression of an equilibrium isotope effect associated with nitrate–nitrite interconversion. Its loss from the combined nitrate + nitrite pool would act to raise the δ15N of nitrate, potentially yielding overestimation of the isotope effect. Here, we revisit the nitrate assimilation isotope effect in the Antarctic Zone with measurements of the δ15N and concentration of nitrate with and without nitrite, using frozen sea water samples from 5 different cruises that collectively cover all sectors of the Southern Ocean. The N isotope effect estimated using nitrate + nitrite δ15N is relatively constant (5.5 ± 0.6‰) across the Antarctic Zone, shows no relationship with mixed layer depth, and is in agreement with sediment trap δ15N measurements. Estimates of the N isotope effect derived from nitrate-only δ15N are higher and more variable (7.9 ± 1.5‰), consistent with an artifact from nitrate-nitrite isotope exchange. In the case of the Southern Ocean, we conclude that the δ15N of nitrate + nitrite better reflects the isotope effect of nitrate assimilation. The stability of this isotope effect across the Antarctic Zone simplifies the effort to reconstruct the past degree of nitrate consumption.

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Language(s): eng - English
 Dates: 2019
 Publication Status: Published in print
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 Rev. Type: -
 Identifiers: ISI: 000456159500016
DOI: 10.1016/j.gca.2018.12.003
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Title: Geochimica et Cosmochimica Acta
  Abbreviation : Geochim. Cosmochim. Acta
Source Genre: Journal
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Publ. Info: Oxford : Pergamon
Pages: - Volume / Issue: 247 Sequence Number: - Start / End Page: 261 - 279 Identifier: ISSN: 0016-7037
CoNE: https://pure.mpg.de/cone/journals/resource/954925401558