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

Fripiat, François and Martínez-García, Alfredo and Fawcett, Sarah E. and Kemeny, Preston C. and Studer, Anja S. and Smart, Sandi M. and Rubach, Florian and Oleynik, Sergey and Sigman, Daniel M. and Haug, Gerald H. (2019) The isotope effect of nitrate assimilation in the Antarctic Zone: Improved estimates and paleoceanographic implications. Geochimica et Cosmochimica Acta, 247 . pp. 261-279. ISSN 0016-7037. http://resolver.caltech.edu/CaltechAUTHORS:20181218-084409721

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Abstract

Both the nitrogen (N) isotopic composition (δ^(15)N) 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 δ^(15)N 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 effectestimates that underpin this hypothesis derive from acid-preserved water samples, from which nitrite would have been volatilized and lost during storage. Nitrite δ^(15)N 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 δ^(15)N 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 δ^(15)N 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 δ^(15)N 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 δ^(15)N measurements. Estimates of the N isotope effect derived from nitrate-only δ^(15)N 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 δ^(15)N 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.


Item Type:Article
Related URLs:
URLURL TypeDescription
https://doi.org/10.1016/j.gca.2018.12.003DOIArticle
ORCID:
AuthorORCID
Fripiat, François0000-0002-2591-6301
Martínez-García, Alfredo0000-0002-7206-5079
Studer, Anja S.0000-0001-7354-8497
Smart, Sandi M.0000-0001-9467-3545
Additional Information:© 2018 Elsevier Ltd. Received 11 June 2018, Revised 28 November 2018, Accepted 5 December 2018, Available online 18 December 2018. The new stable isotope data for the GOSHIP section IO8S and P18S presented in this study will be merged into the IO8S and P18S CCHDO product (https://cchdo.ucsd.edu/). We thank the ship captains, crews, and chief scientists (R.V. Roger Revelle, R.V. Ronald H. Brown, and R.V. SA Agulhas II) who supported the sampling activities during the various field programs that contributed to this study. We are also grateful to M.A. Weigand (Princeton University) for methodological advice and Barbara Hinnenberg (MPIC) for help with sample processing. This study was funded by the Max Planck Society. SEF thanks the South African National Research Foundation and Antarctic Programme (through grants 105539 and 110735). DMS acknowledges US NSF grant 1401489, as well as 0960802, 0992345, 0612198. P.C.K. acknowledges support from the Fannie & John Hertz Foundation and the Department of Defense NDSEG Fellowship.
Funders:
Funding AgencyGrant Number
Max Planck SocietyUNSPECIFIED
National Research Foundation (South Africa)105539
National Research Foundation (South Africa)110735
NSFOPP-1401489
NSFOCE-0960802
NSFOCE-0992345
NSFOPP-0612198
Fannie and John Hertz FoundationUNSPECIFIED
National Defense Science and Engineering Graduate (NDSEG) FellowshipUNSPECIFIED
Subject Keywords:Southern ocean; Nitrate isotopes; Nitrate assimilation; Isotope effect; Nitrite interference; Paleoceanography
Record Number:CaltechAUTHORS:20181218-084409721
Persistent URL:http://resolver.caltech.edu/CaltechAUTHORS:20181218-084409721
Official Citation:François Fripiat, Alfredo Martínez-García, Sarah E. Fawcett, Preston C. Kemeny, Anja S. Studer, Sandi M. Smart, Florian Rubach, Sergey Oleynik, Daniel M. Sigman, Gerald H. Haug, The isotope effect of nitrate assimilation in the Antarctic Zone: Improved estimates and paleoceanographic implications, Geochimica et Cosmochimica Acta, Volume 247, 2019, Pages 261-279, ISSN 0016-7037, https://doi.org/10.1016/j.gca.2018.12.003. (http://www.sciencedirect.com/science/article/pii/S0016703718306823)
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:91885
Collection:CaltechAUTHORS
Deposited By: Tony Diaz
Deposited On:18 Dec 2018 17:41
Last Modified:22 Jan 2019 22:44

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