TY - JOUR
T1 - The isotope effect of nitrate assimilation in the Antarctic Zone
T2 - Improved estimates and paleoceanographic implications
AU - Fripiat, François
AU - Martínez-García, Alfredo
AU - Fawcett, Sarah E.
AU - Kemeny, Preston C.
AU - Studer, Anja S.
AU - Smart, Sandi M.
AU - Rubach, Florian
AU - Oleynik, Sergey
AU - Sigman, Daniel M.
AU - Haug, Gerald H.
N1 - Funding Information:
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.
Funding Information:
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 .
Publisher Copyright:
© 2018 Elsevier Ltd
PY - 2019/2/15
Y1 - 2019/2/15
N2 - 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 effect estimates 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.
AB - 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 effect estimates 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.
KW - Isotope effect
KW - Nitrate assimilation
KW - Nitrate isotopes
KW - Nitrite interference
KW - Paleoceanography
KW - Southern ocean
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U2 - 10.1016/j.gca.2018.12.003
DO - 10.1016/j.gca.2018.12.003
M3 - Article
AN - SCOPUS:85059815105
SN - 0016-7037
VL - 247
SP - 261
EP - 279
JO - Geochimica et Cosmochimica Acta
JF - Geochimica et Cosmochimica Acta
ER -