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Isotopic ratios of nitrite as tracers of the sources and age of oceanic nitrite

Nature Geoscience volume 6, pages 308–313 (2013)Cite this article

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Abstract

Nitrite is a central intermediate in the marine nitrogen cycle, and is generally found at low concentrations in the ocean. However, nitrite accumulates at the base of the sunlit surface ocean. The origin of this ubiquitous feature, known as the primary nitrite maximum, is debated and has been difficult to resolve through short-term isotope tracer incubations. Here, we use measurements of the dual isotopic composition of nitrite to evaluate the sources, sinks and average rates of nitrite turnover in the primary nitrite maximum in the Arabian Sea. We determined the rate of abiotic oxygen isotope exchange between nitrite and water, as well as equilibrium isotope effects, under a variety of conditions in a series of laboratory experiments. We used these data to model nitrite isotope data from several sites in the Arabian Sea. The results suggest that ammonia oxidation is the primary source of nitrite at most sites. Steady-state nitrite turnover times of 33–178 days correspond to ammonia oxidation rates of 9–30 nM per day at these sites. We suggest that a similar approach could be used to determine the rates of nitrite turnover in other environments, including the secondary nitrite maximum in low-oxygen regions.

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Figure 1: A box model diagram illustrating the processes that control δ15N and δ18O of nitrite in the PNM.
Figure 2: Time courses of δ 18 O NO 2 values during three different experiments in which pH, temperature and salinity were varied.
Figure 3: Interpretation of δ 15 N NO 2 and δ 18 O NO 2 values from the Arabian Sea PNM.

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Acknowledgements

We would like to thank M. McIlvin and B. Peters for help collecting data. We would like to thank the chief scientist B. Ward and the captain and crew of the R/V Revelle for assistance in the collection of nitrite isotope samples in the Arabian Sea. Financial support was provided from NSF/OCE 07-48674 and 11-40404 to K.L.C.

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Authors and Affiliations

  1. Massachusetts Institute of Technology/ Woods Hole Oceanographic Institution Joint Program in Chemical Oceanography, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA

    Carolyn Buchwald

  2. Department of Environmental Earth System Science, Stanford University, Stanford, California 94305, USA

    Karen L. Casciotti

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  1. Carolyn Buchwald
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  2. Karen L. Casciotti
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Contributions

C.B. and K.L.C. designed the experiments and the model formulations. C.B. performed the experiments and model calculations. C.B. and K.L.C. authored the paper.

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Correspondence to Carolyn Buchwald or Karen L. Casciotti.

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Buchwald, C., Casciotti, K. Isotopic ratios of nitrite as tracers of the sources and age of oceanic nitrite. Nature Geosci 6, 308–313 (2013). https://doi.org/10.1038/ngeo1745

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