Global Biogeochemical Cycles

A probabilistic estimate of global marine N-fixation and denitrification

Authors

  • Olivier Eugster,

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    1. Environmental Physics, Institute of Biogeochemistry and Pollutant Dynamics, ETH Zurich, Zurich, Switzerland
      Corresponding author: O. Eugster, Environmental Physics, Institute of Biogeochemistry and Pollutant Dynamics, ETH Zurich, CHN E27, Universitätstr. 16, CH-8092 Zürich, Switzerland. (olivier.eugster@env.ethz.ch)
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  • Nicolas Gruber

    1. Environmental Physics, Institute of Biogeochemistry and Pollutant Dynamics, ETH Zurich, Zurich, Switzerland
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Corresponding author: O. Eugster, Environmental Physics, Institute of Biogeochemistry and Pollutant Dynamics, ETH Zurich, CHN E27, Universitätstr. 16, CH-8092 Zürich, Switzerland. (olivier.eugster@env.ethz.ch)

Abstract

[1] We determine the global rates of marine N-fixation and denitrification and their associated uncertainties by combining marine geochemical and physical data with a new two-dimensional box model that separates the Atlantic from the IndoPacific basins. The uncertainties are estimated using a probabilistic approach on the basis of a suite of 2500 circulation configurations of this box model. N-fixation and denitrification are diagnosed in an inverse manner for each of these configurations usingN*, P*, and the stable nitrogen isotope composition of nitrate as data constraints. Our approach yields a median water column denitrification rate of 52 TgN yr−1 (39 to 66 TgN yr−1, 5th to 95th percentile) and a median benthic denitrification rate of 93 TgN yr−1 (68 to 122 TgN yr−1). The resulting benthic-to-water column denitrification ratio of 1.8 confirms that the isotopic signature of water column denitrification has a limited influence on the global mean stable isotopic value of nitrate due to the dilution of the waters with a denitrification signal with the remainder of the ocean's nitrate pool. On the basis of two different approaches, we diagnose a global N-fixation rate of between 94 TgN yr−1 and 175 TgN yr−1, with a best estimate of 131 TgN yr−1 and 134 TgN yr−1, respectively. Most of the N-fixation occurs in the IndoPacific suggesting a relative close spatial coupling between sources and sinks in the ocean. Our N-fixation and denitrification estimates plus updated estimates of atmospheric deposition and riverine input yield a pre-industrial marine N cycle that is balanced to within 3 TgN yr−1 (−38 to 40 TgN yr−1). Our budget implies a median residence time for fixed N of 4,200 yr (3,500 to 5,000 yr).

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