A global marine‐fixed nitrogen isotopic budget: Implications for Holocene nitrogen cycling

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Abstract
A nitrogen stable isotopic model was constructed in order to constrain the Holocene marine‐fixed nitrogen budget. The primary sources and sinks considered were riverine and atmospheric sources, nitrogen fixation, sedimentary and water column denitrification, and sediment burial. The source budget was found to be insensitive to changes in nitrogen fixation rates, and thus could not be used to constrain this term. However, the isotopic value of fixed nitrogen losses was very sensitive to the amount of sedimentary denitrification. If the isotopic value of marine‐fixed nitrogen has not changed during the Holocene, as supported by sedimentary records, then in order to balance the isotopic value of sinks and sources, approximately 280 Tg N yr−1 of sedimentary denitrification is required. If such a high rate of denitrification has been sustained throughout the Holocene, it implies that present‐day estimates of marine nitrogen fixation are grossly underestimated. It also implies that the marine nitrogen budget has a residence time of less than 2000 years.

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