Aerobic respiration along isopycnals leads to overestimation of the isotope effect of denitrification in the ocean water column. (15th January 2017)

• Record Type: Journal Article
• Authors: Marconi, Dario; Kopf, Sebastian; Rafter, Patrick A.; Sigman, Daniel M.
• Is Part Of: Geochimica et cosmochimica acta. Volume 197(2017:Jan. 15)
• Page Start: 417
• Page End: 432
• Publication Date: 2017-01-15
• Subjects: Denitrification -- Isotope effect -- Rayleigh model; Geochemistry -- Periodicals; Meteorites -- Periodicals; Géochimie -- Périodiques; Météorites -- Périodiques; Geochemie; Astrochemie; Electronic journals; 551.905
• DOI: 10.1016/j.gca.2016.10.012
• Languages: English
• ISSNs: 0016-7037

Abstract

Abstract: The nitrogen (N) isotopes provide an integrative geochemical tool for constraining the fixed N budget of the ocean. However, N isotope budgeting requires a robust estimate for the organism-scale nitrogen isotope effect of denitrification, in particular as it occurs in water column denitrification zones ( ε wcd ). Ocean field data interpreted with the Rayleigh model have typically yielded estimates for ε wcd of between 20 and 30‰. However, recent findings have raised questions about this value. In particular, culture experiments can produce a substantially lower isotope effect (∼13‰) under conditions mimicking those of ocean suboxic zones. In an effort to better understand prior field estimates of ε wcd, we use a geochemical multi-box model to investigate the combined effects of denitrification, aerobic respiration, and isopycnal exchange on the δ 15 N of nitrate. In the context of this admittedly simplistic model, we consider three isopycnals extending from the Southern Ocean to the Eastern Tropical North Pacific (ETNP). We show that the data from the ETNP suboxic zone can be reproduced with an ε wcd of 13‰, given a rate of aerobic respiration consistent with the nutrient data on these isopycnals and a plausible range in the δ 15 N of the sinking flux being remineralized. We discuss the limitations of our analysis, additional considerations, as well as possible data-based tests for the proposal of a lower ε wcd than previously estimated. All else held constant, a lower ε wcd would imply a lower global ocean rate of denitrification that is more similar to the estimated rate of N input to the global ocean, providing a major impetus for further investigation.