Marine denitrification rates determined from a global 3-D inverse model

[摘要] A major impediment to understanding long-term changes in the marine nitrogen(N) cycle is the persistent uncertainty about the rates, distribution, andsensitivity of its largest fluxes in the modern ocean. We use a global oceancirculation model to obtain the first 3-D estimate of marine denitrificationrates that is maximally consistent with available observations of nitratedeficits and the nitrogen isotopic ratio of oceanic nitrate. We find a globalrate of marine denitrification in suboxic waters and sediments of120–240 Tg N yr⁻¹, which is lower than many other recentestimates. The difference stems from the ability to represent the 3-D spatialstructure of suboxic zones, where denitrification rates of50–77 Tg N yr⁻¹ result in up to 50% depletion ofnitrate. This depletion reduces the effect of local isotopic enrichment onthe rest of the ocean, allowing the N isotope ratio of oceanic nitrate to beachieved with a sedimentary denitrification rate about 1.3–2.3 times that ofsuboxic zones. This balance of N losses between sediments and suboxic zonesis shown to obey a simple relationship between isotope fractionation and thedegree of nitrate consumption in the core of the suboxic zones. The globaldenitrification rates derived here suggest that the marine nitrogen budget islikely close to balanced.

[学科分类] 地球化学与岩石