Data‐based estimates of suboxia, denitrification, and N₂O production in the ocean and their sensitivities to dissolved O₂

Abstract

Oxygen minimum zones (OMZs) are major sites of fixed nitrogen removal from the open ocean. However, commonly used gridded data sets such as the World Ocean Atlas (WOA) tend to overestimate the concentration of O2 compared to measurements in grids where O2 falls in the suboxic range (O2 < 2–10 mmol m−3), thereby underestimating the extent of O2 depletion in OMZs. We evaluate the distribution of the OMZs by (1) mapping high‐quality oxygen measurements from the WOCE program, and (2) by applying an empirical correction to the gridded WOA based on in situ observations. The resulting suboxic volumes are a factor 3 larger than in the uncorrected gridded WOA. We combine the new oxygen data sets with estimates of global export and simple models of remineralization to estimate global denitrification and N2O production. We obtain a removal of fixed nitrogen of 70 ± 50 Tg year−1 in the open ocean and 198 ± 64 Tg year−1 in the sediments, and a global N2O production of 6.2 ± 3.2 Tg year−1. Our results (1) reconcile water column denitrification rates based on global oxygen distributions with previous estimates based on nitrogen isotopes, (2) revise existing estimates of sediment denitrification down by 1/3d through the use of spatially explicit fluxes, and (3) provide independent evidence supporting the idea of a historically balanced oceanic nitrogen cycle. These estimates are most sensitive to uncertainties in the global export production, the oxygen threshold for suboxic processes, and the efficiency of particle respiration under suboxic conditions. Ocean deoxygenation, an expected response to anthropogenic climate change, could increase denitrification by 14 Tg year−1 of nitrogen per 1 mmol m−3 of oxygen reduction if uniformly distributed, while leaving N2O production relatively unchanged.

Effects of increased isopycnal diffusivity mimicking the unresolved equatorial intermediate current system in an earth system climate model

“…Further, it reduces (1) spurious zonal temperature gradients on both isopycnals and surfaces of constant depth, and (2) spurious zonal oxygen gradients. The latter improvement is associated with a more realistic distribution of suboxia in the ocean and, in turn, effects a 46 TgN y -1 lower global pelagic denitrification rate of 105 TgN y -1 , a value that is closer to the recent estimates (52˙14 TgN y -1 by Eugster and Gruber [2012]; 70˙50 TgN y -1 by Bianchi et al [2012]). …”

“…Hence, we tested, as described above, a range of diffusivities. Figure 2 shows, for each of the simulations, the volume-weighted root mean square deviations of temperature, salinity, and oxygen from respective observations Antonov et al, 2006;Bianchi et al, 2012]. All of these global metrics suggest that a Locarnini et al, 2006;Antonov et al, 2006;Bianchi et al, 2012] and simulations.…”

“…Figure 2 shows, for each of the simulations, the volume-weighted root mean square deviations of temperature, salinity, and oxygen from respective observations Antonov et al, 2006;Bianchi et al, 2012]. All of these global metrics suggest that a Locarnini et al, 2006;Antonov et al, 2006;Bianchi et al, 2012] and simulations. Each symbol refers to a quasi-equilibrated simulation featuring differing levels of added isopycnal, anisoptropic, zonal diffusivity applied below the EUC between 5 ı S and 5 ı N. The colors refer to oxygen (black), temperature (red), and salinity (blue).…”

“…This change is effected by a reduced volume of suboxic water in D5, which catches comparably less export that is then denitrified. [Bianchi et al, 2012;Locarnini et al, 2006;Antonov et al, 2006], Figure 3b Figures 4a and 4b, the reference simulation (black) and the simulation with the added isopycnal anisotropic zonal diffusivity of 50,000 m 2 s -1 (red) are referenced to the quasi-equilibrated state prior to 1800.…”

Enhancement of anammox by the excretion of diel vertical migrators

“…However, the sparseness of synchronous O 2 profiles and acoustic ADCP data over the OMZs necessitates the use of climatological data. Because climatological data are likely to underestimate the extent of anoxic waters (29), the overlap of DVM and anoxic waters could be larger than shown in Fig. 2.…”