Daniel McCoy

and 4 more

Oceanic emissions of nitrous oxide (N2O) account for roughly one-third of all natural sources to the atmosphere. Hot-spots of N2O outgassing occur over oxygen minimum zones (OMZs), where the presence of steep oxygen gradients surrounding anoxic waters leads to enhanced N2O production from both nitrification and denitrification. However, the relative contributions from these pathways to N2O production and outgassing in these regions remains poorly constrained, in part due to shared intermediary nitrogen tracers, and the tight coupling of denitrification sources and sinks. To shed light on this problem, we embed a new, mechanistic model of the OMZ nitrogen cycle within a three-dimensional eddy-resolving physical-biogeochemical model of the ETSP, tracking contributions from remote advection, atmospheric exchange, and local nitrification and denitrification. Our results indicate that net N2O production from denitrification is approximately one order of magnitude greater than nitrification within the ETSP OMZ. However, only ~30% of denitrification-derived N2O production ultimately outgasses to the atmosphere in this region (contributing ~34% of the air-sea N2O flux on an annual basis), while the remaining is exported out of the domain. Instead, remotely-produced N2O advected into the OMZ region accounts for roughly half (~56%) of the total N2O outgassing, with smaller contributions from nitrification (~7%). Our results suggests that, together with enhanced production by denitrification, upwelling of remotely-derived N2O (likely produced via nitrification in the oxygenated ocean) contributes the most to N2O outgassing over the ETSP OMZ.

Daniel J Clements

and 6 more

Export of sinking particles from the surface ocean is critical for carbon sequestration and to provide energy to the deep biosphere. The magnitude and spatial patterns of this export have been estimated in the past by \emph{in situ} particle flux observations, satellite-based algorithms, and ocean biogeochemical models; however, these estimates remain uncertain. Here, we use a recent machine learning reconstruction of global ocean particle size distributions from Underwater Vision Profiler 5 (UVP5) measurements to estimate carbon fluxes by sinking particles (35 $\mu$m - 5 mm equivalent spherical diameter) from the surface ocean. We combine global maps of particle size distribution properties with empirical relationships constrained against \emph{in situ} flux observations to calculate particulate carbon export from the euphotic zone and wintertime mixed layer depths. The new flux reconstructions suggest a less variable seasonal cycle in the tropical ocean, and a more persistent export in the Southern Ocean than previously recognized. Smaller particles (less than 420 $\mu$m) contribute most of the flux globally, while larger particles become more important at high latitudes and in tropical upwelling regions. Export from the wintertime mixed layer globally exceeds that from the euphotic zone, suggesting shallow particle recycling and net heterotrophy in the deep euphotic zone. These estimates open the way to fully three-dimensional global reconstructions of particle fluxes in the ocean, supported by the growing database of \emph{in situ} optical observations.

Daniel J Clements

and 6 more

Daniel J Clements

and 6 more

Export of sinking particles from the surface ocean is critical for carbon sequestration and for providing energy to the deep-ocean biosphere. The magnitude and spatial patterns of this flux have been estimated in the past by satellite-based algorithms and ocean biogeochemical models; however, these estimates remain uncertain. Here, we present a novel analysis of a global compilation of \textit{in situ} ocean particle size spectra from Underwater Vision Profiler 5 (UVP5) measurements, from which we determine particulate carbon fluxes. Using a machine learning algorithm, we extrapolate sparse observations of particle abundance by size to the global ocean from oceanographic variables that are more commonly observed. We reconstruct global maps of particle size distribution parameters for large sinking particles (80 \textmu{}m to 2.6 cm), and combine them with empirical relationships to calculate the sinking carbon flux from the euphotic zone and the wintertime mixed layer depth. Our flux reconstructions are comparable to other estimates, but suggest a less variable seasonal cycle in the tropical ocean, and a more continuous export in the Southern Ocean than previously thought. Because our estimates are not bounded by a specific depth horizon, we reconstruct export at multiple depths, and find that export from the wintertime mixed layer globally exceeds that from the euphotic zone. Our estimates provide a baseline for more accurate understanding of particle cycles in the ocean, and open the way to fully three-dimensional global reconstructions of particle size spectra and fluxes in the ocean, supported by the growing database of UVP5 observations.