UCLA

In contrast to other biogeochemical tracers, nitrogen in the ocean exists in a myriad of chemical forms, each with its own distinct properties and reactivity. These diverse chemical forms of nitrogen, including organic and inorganic compounds, are collectively involved in a unique and dynamic microbially-mediated cycle which is tightly intertwined with the overall functioning of marine ecosystems and has significant implications for the global cycles of carbon, phosphorous, and oxygen. While nitrogen is predominantly cycled between bioavailable forms in the ocean, additional anaerobic metabolic pathways emerge when the concentration of dissolved oxygen drops to suboxic or anoxic levels within the ocean's oxygen-minimum-zones (OMZs). These pathways produce gaseous dinitrogen (N2) and nitrous oxide (N2O), a potent greenhouse agent and contributor to ozone depletion, which together lead to a loss of bioavailable nitrogen from the oceans to the atmosphere. This dissertation provides a comprehensive analysis of these microbial pathways in OMZs, and further explores their sensitivity to both physical and biogeochemical variability.

In Chapter 2, we describe the development of a general algorithm used to expand the observational record of a special class of subsurface, predominantly anticyclonic oceanic eddies known as submesoscale coherent vortices (SCVs). These eddies have been shown to play an oversized role in propagating water masses in the intermediate and deeper parts of the ocean, and were recently identified as hot-spots of N2 and N2O production. By applying the algorithm to the global Argo float array, we detect nearly 4000 new global observations of these eddies. Furthermore, we demonstrate that their formation takes place in regional hot-spots, allowing us to quantify their contributions to local heat and salt anomalies due to their formation and propagation.

In Chapter 3, we incorporate a new model of the oceanic nitrogen cycle into an eddy-resolving 3D regional ocean model of the Eastern Tropical South Pacific, an upwelling region and hot-spot of nitrogen loss and N2O outgassing. The model accurately simulates both aerobic and anaerobic transformations responsible for N2 and N2O production, and provides a realistic representation of the large scale physical circulation. By decomposing the N2O tracer in the 3D model, we are able to attribute contributions from local biogeochemical sources and sinks, explore the role of the physical circulation in supplying N2O to the region, and ultimately quantify the drivers of N2O outgassing to the atmosphere.

Finally, Chapter 4 builds upon the findings of Chapter 3. Specifically, we deploy a higher resolution version of the 3D model to explore how mesoscale-driven heterogeneity governs the production of N2 and N2O in the Eastern Tropical South Pacific. By filtering biogeochemical tracer fields into ``mean'' and ``eddy'' components (e.g., fields governed by low/high frequency and large/small spatial scales, respectively), we demonstrate that oxygen variability induced by ephemeral eddies and filaments stimulates nitrogen loss to the atmosphere, but by preferential producing N2 at the expense of N2O consumption. These findings reveal that the mesoscale circulation plays a critical role in regulating N2O production, and further implies that coarse-grained biogeochemical models may overestimate the fluxes to the atmosphere from these regions.