Global oceanic emissions of nitrous oxide to the atmosphere: from global patterns to regional-scale processes

Seminar by Daniele Bianchi from University of California, Los Angeles

18 September 2019
KST 10:00 – 11:00

The Seminar is being held in Room 1010 (Jasmin) – Integrated mechanical engineering building. Click here for the campus map.

Assessment of the global budget of nitrous oxide (N2O), a major greenhouse gas, is limited by poor knowledge of the oceanic N2O flux to the atmosphere, whose magnitude, variability, and environmental drivers are highly uncertain. In the first part of the talk, I discuss a new, data-based reconstruction of oceanic N2O emissions that captures coastal hotspots, revealing spatial heterogeneity, and a vigorous, globally synchronous seasonal cycle. The reconstruction is based on a supervised learning algorithm trained with over 150,000 N2O measurements from the surface ocean, the largest synthesis to date. We estimate a global annual flux of 4.3 +/- 1.1 TgN/y, consistent with the range reported by the IPCC, but with a much lower uncertainty. A large part of this flux and its seasonal variability are driven by the tropics, with a disproportionate influence of coastal upwelling systems, in particular in the Indian and Eastern Tropical Pacific. In the second part of the talk, I discuss results from numerical simulations with an ocean biogeochemical model designed to resolve N2O cycling in tropical oxygen minimum zones. We incorporate the model in a high resolution simulation of the Eastern Tropical Pacific, resolving the biogeochemical heterogeneity induced by mesoscale eddies. We show that interaction of aerobic and anaerobic processes at the boundaries of oxygen minimum zones enhances the rates of nitrogen cycling and N2O production. We further show that denitrification is a dominant source of N2O, both in the ocean interior and in the coastal upwelling, where a direct conduit for atmospheric outgassing exists.