Nitrous Oxide (N2O) cycling in the Equatorial Tropical South Pacific (ETSP) Oxygen Deficient Zone (ODZ) - evidence for production, consumption and N2O transport.

Nitrous Oxide (N₂O) is an important greenhouse gas, with much of its marine sources originating from the ocean’s oxygen deficient zones (ODZs). The Eastern Tropical South Pacific (ETSP) is one such region characterized by intense N₂O cycling, yet uncertainty exists in understanding the processes driving its production and emission. Here we use concentration and isotopometric measurements of dissolved N₂O collected during the NBP1305 cruise to the ETSP in 2013 to understand the mechanisms that regulate N₂O concentrations in the ODZ. Dissolved N₂O concentrations ranged from 6-20 nmol/L at the core of the ODZ to 42-65 nmol/L outside the ODZ. N₂O in the ODZ core, displays δ¹⁵N_bulk between 14-22‰, and δ¹⁸O between 68-100‰, while site preference (SP) was between 39–60‰, indicating that N₂O production and consumption are both active. Optimum Multi Parameter (OMP) analysis of regional water masses indicates that Equatorial Subsurface Water (ESSW) displays low [N₂O] and high SP, d¹⁵N and d¹⁸O values reflecting N₂O consumption, whereas the combined Subantarctic Water (SAAW) and Antarctic Intermediate Water (AAIW) waters display high [N₂O] and low SP, d¹⁵N and d¹⁸O, reflecting N₂O production processes. Investigations of N₂O concentration and isotope mixing for ODZ samples, defined as those falling within the potential density anomaly range 26.8 to 26.2 kg/m³, provides a tool in which the extent of consumption and production of N₂O in the ODZ can be analyzed. Preliminary estimates suggest that in addition to biogeochemical processes, the mixing between ESSW and AAIW/SAAW end members plays a large role in N₂O concentration and isotope distributions.