Latitudinal variability of excess N2O and its biogeochemical drivers in the tropical Western North Pacific

초록

Nitrous oxide (N2O) is a potent greenhouse gas produced in the ocean through nitrification and denitrification. We investigated the latitudinal variability of excess N2O (Delta N2O, defined as the excess of seawater N2O above atmospheric equilibrium) and its drivers across 29 stations in the tropical western North Pacific (12.5-20.5 degrees N, 126.5-133.5 degrees E) during September-October 2023. Surface waters were slightly supersaturated with N2O, and Delta N2O increased with depth. Across the intermediate layer (200-600 m), Delta N2O was positively correlated with apparent oxygen utilization and nitrate, consistent with nitrification as the dominant source of N2O production. The Delta N2O maxima shoaled toward lower latitudes, coinciding with a shallower oxycline. The elevated Delta N2O maxima levels at lower latitudes may reflect enhanced primary productivity, indicated by elevated depthintegrated chlorophyll-a, and potentially higher N2O yields under lower-oxygen conditions. Dissolved oxygen remained above suboxic levels, with the minimum oxygen concentration of 64 mu mol L- 1, indicating limited denitrification. Air-sea fluxes were consistently positive, with a mean of 1.39 +/- 1.14 mu mol m- 2 d-1, indicating that the region is a net source of N2O to the atmosphere during early fall. Together, these results show that latitudinal gradients in oxygen utilization and primary productivity drive the observed Delta N2O variability.

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