N2O cycling in the Gulf of Mexico from concentration and stable isotopic data

Abstract:

Nitrous oxide (N₂O) is a trace gas with a warming potential approximately 300 times that of carbon dioxide. N₂O is also the primary source of atmospheric nitrogen oxides, which can deplete the stratospheric ozone layer. Marine N₂O production from bacterial processes (nitrification and denitrification) represents a significant natural source of N₂O to the atmosphere. Different pathways of N₂O production and consumption have distinct isotopic signatures due to kinetic isotope fractionation during biological transformations. N₂O samples were collected at various locations and depths in the Gulf of Mexico hypoxic zone during the summers of 2015 and 2016 to investigate N₂O cycling. We measured N₂O concentrations, the stable isotopic composition of N₂O (d¹⁵N and d¹⁸O) and also the site preference (SP), defined as the difference between the d¹⁵N^a (internal) and the d¹⁵N^b (external), to gain further insight into N₂O production and consumption processes. N₂O concentrations showed interannual variability, ranging from 6 to 16 nM in 2015 and 3 to 37 nM in 2016. Highest N₂O concentrations were associated with a relatively low d¹⁵N near the oxycline, indicating net N₂O production. Anoxia was observed at a few stations in 2016, and d¹⁵N of N₂O as well as SP were elevated at these depths, indicating N₂O consumption. In this presentation, we will discuss spatial and temporal changes in N₂O production and consumption in relation to physical and chemical factors.