Nitrous Oxide on the Texas-Louisiana Continental Shelf: No Laughing Matter

Nitrous oxide (N₂O) is a potent greenhouse gas present in trace quantities in the Earth’s atmosphere. N₂O has a warming potential approximately 300 times that of carbon dioxide, and it is the primary source of atmospheric nitrogen oxides, which can deplete the stratospheric ozone layer. The oceans are one natural source of N₂O to the atmosphere as N₂O can be produced during the bacterial processes of nitrification, which occurs under oxygenated conditions, and denitrification, which occurs under poorly oxygenated conditions. More N₂O is produced during denitrification. The Texas-Louisiana shelf, located in the northern Gulf of Mexico, has seasonal occurrences of hypoxia associated with high N₂O concentrations (>50 nM). The goal of this project is to determine the concentrations of N₂O in the water column at various locations and depths along the Texas-Louisiana shelf as an extension of a summer time series.

Samples were collected at various locations and depths in the northern Gulf of Mexico during June 2018 and July 2019 and analyzed using gas chromatography with electron capture detection. In 2018, 10 stations were sampled between June 22 and June 24, one of which was hypoxic. Surface N₂O concentrations ranged from 5 to 7 nM, and water column concentrations ranged from 5 to 9 nM. There was no significant difference in N₂O concentrations between the hypoxic station and the non-hypoxic stations. In 2019, 14 stations were sampled between July 4 and July 6, four of which were hypoxic. Surface N₂O concentrations ranged from 2 to 7 nM, and water column concentrations ranged from 0.4 and 7 nM. For two hypoxic stations, very low and decreasing concentrations of N₂O (0.4 - 1 nM) corresponded with increasing NO₂^- and NO₃^- concentrations in the presence of low oxygen. Generally, N₂O concentrations are lower and more variable in 2019 despite increased observed hypoxia.