Nitrous Oxide Cycling in the Eastern Tropical South Pacific as Inferred from Isotopic and Isotopomeric Data.

Abstract:

Marine sources of nitrous oxide (N₂O), an important greenhouse gas, account for up to 25% of global emissions, out of which 25-75% originates from oxygen minimum zones (OMZs). The Eastern Tropical South Pacific (ETSP) OMZ is characterized by low to undetectable oxygen concentrations within the water column and is known to be a region of intense N₂O cycling. However, the balance of processes regulating N₂O production and emissions is still uncertain. The isotopic composition of dissolved N₂O is a tracer of its production, transport, and consumption processes in the ocean. Here we use concentration, isotopic and isotopomeric measurements of dissolved N₂O collected during cruise NBP1305 to the ETSP in 2013 to examine the processes affecting the distribution of N₂O throughout the water column. Dissolved N₂O concentrations ranged between 42-65 nmol/L at the edges of the oxycline while ranging between 6 -20 nmol/L at the core of the OMZ. The nitrogen and oxygen isotopic composition of dissolved N₂O (reported as δ¹⁵N vs air N₂ and δ¹⁸O vs VSMOW in units of ‰, respectively) displayed maxima coincident with the OMZ core. δ¹⁵N of N₂O ranged between 14 - 22‰, δ¹⁸O of N₂O ranged between 68 - 100‰ while site preference of N₂O ranged between 39 – 60‰ at the OMZ core. Based on the T-S plot and N₂O concentration profiles, there appears to be a strong correlation between N₂O and water mass features within the OMZ. Thus, the differences in δ¹⁵N and δ¹⁸O of N₂O along the north- south transect within the OMZ core may be related to differences in N₂O production-consumption mechanisms along with N₂O transport. Within the OMZ, the δ¹⁸O: δ¹⁵N relationship is also much lower than the 2.5:1 ratio expected for N₂O consumption via denitrification, leading us to believe that both production and consumption processes are likely to be at play.