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

Monday, 15 December 2014: 3:25 PM
Jagruti Vedamati1, Bonnie X Chang2, Brian D Peters1, Matthew Sean Forbes1, Calvin Mordy2, Mark J Warner2, Allan Devol2, Bess B Ward3 and Karen L Casciotti1, (1)Stanford University, Stanford, CA, United States, (2)Univ Washington, Seattle, WA, United States, (3)Princeton University, Geosciences, Princeton, NJ, United States
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.