The Effects of Biogeochemistry and Water Mass Mixing on the Distribution of Nitrous Oxide in the Tropical South Pacific

Understanding the oceanic cycling and transport of the climatically relevant greenhouse gas, nitrous oxide (N₂O), is imperative for interpreting how it could change with environmental conditions. We studied the distributions of N₂O concentration and stable isotopes under biogeochemically and physically diverse environments along the GEOTRACES GP16 section—from the South Pacific oxygen deficient zone (ODZ) into the oligotrophic South Pacific Gyre—in concert with isotopic measurements of nitrate and nitrite, to investigate the interplay of N₂O production, consumption, and water mass mixing. We developed an isotope mixing model to determine the relative contributions of four N₂O endmembers along the section. The model showed that N₂O derived from the ODZ was rapidly diluted outside the ODZ, but was still detected at 152°W, the westernmost extent of the section. Keeling model results similarly confirmed that N₂O can be traced in the thermocline from the ODZ into the Gyre. Outside the ODZ thermocline, water mass mixing (background N₂O) and N₂O produced by ammonia-oxidizing archaea were largely responsible for its distribution. In this study, a moderate SP (22‰) for N₂O production from incomplete denitrification was needed to produce realistic endmember distributions, contrasting the 0‰ value often applied. Further, our newly developed tracer, Δ(SP, 18), which accounts for the isotopic impacts of N₂O consumption, was used to study the pathways of N₂O production within ODZ waters. This approach illustrated distinct patterns of δ¹⁵N^β observations within ODZ waters, highlighting the potential for nitrate and nitrite to contribute differentially to N₂O production in ODZ waters.