The Effects of Biogeochemistry and Water Mass Mixing on the Distribution
of Nitrous Oxide in the Tropical South Pacific
Abstract
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 N2O distributions under biogeochemically and physically diverse
environments along the GEOTRACES GP16 section – from the south Pacific
oxygen deficient zone (ODZ) to the western oligotrophic gyre – in
concert with isotopic measurements of N2O, 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 and distributions of four N2O endmembers. From the model,
we found that partial consumption was an essential determinant of the
isotopic composition of N2O within the ODZ, but the consumption signal
was rapidly diluted outside the ODZ. Keeling model results also
demonstrated how N2O can be traced from the ODZ into the Gyre
thermocline in the absence of strong production or consumption terms.
Outside of the ODZ thermocline, preformed N2O and N2O derived from
ammonia oxidizing archaea were largely responsible for its distribution.
Lastly, as shown in other modelling work, a moderate positive site
preference (~22‰) for N2O production from incomplete
denitrification was necessary to produce realistic endmember
distributions. Further, our newly developed tracer, D(SP,18), which
removes the isotopic impacts of N2O consumption to highlight the role of
production, illustrated a bifurcation of δ15Nβ within ODZ waters,
highlighting the potential for nitrate and nitrite to contribute
differentially to N2O production in on-shelf and off-shelf ODZ waters.