Project: Biogeochemistry of nitrous oxide cycling in the eastern tropical South Pacific

The eastern tropical South Pacific (ETSP) is a hot spot for oceanic nitrogen cycling. This region of upwelling and high productivity fuels high rates of oxygen consumption below the mixed layer, nitrate regeneration from nitrification, and ultimately denitrification of nitrate to N2 gas. The climatically important trace gas nitrous oxide (N2O) also reaches extreme high concentrations in the oxycline and extreme low concentrations in the heart of the oxygen minimum zone (OMZ), indicating active cycling in this region. Despite many years of investigation, the mechanism of N2O production in this hot spot is ambiguous because of the potential overlap or coupling of nitrification and denitrification processes at low oxygen tensions.

The investigators employed novel stable isotopic techniques to identify processes involved with nitrous oxide production and consumption in the water column at multiple sites within the eastern tropical South Pacific. They also sought to map the natural distributions of nitrate, nitrite and nitrous oxide concentrations and isotopes at high spatial resolution in order to develop a dataset with which to constrain ocean models based on their rate measurements.

Incubation experiments were carried out at sea to quantify the rates of nitrification and N2O cycling in samples throughout the oxygen minimum zone. In addition, approximately 1000 samples were collected for nitrate and nitrite isotopic analysis and 500 samples for N2O isotopic analysis. The investigators worked closely with other researchers onboard to work towards developing the most coherent picture of nitrogen cycling in the eastern tropical Pacific to date.