Table of Contents

• Coverage
• Dataset Description
  • Methods & Sampling
  • Data Processing Description
• Data Files
• Parameters
• Instruments
• Deployments
• Project Information
• Funding

Nitrogen and oxygen isotopic data from a 2005 cruise on R/V Knorr (KN182-09) in the Peruvian Oxygen Deficient Zone.

Sampling and Analytical Methodology:
Samples were collected from Niskin bottles, syringe filtered through 0.2 um pore-size capsule filters and frozen immediately in 60 mL HDPE bottles. Samples were stored frozen until analysis. Nitrate and nitrite concentrations were measured on separate aliquots by auto analyzer at the WHOI nutrient facility.

Samples were initially analyzed for nitrate +  nitrite d15N and d18O using the denitrifier method (Sigman et al., 2001; Casciotti et al., 2002), then nitrite d15N and d18O were subtracted out according to Casciotti and McIlvin (2007). Later, the samples were reanalyzed for nitrate d15N and d18O using the denitrifier method after treatment with sulfamic acid using the method of Granger and Sigman (2009). These latter analyses, which we believe to be more accurate and precise, are reported here. Samples were analyzed in duplicate and are reported as the mean and standard deviation of replicate measurements. Typical precision for these analyses is 0.2-0.3 for d15N and 0.3-0.5 for d18O.

Samples were analyzed for nitrite d15N and d18O using the azide method (McIlvin and Altabet, 2005). Nitrite d18O data are not reported because it was determined that samples may have undergone O atom equilibration with water during storage (Casciotti et al., 2007). Typical precision for these analyses is 0.3-0.5 for d15N and 0.3-0.5 for d18O.

All isotopic measurements were conducted on a Thermo Finnigan Delta PLUS XP isotope ratio mass spectrometer.

Data Processing:
Nitrate d15N and d18O analyses were calibrated against aliquots of the nitrate isotope standards USGS32, USGS34, and USGS35 (Bohlke et al., 2003) prepared and analyzed in parallel with each batch of samples, according to McIlvin and Casciotti (2011).

Nitrite d15N analyses were calibrated against aliquots of the nitrite isotope standards RSIL-N23, RSIL-N7373, and RSIL-N10219 (Casciotti et al., 2007) prepared and analyzed in parallel with each batch of samples.

BCO-DMO Processing Notes: modified original parameter names to conform with BCO-DMO naming conventions; replaced blank/missing values with 'nd' to indicate "no data".

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.