Table of Contents

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

Nitrous oxide (N2O) data from the samples are available in the related dataset "ETSP N2O" (https://www.bco-dmo.org/dataset/821268).

Seawater samples were obtained during the R/V Atlantis (AT15-61) and R/V Melville (MV1104) cruises from January to February 2010 and March to April 2011. Water samples were collected at discrete depths using Niskin bottle type rosette samplers equipped with either 24 bottles (10L) or 12 bottles (20L), and an SBE9plus conductivity-temperature-depth (CTD) sensor package (SeaBird Electronics, Bellevue, WA).

The d15N and d18O of NO3- was determined using the "denitrifier" method (Casciotti et al., 2002; Sigman et al., 2001) with updated techniques (McIlvin & Casciotti, 2011). Samples were treated with sulfamic acid to remove NO2-, if present (Granger & Sigman, 2009).

The d15N and d18O of NO2- was determined using the chemical reduction of NO2- to nitrous oxide (N2O) using the "azide method" (McIlvin & Altabet, 2005).

Samples were collected on the R/V Atlantis in January through February 2010, and the R/V Melville in March through April 2011 between 10° and 20° South and between 80° and 100° West, with station locations and sample depths, salinities, sigma theta, oxygen concentrations values, etc. reported in the 'ETSP N2O' dataset. Water column samples were collected by Niskin bottles deployed on a rosette equipped with conductivity-temperature-depth (CTD) sensors. All samples were collected into sample-rinsed HDPE bottles and passed through a 0.2 micrometer (um) filter before collection, and were stored at -20° Celsius until analysis on land.

BCO-DMO Processing:
- Imported original file named "ETSP_ENSO_no3_no2_isotope_BCODMO_230227.xlsx" sheet 1 into the BCO-DMO data system.
- Replaced missing data identifier of 'NaN' with blank/empty values, which is the default for csv files.
- Added columns ISO_DateTime_Local, Latitude, and Longitude from related N2O dataset (https://www.bco-dmo.org/dataset/821268) by joining on key comprised of Year+Station+Depth.
- Named the final file "903891_v1_etsp_no3_and_no2_isotopes.csv".

Description from NSF award abstract:
Closing the marine budgets of nitrate and nitrous oxide are central goals for researchers interested in nutrient-driven changes in primary productivity and climate change. With the implementation of new methods for oxygen isotopic analysis of seawater nitrate, it will be possible to construct a budget for nitrate based on its oxygen isotopic distribution that is complementary to nitrogen isotope budgets. Before we can effectively use oxygen isotopes in nitrate to inform the current understanding of the marine nitrogen cycle, we must first understand how different processes that produce (nitrification) and consume (assimilation, denitrification) nitrate affect its oxygen isotopic signature.

In this study, researchers at the Woods Hole Oceanographic Institution will provide a quantitative assessment of the oxygen isotopic systematics of nitrification in the field and thus fill a key gap in our understanding of 18O variations in nitrate, nitrite, and nitrous oxide. The primary goal is to develop a quantitative prediction of the oxygen isotopic signatures of nitrite and nitrate produced during nitrification in the sea. The researchers hypothesize that oxygen isotopic fractionation during nitrification is the primary factor setting the 18O values of newly produced nitrate and nitrite. Secondly, they hypothesize that oxygen atom exchange is low where ammonia oxidation and nitrite oxidation are tightly coupled, but may increase in regions with nitrite accumulation, such as in the primary and secondary nitrite maxima. They will test these hypotheses with a series of targeted laboratory and field experiments, as well as with measurements of nitrite and nitrate isotopic distributions extending through the euphotic zone, primary nitrite maximum, and secondary nitrite maximum of the Eastern Tropical South Pacific. The results of these experiments are expected to provide fundamental information required for the interpretation of 18O isotopic signatures in nitrite, nitrate, and N2O in the context of underlying microbial processes. A better understanding of these features and the processes involved is important for quantifying new production, controls on the N budget, and N2O production in the ocean -- which should lead to a better understanding of the direct and indirect interactions among the nitrogen cycle, marine chemistry, and climate.