Table of Contents

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

These data were published as Figure 5 in Santoro et al. (2020).

Nitrate (NO3-) and Nitrite (NO2-) d15N and d18O data from the samples are available in the related dataset "ETSP NO3 and NO2 isotopes" (https://www.bco-dmo.org/dataset/903891).

Description of changes/updates contained in version 2 of this dataset:
Most depths were analyzed in duplicates from two bottles (i.e. one measurement of all isotopic variables and a concentration per bottle). This version (v2) contains each measurement in a unique column (i.e. d15n_alpha1, d15n_alpha2, etc.). A quality control (QC) flag was also added for each measurement using the SeaDataNet QC guidelines available at https://www.seadatanet.org/Standards/Data-Quality-Control.

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). This dataset's samples were collected from Niskin bottles into 160-milliliter (mL) glass serum vials, killed with saturated mercuric chloride, and crimp sealed with gray butyl stoppers. Prior to sealing, ~1 mL of sample was removed to allow room for sample expansion and to prevent the sample vessel from shattering upon warming.

In all cases, samples were extracted and analyzed using a custom automated purge and trap inlet system and normalized to an injection of calibrated pure N2O reference gas introduced prior to the elution of each sample peak (McIlvin & Casciotti, 2010). Isotope ratios were referenced initially to the calibrated N2O reference tank to create a set of 'ratio of ratios' (³¹Rsample/³¹Rreference, ⁴⁵Rsample/⁴⁵Rreference, ⁴⁶Rsample/⁴⁶Rreference). Next, the data were size corrected in reference to a calibrated 20 volt-second (Vs) peak area for a mass-to-charge ratio of 44 (m/z 44). Finally, 'scrambling coefficients' were applied to the isotopomer data to correct the measured 15Ra and 15Rb for the rearrangement of nitrogen atoms in N2O when the gas is ionized in the mass spectrometer ion source (Frame et al., 2014; Frame & Casciotti, 2010; Kelly et al., 2021). The isotope ratios of N and O atoms in N2O, ¹⁵Rsample or ¹⁸Rsample, respectively, are expressed in delta notation (δ), where the δ15N and δ18O are defined relative to the isotope ratios of certified standards: δ15N or δ18O = (Rsample/Rstandard - 1) x1000. The Rstandard values used for δ15N and δ18O are the ratios of 15N/14N and 18O/16O in atmospheric N2 and Vienna Standard Mean Ocean Water (VSMOW), respectively.

Data Processing:
CTD sensor data were processed using Seabird Electronics (SBE) Data Processing software using SBE recommended parameters, including the tau oxygen correction and oxygen hysteresis correction. Processing commands were applied in the following order: filter, alignctd, celltm, loopedit, wildedit.

Derived parameters (pottemp, aou, sigma_theta) were calculated in MATLAB using the CSIRO SEAWATER function library for MATLAB, ver 3.3 (22-Sept-2010).

Version 1:
(Version Date: 2020-08-18)
v1 BCO-DMO Processing:
- Added a conventional header with dataset name, PI name, version date.
- Adjusted parameter names to comply with database requirements.
- Combined year, month, day fields and adjusted timezone to create ISO date field.
- Units in parentheses removed and added to Parameter Description metadata section.
- Missing data identifier of 'NaN' replaced with 'nd'.

Version 2:
(Version Date: 2023-07-28)
Description of v2: This includes a further QC'ed version of the v1 data with six additional isotopic variables: d15n_alpha, d15n_beta, Site_preference (SP), d17O, and d18O. N2O samples were analyzed in duplicates, thus each analysis has an associated QC flag.
v2 BCO-DMO Processing:
- Imported original file named "ETSP_ENSO_N2Oisotope_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.
- Renamed "Presure" column to "Pressure".
- Filled in a value of "1" in the "flag_1" column per request of data submitter.
- Named the final file "821268_v2_etsp_n2o.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.