Table of Contents

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

Seawater samples were obtained during the R/V Atlantis (AT15-61) and R/V Melville (MV1104) cruises in Jan-Feb 2010 and Mar-Apr 2011.  Water samples were collected at discrete depths using a standard 24-bottle Niskin rosette sampler equipped with an SBE9plus conductivity-temperature-depth (CTD) sensor package (SeaBird Electronics, Bellevue, WA). 

Ammonium concentration ([NH4+]) was determined on-ship in unfiltered 50 mL seawater samples using o-phthaldialdehyde derivatization (Holmes et al., 1999) and measurement on an Aquafluor 8000 handheld fluorometer (Turner Designs), with modifications as suggested in Taylor et al., 2007. Nitrite ([NO2-]) was determined on-ship in unfiltered 50 mL sample volumes using standard colorimetric methods (Strickland and Parsons, 1968).  NH4+ standards (30 – 300 nM) were freshly prepared for each analysis in duplicate using deep water (> 500 m) from the same station, which consistently had a lower blank than ultrapure water. Detection limits for [NH4+] and  [NO2-] were 0.010 µM, and 0.10 µM for [NO2-]. Measured values are reported even if they are lower than the detection limit.

Full protocol on seawater [NH4+] processing can be found below under Supplemental Files: "Ammonium Concentration in Seawater Protocol Santoro Lab v2.3"

BCO-DMO processing: 
- Adjusted parameter names to comply with database requirements. 
- Combined year, month, day fields into one date field. 
- Units in parentheses removed and added to Parameter Description metadata section. 
- Added a conventional header with dataset name, PI name, version date.
- Missing data identifier of 'NaN' replaced with 'nd'  ('nd' is BCO-DMO system default missing data identifier). 

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.