Table of Contents

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

Measurements of dissolved organic nitrogen concentration and d15N.

NO3- + NO2- concentration and isotopic composition analysis

The NO3- + NO2- concentration of samples was determined using chemiluminescent analysis (Braman & Hendrix, 1989) in a configuration with a detection limit of 0.05 uM, and +/- 0.1 uM for 1 standard deviation (S.D.). The d15N of NO3- + NO2- was determined using the “denitrifier” method (K. L. Casciotti, Sigman, Hastings, Bohlke, & Hilkert, 2002; Sigman et al., 2001) with modifications (McIlvin & Casciotti, 2011) on samples with NO3- + NO2- concentration >0.3 uM (typically <0.2‰ 1 S.D.) (Supp. Table 1) (Knapp, Casciotti, Berelson, Prokopenko, & Capone, 2016).

DON concentration and isotopic analysis

The DON concentration of samples was determined using persulfate oxidation to convert DON to NO3- (Solorzano & Sharp, 1980), adapted according to (Knapp et al., 2005). The resulting NO3- concentration was then measured using chemiluminescence as described above. In cases where NO3- + NO2- (and/or ammonium, NH4+) was above the detection limit, DON was determined by subtracting the concentration of NO3- + NO2-+NH4+ from the concentration of total dissolved N (TDN). The average standard deviation for duplicate DON concentration analyses of individual samples that have undetectable levels of NO3- in the sample was +/- 0.30 uM, and the propagated error for DON concentration in the presence of detectable NO3- was +/- 0.32 uM.

The d15N of DON was determined according to (Knapp et al., 2005), where DON samples were oxidized to NO3- by persulfate oxidation (as described above in section 2.2), acidified to a pH range of 3 to 4, and measured as NO3- by the denitrifier method. In samples with measurable NO3- + NO2-, the d15N of DON is calculated by mass balance by subtracting the NO3- + NO2- concentration and d15N of NO3- + NO2- from the TDN concentration and TDN d15N measurements. In surface samples with undetectable NO3- + NO2- concentration, the standard deviation of duplicate analyses of DON d15N in a sample is +/- 0.3‰. For subsurface samples with NO3- + NO2- concentration approximately equal to the DON concentration, the propagated error for the calculation of DON d15N using a Monte Carlo method (Press, Teukolsky, Vetterling, & Flannery, 1992), and assuming duplicate analysis of a single sample and the standard deviations for TN concentration, NO3- + NO2- concentration and d15N of NO3- + NO2- given above, is +/- 0.6 0/00. The 15N of DON in samples with NO3- + NO2- concentration exceeding DON concentration, and/or with NH4+ concentration > 0.2 uM, was not determined (i.e., Stations 9, 10, 11, and 12 from the 2010 cruise).

Sampling

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 ºS and 80º W and 100º W (Fig. 1), with station locations and sample depths, salinities, sigma theta values, chlorophyll a concentrations, nitrate+nitrite concentration, NO3-+ NO2- d15N, DON concentrations, and DON d15N reported in Supplementary Information Table 1. Water column samples were collected by Niskin bottles deployed on a rosette equipped with conductivity-temperature-depth (CTD) sensors. All samples were collected into acid-washed, sample-rinsed HDPE bottles, and samples from the upper 400 m passed a 0.2 um filter before collection, and were stored at -20º C until analysis on land.

BCO-DMO Processing:

- dates reformatted to yyyy/mm/dd
- no data replaced with nd
- N/A replaced with NA
- blanks replaced with nd
- converted from wide to long format

Description from NSF award abstract:
Several independent lines of geochemical and remote sensing evidence suggest that dinitrogen (N2) fixation may be associated with surface waters downstream of major oxygen minimum zones (OMZs) and in particular in the Eastern Tropical South Pacific (ETSP). However, little direct evidence supports these inferences. Besides substantiating these indirect assessments, documenting significant N2 fixation in the ETSP would provide insight into two longstanding controversies: Is the marine N budget balanced, as implied by modeling and paleoceanographic data, and if so, how are the processes that add and remove N spatially, and thus temporally coupled?

In this project researchers at the University of Southern California and the University of Miami will test the hypothesis that fixation occurs in the ETSP at areal rates that equal or exceed those previously documented in more well-studied regions such as the oligotrophic waters of the sub/tropical North Atlantic. If scaled to the surface area of ETSP waters, this could add an additional 10-50 Tg N per year of inputs to the global marine N budget. They will undertake two cruises in the ETSP during early and late summer in two consecutive years to assess the quantitative significance of N2 fixation as a source of new N to surface waters using complementary biological and geochemical tools. N2 fixation rates will be evaluated on two temporal/spatial scales: daily/local (bottle 15N2 incubations and floating sediment traps); and seasonal/regional (d15N budget using moored sediment traps and water column TDN d15N). These estimates provide detailed observations of potential N2 fixation during station occupation in two summer seasons, when rates are expected to be greatest, as well as prolonged observation over lower expected N2 fixation periods. A combination of these different estimates will aim to determine if N2 fixation in this region can help balance the marine N budget. If all goes as planned, this study will determine the quantitative importance of N2 fixation in the ETSP, and whether these previously undocumented rates can help resolve the marine N budget. Implications include the ability of the marine N cycle to maintain homeostasis, and thus the global C cycle on glacial/interglacial time scales.

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.