Table of Contents

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

Vertical profiles from the Mexican OMZ of N2 gas concentrations in excess of equilibrium values along with the isotopic composition of total N2, NO3- and NO2-.

Sampling at sea was done using a standard SeaBird CTD/Rosette system. Hydrographic data was processed using SeaBird software and standard procedures. O2 was measured by an SBE43 sensor and final data were calibrated against Winkler O2 determinations

NO3- and NO2- concentrations were measured in the laboratory on frozen samples using a SmartChem autoanalyzer using standard chemical methods

N2 excess were determined from N2/Ar ratios measured using the procedures described by Charoenpong et al. (2014) with the exception that the measurements were made at sea using a Pfieffer 400 series quadrupole mass spectrometer system. Data acquisition and initial data processing used custom LabView software. Biogenic N2 concentrations were determined by subtraction of estimated background excess N2 as described in Chang et al. (2012). The d15N of N2 was determined in the laboratory using similar methodology but using an IsoPrime Isotope Ratio Mass Spectrometer (IRMS) as described by Charoenpong et al. (2014) using IonVantage software. 

The isotopic composition of nitrate and nitrite was determined on samples returned to the laboratory using procedures described by McIlvin and Casciotti (2011) and McIlvin and Altabet (2005). Samples for nitrate isotope analysis were preserved by mild acidification and addition of sulfamic acid to remove nitrite. Nitrite samples were preserved at high pH with NaOH to retain its d18O signature. An IsoPrime IRMS running IonVantage was used to make these measurements.

Final data reduction and organization was done using Microsoft Excel.

See above for data processing.

BCO-DMO processing:
- modified parameter names to conform with BCO-DMO naming conventions (replaced # with "num", replaced hyphens with underscores);
- re-formatted date to mm/dd/yyyy;
- re-formatted time to HH:MM:SS;
- added ISO date/time field using original date and time fields;
- replaced "#N/A" and blanks (missing data) with "nd";
- there were 12 rows where lon values were degrees and decimal mins (CTD 11F02; 107 53.218). Converted those to decimal degrees.

NSF Award Abstract:
Intense oxygen minimum zones (OMZ) of the world's oceans, though constituting a small fraction of total oceanic volume, host critical biogeochemical processes and are central to understanding the ocean's N cycle and its biogeochemical and isotopic signatures. OMZ's are sites for a large portion of marine combined N loss to N2 (25 to 50%) and dominate the ocean N isotope budget through cogeneration of 15N and 18O enriched NO3 -. Major outstanding issues include the magnitude of this N sink, the stoichiometry between NO3 - loss and the production of biogenic N2, the microbial pathways leading to N2 production, as well as the interaction between these OMZ processes and the surface export of organic matter as well as physical circulation.

The PI's request funding to develop a new, in situ, autonomous tool for studying N loss in OMZ's. It will allow observation of variability over a range in temporal and spatial scales that are critical for understanding controlling processes and better estimating the magnitude of N-loss. The sustained deployments possible with autonomous platforms will be critical for detecting any response of OMZ's to climate change.

Broader Impacts: Nitrogen is often the limiting nutrient for biological production in the oceans, and the current global marine nitrogen balance has been in much debate due to a number of uncertainties and questions. A successful development of this proposed sensor-float package may help in resolving some of the important questions on the spatial and temporal variabilities of the OMZs. In turn, such knowledge is essential in assessing the global nitrogen balance in the current and future oceans. This proposed project would involve active participation of undergraduates, graduates and postdocs, as well as the training of a K-12 science teacher. This project would also foster collaboration with international researchers. The PI's have partnered with Ocean Explorium at New Bedford Seaport to provide an educational outreach component designed to aid teacher development and create a field trip program for teachers in the south coast of Massachusetts. The proposal will support post-doc, graduate and undergraduate students.