Table of Contents

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

Wintertime nitrate delta 15N and delta 18O from the Southern Ocean south of Africa from hydrocasts taken on the July 2012 R/V S.A. Agulhas II VOY03 expedition. The dataset includes hydrocast (depth-profile) data along with the hydrographic data accompanying each CTD.

Note: In addition to the awards identified below, the dataset was also supported by funding from the South African National Research Foundation (NRF), Applied Centre for Climate and Earth Systems Science (ACCESS), University of Cape Town (UCT), and the Grand Challenges Program of Princeton University.

Samples were collected aboard the R/V S.A. Agulhas II and analyzed in the Sigman lab at Princeton University using the "denitrifier method" to convert sample nitrate to N2O gas. The isotopic composition of N2O was measured by GC-IRMS using a Thermo MAT 253 mass spectrometer with a purpose-built on-line N2O extraction and purification system. For the nitrate+nitrite data, the pooled sample standard deviation is 0.05‰ for delta 15N (n = 3-7) and 0.17‰ for delta 18O (n = 3-5). For the ‘nitrate-only’ data, the pooled sample standard deviation is 0.05‰ for delta 15N (n = 3-7) and 0.13‰ for delta 18O (n = 3-7). See methods section of Smart et al. (2015) for details of sample collection, isotope analysis and data quality.

Related Publications:
Smart, S.M., S.E. Fawcett, S.J. Thomalla, M.A. Weigand, C.J.C. Reason, and D.M. Sigman. 2015. Isotopic evidence for nitrification in the Antarctic winter mixed layer, Global Biogeochem. Cycles, 29, doi: 10.1002/2014GB005013.

Individual sample analyses were referenced against automated injections of N2O from a gas cylinder; however, the N2O cylinder is not used as the absolute reference. Rather, each sample run included replicates of two international reference materials, IAEA-N3 and USGS-34 that were used to calibrate isotope ratios to that of N2 in air (for delta 15N) and VSMOW (for delta 18O). An in-house N2O standard was also run with each batch of samples to monitor mass spectrometry. See methods section of Smart et al. (2015) for a description of the procedures followed.

BCO-DMO Processing Notes:
- Modified parameter names to conform with BCO-DMO naming conventions.
- Replaced blanks (missing data) with 'nd', meaning 'no data'.

Description from NSF award abstract:
The high concentration of the major nutrients nitrate and phosphate is a fundamental characteristic of the Antarctic Zone in the Southern Ocean and is central to its role in global ocean fertility and the global carbon cycle. The isotopic composition of diatom-bound organic nitrogen is one of the best hopes for reconstructing the nutrient status of polar surface waters over glacial cycles, which in turn may hold the explanation for the decline in atmospheric carbon dioxide during ice ages. The PIs propose to generate detailed diatom-bound nitrogen isotope (δ15Ndb) records from high sedimentation rate cores from the Kerguelen Plateau. Because the cores were collected at relatively shallow seafloor depths, they have adequate planktonic and benthic foraminifera to develop accurate age models. The resulting data could be compared with climate records from Antarctic ice cores and other archives to investigate climate-related changes, including the major steps into and out of ice ages and the millennial-scale events that occur during ice ages and at their ends. The records generated in this project will provide a critical test of hypotheses for the cause of lower ice age CO2.

This study will contribute to the goal of understanding ice ages and past CO2 changes, which both have broad implications for future climate. Undergraduates will undertake summer internships, with the possibility of extending their work into junior year projects and senior theses. In addition, the PI will lead modules for two Princeton programs for middle school teachers and will host a teacher for a six-week summer research project.