Table of Contents

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

Planktic foraminifer shell d15N from sediment traps in the Sargasso Sea (off Bermuda) in the western subtropical North Atlantic. Sediment traps from 500 m, 1500 m, and 3200 m water depth (31°50’N, 64°10’W).

Data published in:
Smart, S.M., H. Ren, S.E. Fawcett, R. Schiebel, M. Conte, P.A. Rafter, K.K. Ellis, M.A. Weigand, S. Oleynik, G.H. Haug, D.M. Sigman (2018). Ground-truthing the planktic foraminifer-bound nitrogen isotope paleo-proxy in the Sargasso Sea, Geochimica et Cosmochimica Acta, doi: https://doi.org/10.1016/j.gca.2018.05.023

Planktic foraminifer shells were picked from Oceanic Flux Program (OFP) sediment traps at 500 m, 1500 m, and 3200 m water depth (Conte et al., 2001; Conte and Weber, 2014). After crushing and the appropriate cleaning steps (Ren et al., 2012; Ren et al., 2015), shell-bound N was released by dissolution with hydrochloric acid, and organic N converted to nitrate using a basic persulfate oxidising reagent (POR; a potassium persulfate/sodium hydroxide solution) (Nydahl, 1978; Knapp et al., 2005). The "denitrifier method" was used to convert sample nitrate to N₂O gas for isotope analysis (Sigman et al., 2001; Casciotti et al., 2002; Weigand et al., 2016). See methods section of Smart et al. (2018) for more details of sample collection, isotope analysis and data quality.

The isotopic composition of N₂O was measured at Princeton University by GC-IRMS using a Thermo MAT 253 mass spectrometer with a purpose-built on-line N₂O extraction and purification system. See methods section of Smart et al. (2018) for more details of sample collection, isotope analysis and data quality.

Data Processing: Isotope ratio measurements were referenced against automated injections of N₂O from a gas cylinder, and then calibrated to N₂ in air using two international reference materials IAEA-N3 and USGS-34 that were included in every run. An in-house N₂O standard was also measured along with each batch of samples to monitor mass spectrometry. Oxidized samples were additionally corrected for the contribution of the N blank associated with the POR.

Missing data identifier = -999

BCO-DMO Processing:
- modified parameter names (replaced spaces w/ underscores);
- formatted date from mm/dd/yyyy to yyyymmdd.

NSF Award Abstract:
Nitrogen (N) and phosphorus are the two nutrients required in large quantity by phytoplankton in the ocean, and together they limit productivity throughout most of the tropical, subtropical, and temperate ocean. Both the cycling of N and its input/output budget have been argued to control the fertility of the ocean and the ocean's role in setting atmospheric CO2. The CaCO3 tests of foraminifera can represent a substantial fraction of marine sediments and have been used extensively in paleoceanography; they are an obvious target for isotopic analysis of microfossil-bound organic matter.

In recent years, researchers at Princeton have developed a protocol for the isotopic analysis of foraminiferal shell-bound N. The current protocol is at least 100 times more sensitive than typical on-line combustion, allowing for rapid progress with a N isotope archive that was previously not feasible to measure. Measurements on surface sediments and a downcore record from the Caribbean show the promise of foraminifera-bound del15N (fb-del15N) to provide both a robust N isotope archive for paleoceanography, and one with a unique potential of richness, given the existence of multiple foraminiferal species with different depth habitats and behaviors. Moreover, the finding from the Caribbean Sea record -- reduced N fixation in ice age Atlantic -- has changed the scientific conversation about the nature of the input/output budget of oceanic fixed N and its potential to change ocean fertility and atmospheric CO2. However, the controls on fb-del15N have not yet been adequately studied.

In this project, as a first major step in developing a foundation for the paleoceanographic application of fb-del15N, the same Princeton University team will study its genesis in the water column, transport to the seafloor, and early diagenesis. They will undertake this study in the Sargasso Sea south of Bermuda. This is one of the best studied regions of the ocean, in general and with respect to foraminifera, and a region that has been has been a focus of the N isotope research of the PI for the last decade and others previously. Moreover, its significant seasonality -- in physical oceanography, biogeochemistry, and foraminiferal species abundance -- will facilitate the effort to understand the controls on fb-del15N at a mechanistic level. The research team will participate in six Bermuda Atlantic Time-series Study (BATS) cruises over two years, collecting foraminifera and other N forms likely to provide insight into the controls on fb-del15N. From the nearby Oceanic Flux Program (OFP) moored sediment traps and from shallow sediments collected in the region, they will pick foraminifera shells and again make relevant ancillary measurements. This work will establish the relationship of foraminiferal biomass to shell-bound del15N for different species, and comparison of the foraminiferal isotope data with the upper ocean N pools will yield empirical isotopic relationships and work toward a mechanistic insight of fb-del15N (e.g., the importance of different N pools to the diets of different foraminifera; the role of algal symbionts). The sediment trap and surface sediment data will support the plankton tow data by integrating over longer time scales and will also address questions regarding late stage (e.g., gametogenic) calcification and the early diagenesis of fb-del15N and fb-N content.

Broader Impacts: This study will yield an improved understanding of the nutrient dynamics of foraminifera, a class of organisms whose shells are a central tool in micropaleontology and paleoclimatology. The project will also build on the principal investigator's involvement in the Bermuda Institute of Ocean Sciences as an asset for integrating ocean-related education and research at both the undergraduate and graduate levels.