Table of Contents

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

Samples spanned basins and water depths, including the North Atlantic Gyre, Northeast Pacific oxygen minimum zone, Northeast Pacific Shelf, North Pacific Gyre, San Pedro Basin, Caeté Estuary, South Pacific Gyre, and the North Sea. Samples spanned depths of 0 to 4800 meters. Exact location data, including latitude, longitude, and collection depth are provided in the dataset. Most data in this dataset represent measurements of aliquots of archived samples from collaborators; these were collected on many different cruises, on many different dates, around the world. New samples represented in this dataset include: 12 samples collected at station HOT on the R/V Kilo Moana in November 2020 (KM2013) and 12 samples collected at station BATS in September 2020 (AE2014).

Seawater samples (of ∼5 to 20 liters (L) were collected from Niskin bottles into acid-washed polyethylene containers and filtered through a 0.80/0.45 micrometer (μm) capsule filter (AcroPak 500) prior to acidification to pH 2 with reagent grade 12 N hydrochloric acid. Samples were then stored at 4 degrees Celsius (°C) in the dark until back on shore. In the lab, dissolved organic carbon (DOC) concentrations were measured prior to solid phase extraction (SPE) via high-temperature combustion on a Shimadzu Total Organic Carbon Analyzer (Shimadzu Corp). Dissolved organic matter (DOM) was concentrated using SPE with Bond Elute PPL cartridges (Agilent; 1 gram, 6 milliliter (mL) size) following Dittmar et al. (2008). DOM samples were eluted in GC-grade methanol. Extracts were dried under a stream of N2 gas and transferred to 2 mL GC vials and then to 150 microliter (μL) glass inserts. Aliquots corresponding to ∼4.5 micrograms (μg) sulfur (∼350 μg DOC) were transferred in methanol from 2 mL GC vials into smooth-walled tin EA capsules (~ 2.9 millimeters (mm), OEA Laboratories). Methanol was evaporated at room temperature (∼2 hours) prior to folding. Tin capsules containing dried DOM were folded closed with clean tweezers, loaded into an autosampler, and then combusted and analyzed by combustion elemental analyzer / isotope ratio mass spectrometer following Phillips et al. (2021).

Isotope data were recorded using Isodat v3.0 (Thermo Scientific). Isodat calibrates the integrated ion-current ratios against internal reference gas standards, and reports them as δ13C and δ34S values relative to the VPDB and VCDT isotopic scales, respectively. All δ values are reported as ‰ (permil, ppt) deviations from the relevant zero points. Subsequent data processing in spreadsheets included subtraction of capsule blanks, discarding outliers, linearity (peak size) correction, and further calibration to external standards. Sulfur isotope and concentration standards included a methionine working standard, seawater sulfate, and silver sulfide reference materials (IAEA S1, S2, S3). Additionally, a working standard of DOMSPE extracted in a large batch (∼200 L) from the Scripps Institution of Oceanography (SIO) pier was run in at least triplicate with each sample set. C:S ratios were obtained by dividing corrected carbon and sulfur amounts (molar ratio), as calculated from EA peak areas. DOS concentrations were calculated as the product of measured total DOC concentration and DOMSPE C:S ratio.

Extracts supplied by collaborators were often limited by sample size to single or duplicate analyses, so we could not always estimate precision directly for each sample. Instead, uncertainties in isotopic compositions and C:S ratios for samples are reported using the SD of our DOM SIO pier standard measurements, divided by the square root of number of sample replicates analyzed. These SEs (1σ) were ≤0.2‰ for δ34S values and δ13C values and ≤6 for C:S ratios, and we therefore refer to DOSSPE concentrations as apparent or calculated throughout.

- Imported original file "pnas.2209152119.sd01.xlsx" into the BCO-DMO system.
- Renamed fields to comply with BCO-DMO naming conventions.
- Converted the Month column from string to numeric.
- Saved final file as "927046_v1_dom_sulfur_and_carbon_analysis.csv".

NSF Award Abstract:
Organic molecules that contain sulfur in the ocean are poorly understood. Organic sulfur is abundant, and likely helps stabilize dissolved organic matter, binds important trace metals, and provides trace nutrients to phytoplankton. However, it is not known whether organic sulfur molecules derive from biological processes, or from non-biological “sulfurization” reactions involving hydrogen sulfide (H2S). Existing knowledge predicts that the two pathways should generate very different -- and easily measurable -- distributions of sulfur isotopes (i.e., elemental sulfur with varied atomic mass). This study will use a new, highly sensitive method to measure the sulfur isotope values of dissolved organic molecules to distinguish between the two formation pathways. If successful, this method will be used to calculate how much dissolved organic sulfur results from each pathway. This project will provide an opportunity for a local high school student to engage in laboratory research during a one year internship, in partnership with a local all-girls high school. Additionally, the graduate student supported by this project will host a “Day in the Life” information session on the Women Doing Science Instagram social media platform. She will answer questions about oceanography and marine science careers from followers of this Instagram account while on a research cruise collecting samples for the project.

Several lines of evidence have recently converged to focus interest on marine dissolved organic sulfur (DOS), including the recognition that many heterotrophic bacteria require exogenous sources of ‘fixed’ organic sulfur; that organosulfur molecules -- particularly thiols -- play a major role in binding and sequestering trace metals; and that at 6700 Tg S, DOS is the second largest (after dissolved sulfate) sulfur pool in the oceans. Perhaps most importantly, DOS is thousands of years old, implying that it is not rapidly recycled and challenging the expectation that DOS derives mainly from labile biomolecules such as cysteine, methionine and dimethylsulfoniopropionate (DMSP). A plausible alternative is that abiotic sulfurization reactions with H2S from anoxic porewaters and the water column is a source of recalcitrant marine DOS. Distinguishing between these formation pathways is critical for understanding DOS dynamics, but thus far has been difficult. Based on existing data, it is thought that DOS formed from marine phytoplankton should have a sulfur isotope value (δ34S) near +20‰ (VCDT). Alternatively, DOS formed from abiotic sulfurization should be more variable, with average values of -20‰. Because these isotope values are distinct, they should be able to distinguish between the two pathways. The primary obstacle to analysis has been the low (µM) concentration of DOS, and difficulties associated with concentrating it in seawater. This study will use new analytical techniques developed at Caltech to measure the δ34S values of DOS at several locations, and compound-specific δ34S values for cysteine and methionine from the Bermuda (BATS) and Hawaii (HOT) time series locations. This work will contribute new information about the marine sulfur cycle, and improve understanding of the role that DOS plays in stabilizing carbon in the deep oceans.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.