Table of Contents

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

Core collection: Cores were collected via three different methods: a multicorer (MUC), a gravity corer (Gravity), or push cores via the HOV Alvin (HR23) or the ROV Jason (HR24; push). Based on pre-discussed project needs, the core collected was then allocated for a specific purpose and assigned a flux regime based on location. When possible, the overlying water on top of the core was siphoned off for major anions and assigned a number.

Core sectioning and porewater extraction:
HR23: Push core material was sectioned through extrusion out the top at 3 centimeters (cm) resolution, gravity core material was sectioned with a tubing cutter, and MUC cores were sectioned by extrusion, typically at 5 cm resolution. First, (2) 3-milliliter (mL) sediment plugs were taken for CH4/CO2 and (1) 3 mL sediment plug was taken for H2 if the Core Allocation = Radiocarbon. A small scoop of sediment was taken for porosity and placed in a Petri dish. Then, the remaining sediment was placed in Reeburgh squeezers (Reeburgh, 1967) to extract the porewater into acid-washed 60 mL syringes equipped with stopcocks. Porewater was then filtered through 0.45 micrometer (um) polyethersulphone syringe filters.

HR24: Push core material was sectioned through extrusion out the top at 5 cm resolution. (2) 3 mL sediment plugs were taken for CH4/CO2 and a small scoop of sediment was taken for porosity and placed in a Petri dish. Then, the remaining sediment was packed into 60 mL acid-clean centrifuge tubes and a rhizon sampler (Rhizosphere Research Products) was inserted. A vacuum was pulled with an acid-clean 60 mL syringe to extract the porewater.

Sediment/porewater sampling:
CH4, CO2, δ13C–CH4, δ13C-CO2: 10 mL glass serum vials containing 3-6 mL sediment preserved with 5 mL saturated brine were subsampled using the headspace equilibration method following Magen et al. (2014). Briefly, 8 mL of ultra-zero purity air (UZA) was injected into the sediment vial with a plastic syringe equipped with a 22-gauge needle. Vials were shaken and the headspace was mixed for 2 minutes, after which 8 mL of headspace gas sample was removed. Check standards for concentration were run approximately every 5 samples and corrected as described by Pohlman and others (2021). For samples measured by cavity ring-down spectroscopy, dissolved concentrations were calculated using the following equation:

((pCH4 * 10^-6 * Vg) / (R * T_extraction) + (Vw * pCH4 * 10^-6 * Sol_CH4)) / Vw * 10^6 = [CH4]

Where pCH4 is the headspace concentration of methane in ppm, Vg is the volume of gas sample in mL, R is the gas constant in L-atm/mol-K, T_extraction is the extraction temperature in Kelvin, Vw is the volume of water sample that was extracted in mL, Sol_CH4 is the solubility of methane in mol/L-atm (Wiesenburg & Guinasso, 1979; Yamamoto et al., 1976), and [CH4] is the dissolved concentration of methane in the original natural water sample, in micromoles per liter.

Porosity: Fresh sediment was placed in pre-weighed Petri dishes, sealed, and refrigerated until measurement in the lab. Petri dishes were weighed upon return to shore, then were placed in a drying oven (45 degrees Celsius, 1 week). Petri dishes with dried sediment were weighed again. The difference between the wet and dry sediment is the water weight, and the ratio of the water volume to the total volume is the porosity, assuming a dry bulk density of 2.5.

DIC and δ13C-DIC: 1 mL porewater was injected into a helium-filled 12 mL exetainer vial pre-filled with 1mL 85% phosphoric acid. The stable carbon isotope ratios and concentrations were determined from CO2 by isotope ratio mass spectrometry. Measurements are standardized with lithium carbonate reference material, and isotope ratios are reported in the standard d-notation relative to VPDB.

DOC and δ13C-DOC: 40 mL amber VOA vials containing 1 mL porewater sample acidified to pH 2 with trace metal clean HCl were measured with the wet oxidation method, similarly to the setup detailed in Lalonde et al. (2014). Samples were acidified and sparged to remove inorganic carbon, then reacted with Sodium persulfate as wet oxidation. Data were normalized using two different internal organic standards (precision ± 0.5 ppm for concentrations and 0.2‰ for isotopes).

SO4 and Cl: 2 mL microcentrifuge tubes containing 40 uL porewater sample acidified to pH 2 with 0.1M H3PO4 were diluted (135x) with Milli-Q water prior to analysis. IAPSO certified seawater standards (Ocean Scientific International Ltd.) were used for all samples and precision is ±1.5%.

TOC and δ13C-TOC: Sediment samples from all cruises were dried at 60 degrees Celsius for three days, then ground with a mortar and pestle. For HR23, homogenized sediment was weighed into silver capsules for acid fumigation. Samples were acid fumigated to remove carbonate and dried at 60 degrees Celsius for 2 to 4 hours, then wrapped into tin capsules. For HR24, homogenized sediment was acidified with direct acid application to remove carbonate, then rinsed, dried, and wrapped into tin capsules. Samples were run on an elemental analyzer interfaced with an isotope ratio mass spectrometer. Samples were normalized to an acetanilide, protein, and bass standard calibrated to standards USGS 40 and USGS 41. Stable carbon isotope values are reported in d notation relative to VPDB.

H2S: Porewater sulfide (ΣH2S) was determined by sparging the sample aliquot after acidification with 25 % phosphoric acid, and trapping evolved H2S in sulfide antioxidant buffer (SAOB) solution for measurement with a sulfide-specific electrode. Standards were prepared from a sodium bisulfide stock solution that was titrated with lead nitrate to determine its concentration daily. Also on a daily basis, the stock solution was serially diluted with SAOB solution to produce a five-level calibration.

NH3: Porewater sample was diluted (4-5x) with Milli-Q water prior to analysis. The SM4500-NH3 G-2011 method was used. All reported values were above the 2024 method detection limit (0.009 mg N/L).

PO4: Porewater sample was diluted (2x) with Milli-Q water prior to analysis. The EPA 365.1 Rev 2.0, 1993 method was used. All reported values were above the 2024 method detection limit (0.0034 mg P/L).

NO2+NO3: Porewater sample was diluted (4-5x) with Milli-Q water prior to analysis. The ASTM D-7781-14 method was used. All reported values were above the 2024 method detection limit (0.0009 mg N/L).

Methane, carbon dioxide, and dissolved inorganic carbon concentrations and stable carbon isotope ratios: Software for Picarro was used to obtain the CH4, CO2, and DIC concentrations (ppm) as well as stable carbon isotope ratios (delta notation; per mille). Concentrations were converted to dissolved concentrations (mM) and stable carbon isotope ratios were corrected for the machine offset using Microsoft Excel.

Sulfate and chloride: Chromeleon 7 software was used to obtain areas for SO4 and Cl. Areas were converted to concentrations (mM) using Microsoft Excel.

Particulate organic carbon concentrations and stable carbon isotope ratios: EAS Clarity 3.0.0.154 and Isodat Acquisition Version 3.0 were used to obtain POC concentrations (%) and stable carbon isotope ratios (delta notation; per mille). Concentrations and stable carbon isotope ratios were corrected for machine drift using Microsoft Excel.

- Imported sheet 1 of the original Excel file "HR23_HR24_combined_BCO_DMO_v2.xlsx" into the BCO-DMO data processing system.
- Renamed column OLW_# to OLW_num to comply with BCO-DMO naming conventions.
- Converted Date_Collected column from string format %m-%d-%y to date type with output format %Y-%m-%d.
- Renamed column NO3_mgN_L to NO2_NO3_mgN_L.
- Saved the final file as "1002509_v1_porewater_geochem_hr23_hr24.csv".

NSF Award Abstract:
Dissolved organic carbon (DOC) is a key component of the ocean’s food web and carbon cycle, and carbon exchanged between oceanic DOC and the atmosphere has influenced atmospheric CO2 levels on timescales ranging from recent decades to the geologic past. Production by marine algae in the surface ocean is the largest source of DOC and its effects on the ocean carbon cycle are widely appreciated. However, the contribution of DOC from additional sources such as rivers, hydrothermal vents, and methane seeps and their impact on ocean ecology and chemistry are not well understood. Each source differs in terms of its biological utilization and age, which affects the storage and distribution of DOC among the ocean basins. Methane seeps located along continental margins are particularly significant because they may transfer globally significant quantities of carbon stored below the seafloor as natural gas and gas hydrate to the oceans. This project will investigate the production, flux, composition and potential for biological utilization of DOC at Hydrate Ridge, located offshore Oregon. Hydrate Ridge is a prominent methane seep with massive accumulations of gas hydrate and a node of the Ocean Observatories Initiative telecommunications cabled array on the Juan De Fuca tectonic plate, which provides a continuous stream of real-time regional oceanographic data. We will sample and chemically characterize methane, DOC, and other materials to provide information about where the materials originated (deep vs shallow), how they have been chemically altered, to what extent they may feed deep ocean organisms, or contribute to the long term storage of DOC in the ocean. Experiments and analysis will be conducted using sediment cores and bottom water samples collected using either the remotely operated vehicle Jason or the human occupied vehicle Alvin during a 7-day ocean expedition. Additionally, this project will place osmotically-driven pumps on the seafloor to continuously sample fluids for approximately one year, thereby allowing us to monitor the movement of methane and DOC expelled from the seafloor to the ocean and constrain processes that regulate the release of carbon to the oceans at methane seeps. This project will support one graduate student and several undergraduates from a community college in Maryland and a college located in a lower-income urban center in southeastern Massachusetts. We will disseminate project findings to the public with a series of videos for public TV.

This study will investigate the production, flux and reactivity of methane-derived dissolved organic carbon (DOC) from methane (CH4) seeps at Hydrate Ridge, Offshore Oregon. The study will address four fundamental questions to determine the significance of CH4-derived DOC within the ocean carbon cycle: (1) How much CH4-derived fossil DOC do seeps contribute to the oceans? (2) To what extent is CH4-derived C incorporated into DOC during anaerobic oxidation of CH4? (3) Is seep DOC bioavailable or recalcitrant when released into the deep ocean? (4) How does the flux of DOC to the water column vary over time? We will employ an interdisciplinary strategy that includes in situ sampling, laboratory incubations, and a comprehensive analytical geochemistry program. Data from the Ocean Observatories Initiative Regional Cabled Array at Southern Hydrate Ridge will be used to provide context for field and experimental data. The composition and abundance of organic and inorganic chemical species along with the stable and radiocarbon isotope composition of pore water, bulk sediment, and water column C pools will be used to identify DOC sources and quantify fluxes from cold seeps characterized by a range of advection rates. The centerpiece of the investigation will be a 7-day research cruise to Hydrate Ridge to collect sediments, pore fluids, and water column samples, and deploy OsmoSamplers for continuous time series fluid sampling. The results will form the foundation for estimating the contribution of CH4-derived DOC to the oceanic DOC pool.

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.