Dataset: Porewater geochemical data from methane seeps in the Astoria Canyon and Hydrate Ridge along the Cascadia Margin collected during R/V Atlantis cruises in 2023 and 2024

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).