Porewater dissolved organic carbon and associated geochemical data for methane seeps in the Cascadia Margin: Astoria Canyon, Barkley Canyon, Hydrate Ridge, and Bullseye Vent

Sediment and porewater handling; PCG02-08- Core material was sectioned and pressure-squeezed with a modified Reeburgh squeezer (Reeburgh, 1967) into acid-washed all-plastic syringes. Porewater was then filtered through 0.45 um polyethersulphone syringe filters. Hydrates2004- Core material was sectioned and pressure-squeezed with a modified Reeburgh squeezer (Reeburgh, 1967) into acid-washed all-plastic syringes. Porewater was then filtered through 0.2 um arodisc polyethersulphone syringe filters. AT50-14- Core material was sectioned and pressure-squeezed with a modified Reeburgh squeezer (Reeburgh, 1967) into acid-washed all-plastic syringes. Porewater was then filtered through pre-combusted 0.4 um glass fiber filters. AT50-29B- Core material was sectioned into acid-washed falcon tubes and refrigerated. Porewater was extracted using Rhizons with a 0.2 um filter size. The process step took place over a time period of 2002 to 2024. The cores were collected and porewater was squeezed during field expeditions in 2002, 2004, 2023, and 2024. The process date represents the most recent completion of the work.
Reference:
Reeburgh, W.S., 1967, An improved interstitial water sampler: Limnology and Oceanography, 846 12(1), 163–165, https://doi.org/10.4319/lo.1967.12.1.0163

Porosity; Fresh sediment was placed in pre-weighed petri-dishes, sealed, and refrigerated until measurement in lab. Petri dishes were weighed upon return to shore, then were played in a drying oven (45 degress Celsius, 1 week). Petri dishes with dried sediment were weighed again. The difference between the wet and dry measurements is the water weight, and the difference between the dry weight and the petri dish is the sediment weight. Assuming a constant water density of 1.025 g/cc and a constant sediment grain density of 2.5 g/cc, the water and sediment masses are converted to volumes. The ratio of the water volume to the total volume is the porosity. The process step took place over a time period of 2002 to 2024 where the analyses were run in the 1 year post core collection for each field expedition. The process date represents the most recent completion of the work.

SO4_mM (Pore water dissolved sulfate); 0.5 mL porewater was sub-sampled into 2 mL microcentrifuge tubes and dosed with 50 uL 0.1M phosphoric acid. Samples were measured using ion chromatography. IAPSO certified seawater was used for standardization, and precision is ±1.5%. The process step took place over a time period of 2002 to 2024 where the analyses were run in the 1 year post core collection for each field expedition. The process date represents the most recent completion of the work.

CH4_uM (Pore water dissolved methane); Sediment plugs (volume = 3-6 cm3) were sampled and stored in serum bottles sealed with 1-cm-thick butyl rubber septa. 5mL of saturated brine was added to the serum bottles and shaken to equilibrate the headspace. The samples were refrigerated until analysis. For cruises PCG02-08, Hydrates2004, and AT50-14, methane concentrations of serum bottle headspaces were determined by gas chromatography flame ionization detection. These headspace concentrations were processed into dissolved concentrations of methane in porewater. For samples measured by GC, dissolved concentrations were calculated following Hoehler et al. (2000). For cruise AT50-29B, methane concentrations were determined using cavity ringdown spectroscopy. Check standards for concentration were run approximately every 5 samples and corrected as described by Pohlman and Casso (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), and [CH4] is the dissolved concentration of methane in the original natural water sample, in micromoles per liter. The process step took place over a time period of 2002 to 2024 where the analyses were run in the 1 year post core collection for each field expedition. The process date represents the most recent completion of the work.
References:
Hoehler, T., Borowski, W., Alperin, M., Rodriguez, N., and Paull, C., 2000, Model, stable isotope, and 727 radiotracer characterization of anaerobic methane oxidation in gas hydrate-bearing sediments 728 of the Blake Ridge: Scientific Results, 164, https://doi.org/10.2973/odp.proc.sr.164.242.2000.
Pohlman, J.W., and Casso, M., 2021, Comparison of methane concentration and stable carbon isotope data for natural samples analyzed by discrete sample introduction module - cavity ring down spectroscopy (DSIM-CRDS) and traditional methods: U.S. Geological Survey data release, https://doi.org/10.5066/P99B34V1.
Wiesenburg, D. A., and Guinasso Jr, N.L., 1979, Equilibrium solubilities of methane, carbon monoxide, and hydrogen in water and sea water: Journal of Chemical and Engineering Data, 24(4), 356-360, https://doi.org/10.1021/je60083a006.

DOC_uM (Pore water dissolved organic carbon) and DOC_d13C_permille (Bulk stable carbon isotope value of pore water dissolved organic carbon); Samples (2-4 mL) were stored in combusted borosilicate glass vials with acid-cleaned teflon-lined septa caps. Vials were pretreated with 20 percent hydrochloric acid (12 microliters per milliliter of sample) to decrease the pH of the sample to 2 or less, and samples were stored at 4 degrees Celsius until analysis. Concentrations were determined with a TOC Analyzer using wet chemical oxidation (cruises PGC02-08 and Hydrates2004) (Heuer et al., 2009; Pohlman et al., 2011) and high temperature catalytic oxidation (cruises AT50-14 and AT50-29B) to convert DOC to CO2. For all cruises, nondispersive infrared detection was used to quantify this CO2. Potassium hydrogen phthalate (KHP) calibration standards were run to quantify DOC concentration. Stable carbon isotopes (d13C-DOC) were measured by isotope ratio mass spectrometry. Stable carbon isotope ratios are reported in standard d notation relative to Vienna Pee Dee Belemnite (VPDB). Quality assurance included replicate analysis of natural reference materials, field samples and calibration standards. Uncertainty in DOC concentration was 5 percent or less, and uncertainty in d13C was 0.2 permille or less, based on replicate measurements. The process step took place over a time period of 2002 to 2024 where the analyses were run in the 1 year post core collection for each field expedition. The process date represents the most recent completion of the work.
References:
Heuer, V.B., Pohlman, J.W., Torres, M.E., Elvert, M., and Hinrichs, K.U., 2009, The stable carbon isotope biogeochemistry of acetate and other dissolved carbon species in deep subseafloor sediments at the northern Cascadia Margin: Geochimica et Cosmochimica Acta, 73(11), 3323-3336, https://doi.org/10.1016/j.gca.2009.03.001.
Pohlman, J.W., Bauer, J.E., Waite, W.F., Osburn, C.L., and Chapman, N.R., 2011, Methane hydrate-bearing seeps as a source of aged dissolved organic carbon to the oceans: Nature Geoscience, 4(1), 37-41, https://doi.org/10.1038/ngeo1016.

DIC_mM (Porewater dissolved inorganic carbon) and DIC_d13C_permille (Bulk stable carbon isotope value of porewater dissolved inorganic carbon); Samples for DIC concentration for cruises PCG02-08 and Hydrates2004 were collected as 2 ml aliquots in 5 ml serum vials, sealed with butyl rubber septa, and quantified by coulometry. Samples for stable carbon isotopes (d13C-DIC) for cruises PGC02-08 and Hydrates2004 were collected as 1 ml aliquots in 2 ml serum vials, sealed with butyl rubber septa and analyzed as CO2 by isotope ratio mass spectrometry following acidification of the sample (Pohlman et al., 2013). Samples for DIC concentration and stable carbon isotopes for DIC for cruises AT-50-14 and AT50-29B were collected by injecting 1mL porewater 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 delta notation relative to VPDB. The process step took place over a time period of 2002 to 2024 where the analyses were run in the 1 year post core collection for each field expedition. The process date represents the most recent completion of the work.
Reference:
Pohlman, J.W., Riedel, M., Bauer, J.E., Canuel, E.A., Paull, C.K., Lapham, L., Grabowski, K.S., Coffin, R.B., and Spence, G.D., 2013, Anaerobic methane oxidation in low-organic content methane seep sediments: Geochimica et Cosmochimica Acta, 108, 184-201, https://doi.org/10.1016/j.gca.2013.01.022.

TOC_wt.% (Sediment total organic carbon content) and TOC_d13C_permille (Bulk stable carbon isotope value of pediment total organic carbon content); Sediment samples from all cruises were freeze-dried and ground with a mortar and pestle. Homogenized sediment was weighed into silver capsules for acid fumigation. Samples were acid fumigated to remove carbonate and dried at 45 degrees C for 24 hours, then wrapped into tin capsules. Samples were run on an elemental analyzer interfaced with an isotope ratio mass spectrometer. Samples from PGC02-08 were normalized to an acetanilide standard and samples from AT50-14 were run with a peptone reference material, as well as standards USGS 40 226 and USGS 41. Stable carbon isotope values are reported in d notation relative to VPDB. The process step took place over a time period of 2004 to 2024 where the analyses were run in the 1 year post core collection for each field expedition. The process date represents the most recent completion of the work.

H2S_mM (Pore water dissolved sulfide); 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 to produce a five-level calibration. The process step took place over a time period of 2023 to 2024 with the analyses for a particular cruise run onboard the ship during the cruise. The process date represents the most recent completion of the work.