Core collection: Cores were collected via a gravity corer (1 meter) in July 2022 during R/V Carson cruise CB22 (R/V Carson is operated by the University of Maryland Center for Environmental Science, https://www.umces.edu/research-fleet/).

Sediment and porewater extraction: For CH4, CO2, δ13C-CH4, and δ13C-CO2 measurements, a cutoff 3 milliliter (mL) syringe was inserted into pre-drilled holes in the core liner at 5 centimeters (cm) resolution. Two 3 mL sediment plugs were taken for CH4/CO2 and placed in a whirlpak bag. For porewater extraction, rhizons (19.21.23 F; Rhizosphere Research Products) were inserted into pre-drilled holes offset from the sediment holes in the core liner. A vacuum was pulled with an acid-clean 30 mL syringe to extract the porewater, then pipetted into respective containers for DIC, δ13C-DIC, DOC, δ13C-DOC, SO4, and Cl.

Sediment/porewater sampling:
CH4, CO2, δ13C-CH4, δ13C-CO2: 20 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.

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 are acidified and sparged to remove inorganic carbon then reacted with Sodium persulfate as wet oxidation. Data was 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 C for three days, then 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 60 degrees Celsius for 2-4 hours, then 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.