Dataset: Pore water data from sediment cores collected from R/V Oceanus cruise OC1906A and R/V Sikuliaq cruise SKQ202016S off the coast of California in 2019 and 2020

Field sampling methods:
Sediment cores were collected using a gravity corer and a multicorer on both cruises. Following recovery, the gravity cores were secured horizontally on the ship's deck and sampled from the bottom of the core upwards. This was done by sequentially removing 10-centimeter (cm) sediment intervals by cutting the core liner using a pipe cutter. Freshly exposed sediment was immediately subsampled using 3- to 60-milliliter (mL) push corers made of plastic syringes with the tips removed. All subcores were immediately transferred to an N2 filled glove bag in a refrigerated van for further processing. Multi cores were transferred to a refrigerated van immediately upon recovery, and within 12 hours were either sampled using pre-cleaned Rhizon samplers (Seeberg-Elverfeldt et al., 2005) or extruded and sectioned in an N2 atmosphere at intervals of 0.5 to 2 cm depth over the depth of the core (generally 30 to 40 cm total length).

While it is possible to recover intact sediment-water interfaces using multi-corers, loss of surface sediments is typical during gravity coring, making it impossible to directly quantify absolute depths below the sediment-water interface in a gravity core. We therefore determined absolute depths of sediment sample intervals in gravity cores by aligning DIC, SO42-, NH4+, and porosity gravity core profiles to multicore profiles from the same site (Berelson et al., 2005; Iversen and Jørgensen, 1985; Komada et al., 2016).

Sediment aliquots from both gravity cores and sectioned multi cores were centrifuged at 6 degrees Celsius (°C) in polycarbonate tubes, and the supernatant was collected into all-polypropylene syringes with stainless steel needles. Rhizon samples were also collected in all poly-propylene syringes. All pore water samples were then filtered through disposable 0.2 micrometer (µm) nylon filters with 0.7 µm GF/F pre-filters (Whatman 6870-2502). The first 3 mL were discarded. To minimize the DOC blank, 100 mL of UV-irradiated deionized water (DIWUV) were pushed through each disposable filter prior to use.

Pore water samples for DIC concentration were placed in 2-mL serum vials without headspace, immediately crimp sealed, and refrigerated until analysis (Burdige and Homstead, 1994). Alkalinity samples were collected in 3-mL plastic syringes, sealed with 3-way stopcocks, and titrated on-board the ship within 24 hours. Titrated alkalinity samples were placed in snap cap vials and later used for sulfate and ammonium determinations.

Pore water samples for DOC concentration were acidified to pH < 2 with 6 N trace metal grade HCl and flame-sealed in pre-combusted glass ampules under a stream of UHP N2 gas and refrigerated.

Pore water samples for total dissolved sulfide (∑H2S = [H2S] + [HS-] + [S2-]) analysis were fixed onboard ship by adding pore water to an N2-degassed solution containing 4 mL of 5 millimolar (mM) ZnCl2 and and 4 mL of 10 mM NaOH (Ingvorsen and Jørgensen, 1979) in a 10 mL serum bottle. At the basic pH of this "fixing" solution, all dissolved inorganic sulfide precipitates out as ZnS. The headspace was then degassed with N2 and the bottle was crimp sealed with plug-style stoppers and refrigerated. In SBB pore water samples near the sediment surface (upper 10 cm) where sulfide levels are low, 1 mL of pore water was added to the fixing solution, whereas for deeper samples (with higher sulfide levels) 0.1 mL pore water was added to the fixing solution. In station D and K pore water samples, 1-2 mL pore water was added to the fixing solution. ∑H2S was not determined in Cat pore water samples.

Bottom-water samples were collected from 10 meters (m) above the seafloor with a Go Flo bottle. All tools and parts used in all sampling were first cleaned with household dish soap, then acid rinsed (exclusive of metal parts). Plasticware was air dried.

Analytical methods:
Alkalinity samples were titrated on-board the ship by automated Gran titration (Burdige et al., 2010). pH values listed here are the initial pH values from the titrations and are expressed on the NBS scale. Concentrations of DIC and ammonium (SBB and Cat samples) were analyzed by flow injection analysis (Hall and Aller, 1992; Lustwerk and Burdige, 1995). Ammonium (stations D and K) was determined by the fluorometric technique of Holmes et al. (1999). Sulfate was determined by ion chromatography with conductivity detection (Komada et al., 2016). ∑H2S was determined spectrophotometrically using the methylene blue technique (Cline, 1969). All reagents were added directly to the serum bottle containing the ZnS suspension (see the section above for details).

Concentrations of DOC were determined by high-temperature combustion using a Shimadzu TOC-V total carbon analyzer (Burdige and Gardner, 1998). DOC consensus reference materials (D. Hansell, RSMAS) were run along with samples, and measured values agreed to within <10% with the reported consensus values.