Push cores were collected across multiple dives of HOV Alvin during cruises AT18-02 and AT26-13 aboard the R/V Atlantis. Cores were also collected during cruises EN527, EN528, EN559, and EN586 aboard the R/V Endeavor.

Cores were collected immediately upon retrieval and stored at 4 degrees C until further processing. Sediment samples were collected from select cores in 3 cm intervals (reported as average depth below seafloor, in centimeters) for geochemical and rate analysis, using aseptic techniques at in situ temperature (4 degrees C). Methane samples were collected first: A 3 ml subsample was collected in a cut-end syringe, placed in a 20 ml serum vial containing 4 ml 2M NaOH to stop microbial activity, sealed with a butyl rubber stopper, crimp sealed, vortexed to homogonize, and stored at room temperature until analysis. Porewater samples were extracted from whole sediment under pressure into acid-washed syringes using a custom argon-purged squeezer as described by Joye et al., 2004. A subsample was collected for sulfide determination and analyzed on board using Cline (1969) colorometric methods. DIC samples were collected in 20 ml Hungate tubes and sealed with butyl stoppers. Samples were preserved with 1 ml saturated CuSO4 and 1 ml of 56 mM NaMoO4 in 10% v/v H3PO4. The headspace DIC was methanized and analyzed by GC-FID. Subsamples were collected for determination of porewater pH and salinity: pH was measured on board using a ROSS pH electrode, calibrated with salinity corrected buffers (pH 4, 7, and 10) chilled to in situ temperature (Bowles et al., 2011), and salinity was measured visually on a 500ul subsample using a handheld refractometer (Cole-Parmer RSA-BR60). The remaining sample was filtered-sterilized through a washed 0.2 um Target filter and subsampled further for ammonium, major ions, and nutrients.

A 2 ml ammonium subsample was immediately analyzed on board using the method of Soloranzo (1969). A 5 ml subsample for major ions (sodium, magnesium, potassium, calcium, sulfate, and chloride) was preserved with concentrated nitric acid (0.1 micromole per L final concentration) and stored at room temperature until analysis by ion chromatography (Joye et al., 2004). The remaining nutrient subsample was frozen at -20 degrees C until shore-based laboratory analysis. Total dissolved nitrogen (TDN) was analyzed via the oxidative combustion-chemolumiescence technique of Salgado and Miller (1998) using a Shimadzu TOC 5000 coupled to an Antek model 7020 NO analyzer (Joye et al., 2004). NOx (nitrate + nitrite) was analyzed on a Lachat FIA 8000 Autioanalyzer using method 31-107-04-1-A, phosphate via the molybdate blue colorometric method, total dissolved phosphate (TDP) by high-temperature combustion and hydrolysis (Monaghan and Ruttenberg 1999), dissolved inorganic carbon (DIC) by gas chromatography (flame ionization detector; GC-FID), dissolved organic carbon (DOC) by oxidative combustion-infrared analysis, methane by GC-FID, and stable 13C isotopes of DIC by headspace analysis using a Picarro G2201-i isotope analyzer (Joye et al., 2004, Bowles et al., 2016). Whole sediment subsamples for radiotracer sulfate reduction rates and anaerobic oxidation of methane rates were collected in triplicate from a parallel core and were handled, injected, incubated, and analyzed using the methods described in Joye et al., 2004.