Sediment samples were collected by the DSV Alvin using PVC push cores. Upon arrival at the surface the cores were described and cataloged prior to being sectioned into discrete depth intervals. Porewater was separated from the sediment using a manually-actuated pore water press. Porewater and sediment samples were preserved and analyzed as follows:

1) Nutrients (DOC, TDN, NOx, NO2, NH4, PO4, TDP): Porewater sample was filtered through a pre-rinsed 0.2 um regenerated cellulose Target2 syringe filter (Thermo Scientific, Prod. No. F25047), collected into an HDPE bottle and stored frozen at -20°C until analysis. Individual analytes were analyzed as follows:

DOC was determined using high temperature catalytic combustion and an NDIR detector following the method described in Sugimura and Suzuki, 1988 (A high-temperature catalytic oxidation method for the determination of non-volatile dissolved organic carbon in seawater by direct injection of liquid sample. Mar. Chem., 24: 105-131).

TDN was determined using high temperature combustion and a chemiluminescence detector following the method described in Watanabe et. al, 2007 (Conversion efficiency of the high-temperature combustion technique for dissolved organic carbon and total dissolved nitrogen analysis. Intern. J. Environ. Anal. Chem., 87: 387-399).

NOx was determined using chemical reduction and a nitric oxide detector following the method described by Garside, 1982 (A chemiluminescent technique for the determination of nanomolar concentrations of nitrate and nitrite in seawater. Mar. Chem. 11: 159-167).

NO2 was determined using the colorimetric method described by Bendschneider and Robinson, 1952 (A new spectrophotometric method for the determination of nitrite in sea water. J. Mar. Res., 11: 87) as reproduced by Parsons, Marta, and Lalli, 1984 (Determination of Nitrite. A manual of chemical and biological methods for seawater analysis pp. 7-9).

NH4 was determined using the colorimetric method described by Solorzano, 1969 (Determination of ammonia in natural waters by the phenolhypochlorite method. Limnol. Oceanogr., 14: 799-801).

PO4 was determined using the colorimetric method described by Strickland and Parsons, 1972 (Determination of reactive phosphorus. A practical handbook of seawater analysis. Fisheries Research Board of Canada, pp. 49-52).

TDP was determined using the colorimetric method described by Solorzano and Sharp, 1980 (Determination of total dissolved phosphorous and particulate phosphorous in natural waters. Limnol. Oceanogr., 25: 754-758).

Sediment pH was determined using a sediment probe (Oakton, Prod. No. 35634-50) and measured immediately during core sectioning; the probe was calibrated using NBS standards.

Pore water pH was determined using a Ross electrode (Thermo Fisher, Prod. No. 8103BNUWP) that was calibrated using NBS standards.

2) Alkalinity: Porewater alkalinity was determined using the spectrophotometric method described by Sarazin et al., 1999 (A rapid and accurate spectroscopic method for alkalinity measurements in sea water samples. Water Research 33:290-294)./

3) CH4: Sediment samples (3 cubic centimeters) were collected into a glass serum vial, preserved with 1M NaOH (3 milliliters), crimp-sealed with a butyl rubber stopper and stored at room temperature until analysis. CH4 was determined by headspace analysis using an SRI 8610C gas chromatograph equipped with a flame ionization detector and SRI Hayesep D 6’x1/8” column (Prod. No. 8600-PKDB).

4) H2S: Porewater sample was collected into a 15 mL centrifuge tube containing 2M zinc acetate and stored at 5°C until analysis. H2S was determined using the colorimetric method described by Cline, 1969 (Spectrophotometric determination of hydrogen sulfide in natural waters. Limnol. Oceanogr., 14: 454-458).

5) Sulfate (SO4): Porewater sample was filtered through a pre-rinsed 0.2 μm regenerated cellulose Target2 syringe filter (Thermo Scientific, Prod. No. F25047) into a 7 mL scintillation vial. Samples were acidified with 10 μL of concentrated HNO3 per 1 mL sample, sealed with a PTFE lined cap, and stored at room temperature until analysis. Sample analysis was performed using KOH eluent supplied by a Dionex EGC III KOH Eluent Generator Cartridge (Prod. No. 074532), Dionex CR-ATC Continuously Regenerated Trap Column (Prod. No. 060477), Dionex AERS 500 Electronically Regenerated Suppressor (Prod. No. 082541), Dionex IonPac AG19 Guard Column (Prod. No. 062888), Dionex IonPac AS19 Analytical Column (Prod. No. 062886) and Dionex CRD 200 RFIC Carbonate Removal Device (Prod. No. 062986). Reference - Weston et al. 2006, Biogeochemistry 77: 375-408.

6) Chloride (Cl): Porewater sample was filtered through a pre-rinsed 0.2 μm regenerated cellulose Target2 syringe filter (Thermo Scientific, Prod. No. F25047) into a 7 mL scintillation vial. Samples were acidified with 10 μL of concentrated HNO3 per 1 mL sample, sealed with a PTFE lined cap, and stored at room temperature until analysis. Sample analysis was performed using KOH eluent supplied by a Dionex EGC III KOH Eluent Generator Cartridge (Prod. No. 074532), Dionex CR-ATC Continuously Regenerated Trap Column (Prod. No. 060477), Dionex AERS 500 Electronically Regenerated Suppressor (Prod. No. 082541), Dionex IonPac AG19 Guard Column (Prod. No. 062888), Dionex IonPac AS19 Analytical Column (Prod. No. 062886) and Dionex CRD 200 RFIC Carbonate Removal Device (Prod. No. 062986). Reference - Weston et al. 2006, Biogeochemistry 77: 375-408.

7) Calculated Values: Values for NO3, DIN, DON, DOP were calculated as follows:
NO3 = NOx - NO2
DIN = NOx + NH4
DON = TDN - DIN
DOP= TDP - PO4