Sampling and Analytical Methodology: Freshly collected sediment cores were sliced on the ship into 3 cm layers; porewater was obtained by gently centrifuging freshly collected sediment in 50 ml conical Falcon tubes for ca. 5 to 10 minutes until the sediment had settled; one Falcon tube produced ca. 8 to 10 ml of porewater. For porewater sulfate measurements, 1 ml subsamples of the overlying porewater were drawn into syringes and injected through 0.45 μm filters into screw cap Eppendorf vials, each acidified with 50 µl of 6N HCl, and then gently bubbled with nitrogen for 4 min to remove sulfide; the samples were then stored at 4ºC before shipping and analysis.

Geochemical Analyses: SO4 was analyzed on a Dionex ICS-1000 using a RFIC IonPac AS22 4 X 250mm column and a RFIC IonPac AG22 Guard column 4 X 50 mm. The IC was calibrated using a 5 point std curve and a minimum r^2 value of 0.999. The MDL measured at 0.10 ppm SO4. A check standard from a 1000 ppm was made fom a Dionex 7 anion standard to a concentration of 15 ppm. An external Standard from HACH 100 ppm SO4 was used to make a 20 ppm QC standard. In general, QCs were analyzed for 10% of the samples.

Data Processing: The porewater data of Guaymas Basin sand Sonora Margin piston cores were tabulated in Excel sheets.

Quality Control: Note that Sample_IDs "D7A_10X_CKS" are internal quality control standards (15.5 mg/L) and Sample_IDs "DI_SPK" are external quality control standards (20.0 mg/L).