Dataset: Porewater geochemistry, including methane, CO2, sulfate, and sulfide from Guaymas Basin hydrothermal sediments collected on R/V Atlantis cruise AT15-56 in Nov-Dec 2009

Geochemical Analyses:
Sulfate concentration measurements were completed shipboard; after centrifuging sediment-filled 15-milliliter (ml) tubes, the overlying porewater was filtered through 0.45-micrometer (um) filters, acidified with 50 microliters (ul) of 50% HCl and bubbled with nitrogen for 4 minutes to remove sulfide. Sulfate concentrations were then measured shipboard using a 2010i Dionex Ion Chromatograph (Sunnyvale, CA, USA) through Ag exchange columns (Dionex) so as to remove Cl (Martens et al., 1999). For sulfide, 1 ml porewater samples were combined with 0.1 molar (M) zinc acetate and concentrations were analyzed spectrophotometrically on the ship (Cline, 1969).

Headspace methane concentrations were determined onboard by standard gas chromatography with a flame ionization detector (FID), specifically using a HACH Carle Series 100 AGC Gas Chromatograph (GC) with an Alltech Molecular Sieve 5A packed column (80/100 mesh, 3.05 meters (m) length, 3.2 millimeter internal diameter (mm ID)) and an 80 degree Celsius (C) isothermal temperature profile. Stable isotopic compositions of the same methane samples were measured post-cruise at the University of North Carolina (UNC) via gas chromatography-combustion-isotope ratio mass spectrometry (GC-C-IRMS) on a Finnigan MAT 252 Isotope Ratio Mass Spectrometer, using an HP 5890 Series II Gas Chromatograph with an HP Plot Q column (30 m length, 0.32 mm ID, 20 um film thickness) and a 30 degree C isothermal temperature profile.

To measure DIC, 2 ml of unamended porewater from each sediment horizon were injected into evacuated serum vials (30 ml) and stored upside down at -20 degrees C. At UNC, the samples were thawed, and DIC was reacted to gaseous CO2 by adding 1 ml of a 30% phosphoric acid solution to each serum vial and shaking vigorously before GC analysis (Kelley et al., 1990). Stable isotopic values and concentrations of DIC were analyzed via coupled GC (Hewlett Packard 5890) and Isotope Ratio Mass Spectrometer (Finnigan MAT 252).

Porewater concentrations of dissolved organic acids were measured via High-Pressure Liquid Chromatography (HPLC) after the cruise, using a Beckman Model 332 gradient liquid chromatograph combined with an ISCO V4 UV/VIS detector and a Shimadzu CR3-A integrator (Albert and Martens, 1997).