Porewater geochemistry data for sediment samples included in global seep survey from R/V Atlantis cruise AT15-40 and Alvin dives in the Guaymas Basin in 2008 (Guaymas Basin interactions project) (NCEI Accession 0277890)

Acquisition Description:
Geochemical Analyses:
Sulfate concentration measurements were completed shipboard; after centrifuging sediment-filled 15 ml tubes, the overlying porewater was filtered through 0.45 um filters, acidified with 50 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 0.1M zinc acetate and concentrations were analyzed spectrophotometrically on the ship (Cline 1969).

For methane, sediment subsamples were added to serum bottles containing a 0.1 M sodium hydroxide solution. Headspace methane concentrations were determined onboard using standard gas chromatography using a flame ionization detector. Stable isotopic compositions of methane was measured via gas chromatography/combustion/isotope ratio mass spectrometry (GC/C/IRMS) on a Finnigan MAT 252 Isotope Ratio Mass Spectrometer.

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 volatilized by adding 1 ml 30% phosphoric acid 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).