Methodology: Porewater Methane concentrations were determined by headspace Gas Chromatography - flame ionization detection, and d13C signatures of methane were determined by Gas Chromatography coupled to Mass Spectrometry, using a Finnigan MAT Delta S isotope ratio Mass Spectrometer inlet system.
Sampling and Analytical Procedures: For combined concentration and δ13C analysis of methane, 2 ml sediment subsamples were added to 30 ml serum vials containing 2 ml of 1 M sodium hydroxide solution, sealed with thick butyl rubber stoppers, crimped with aluminum seals and stored at 4°C. Due to shipping problems and a resulting shortage of serum vials, only selected sediment cores were sampled for methane. Since cores were retrieved unpressurized, outgassing may have impacted in particular the measurements of methane concentrations near and above saturation, 1.5 mM. After the cruise, the methane samples were analyzed by headspace gas chromatography-flame ionization detection (GC-FID) at Florida State University (Magen et al., 2014). Gas samples were analyzed for δ13C by injecting 0.1 to 0.5 ml of sample into a gas chromatograph interfaced to a Finnigan MAT Delta S isotope ratio Mass Spectrometer inlet system as previously described (Chanton and Liptay 2000). Values are reported in the per mil (‰) notation relative to Vienna Pee Dee Belemnite (VPDB). Sampling site names are based on Teske et al. 2016 and Teske et al. 2021.
Known Problems/Issues: Problems with Mexican customs and the agent used by WHOI at the time have resulted in limited availability of sampling gear and sampling vials on the ship. Transport problems during the return trip have caused sample losses among the porewater samples, which are evident in occasional gaps in porewater profiles or short profiles.
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