Sediment cores were collected via wireline coring tools (advance piston coring, extended core barrel coring, and pressure coring) and cut into sections at sea. Hole H002 is located at 26.66228° latitude and -91.67601° longitude at the seafloor. Hole H003 is located at 26.66263° latitude and -91.67599° latitude at the seafloor. Void gases were collected using a syringe and a puncture tool that penetrates the core liner. Pressure core gases were collected from an inverted graduated cylinder as part of the quantitative degassing system that collects gases produced from the core storage chamber during depressurization. Hydrate gases were collected from cryogenically frozen samples that were allowed to dissociate within a syringe. Void gas, pressure core gas, and hydrate gas were collected in Cali-5 Bond gas bags and were analyzed for gas composition and methane stable carbon isotopes at the U.S. Geological Survey Woods Hole Coastal and Marine Science Center using the Automated Sample Introduction Module (Auto-SIM) Gas Chromatography/Cavity Ring-Down Spectroscopy System (Auto-SIM GC/CRDS). Gas samples in Cali-5 Bond bags were attached to the 16-port Auto-SIM that automatically transfers a portion of each sample to a series of samples loops of the Auto-SIM GC/CRDS that includes the USGS Discrete Sample Introduction Module (DSIM) with a cavity ring-down spectrometer and a Scientific Research Instruments (SRI) MultipleGas#5 (MG5) gas chromatograph (GC) equipped with sample loops and injection valves for parallel analysis by flame ionization detection (FID) and thermal conductivity detection (TCD).
The DSIM contains a fixed (low-volume) internal sample loop and a replaceable external sample loop (larger-volume) that allows the user to properly dilute gases so they’re within the ideal analytical range of a Picarro G2201-I CRDS (100-600 ppm). The CRDS measures methane and CO2 concentrations and stable carbon isotopes of each (see Pohlman and others (2021) for additional details).
Isotope ratios measured by the CRDS are reported in the -notation (δ13C) relative to a Vienna Pee Dee Belemnite (VPDB) standard calibration. Measurement precision for methane isotope ratios improves from 1‰ at 10 ppm to less than 0.2‰ at 30 ppm and remains constant through the range of measurements made. The average sample precision for this project was 0.18‰ for methane carbon isotope ratios.
Additional details of the gas collection and analysis procedures are described in Flemings and others, 2025.
Reference:
Flemings, P.B., Thomas, C., Phillips, S.C., Collett, T.S., Cook, A.E., Solomon, E., Colwell, F.S., Johnson, J.E., Awwiller, D., Aylward, I., Bhandari, A.R., Brooks, D., Buser-Young, J.Z., Cardona, A., Casso, M., Coyte, R., Darrah, T., Davis, M., Dugan, B., Duncan, D., Germaine, J.T., Holland, M., Houghton, J., Mills, N.T., Mimitz, M., Minarich, D., Morono, Y., Murphy, Z., O’Connell, J., Petrou, E., Pettigrew, T., Pohlman, J.W., Portnov, A., Phillips, M.P., Redd, T., Sawyer, D. E., Schultheiss, P., Shannon, K., Sullivan, C., Small, C., Tozier, K., Tsang, M.-Y., Maal, C.V.D., Waite, W.F., and Walton, T., 2025, Expedition UT-GOM2-2 Methods, in Flemings, P.B., ed., Proceedings of the Deepwater Hydrate Coring Expedition UT-GOM2-2: The University of Texas at Austin, University of Texas Institute for Geophysics,
https://doi.org/10.5281/zenodo.13971228.
Pohlman, J.W., Casso, M., Magen, C., and Bergeron, E., 2021, Discrete Sample Introduction Module for Quantitative and Isotopic Analysis of Methane and Other Gases by Cavity Ring-Down Spectroscopy: Environmental Science and Technology, v. 55, p. 12066-12074,
https://doi.org/10.1021/acs.est.1c01386
