Concentrations of gas dissolved in seawater and in gas bubbles collected in the northern Gulf of Mexico, April 12-20, 2015

Seawater and gas samples were collected in the Gulf of Mexico at MC118, located at 28° 51.129’N, 88° 29.51’W, from 12 - 20 April 2015 aboard the E/V Nautilus. The seawater samples were collected directly from waters impacted by seafloor methane seeps. This was carried out using the SUPR sampler [Breier et al., 2014] mounted to the ROV Hercules. The SUPR sampler is an in-situ seawater pumping system and was developed to sample dynamic, high gradient, ocean geochemical features at areas such as seep sites. The inlet of the SUPR sampler was attached with tubing to the ROV arm and is pumped seawater directly into sampling bottles mounted to the ROV chassis. The samples were taken from waters visibly impacted by methane bubbles, so an adapter was placed on the inlet of the sampling tube to collect seawater but not bubbles. The seawater samples were measured for temperature and salinity, the concentrations of dissolved oxygen, methane, ethane, propane, and butane, as well as the stable carbon isotopes of methane (δ13C-CH4). The measurements of temperature, salinity, and dissolved oxygen were measured in-situ with probes mounted to the ROV; a Seabird FastCat 49Plus was used for the temperature and salinity measurements, while an Aanderaa Oxygen Optode 3830 was used for the dissolved oxygen measurements. Measurements of dissolved methane, ethane, propane, and n-butane concentrations were conducted on recovered samples while at sea on the E/V Nautilus. Samples for the analysis of δ13C-CH4 were preserved for later analysis in a land-based laboratory. The procedures for dissolved methane, ethane, propane, and butane concentrations as well as δ13C-CH4 are previously published [Leonte et al., 2017] and are being updated, focusing more specifically on the analyses of these samples [Leonte et al., 2018]. Samples of gas bubbles were collected using a custom built gas sampler made up of two plastic cylinders and a funnel. A primary cylinder was open at the bottom with a funnel attached underneath it to increase the surface area over which gas bubbles were collected. The primary cylinder and secondary cylinder were connected by a tube attached to the top of each cylinder. The secondary cylinder also had a drain tube attached to the bottom. Gas samples were collected by positioning the ROV within the bubble plume and using the funnel to direct gas bubbles into the primary cylinder. As the ROV ascended to be brought back on board, decreasing water pressure allowed the gas sample in the primary cylinder to expand into the secondary cylinder, driving out water through the drain tube. By the time the ROV was recovered, minimal water remained inside the secondary container. The collected gas samples were analyzed on shore for the mole percentages of Nitrogen, Methane, Carbon Dioxide, Ethane, Propane, Iso-Butane, n-Butane, Neopentane, Iso-Pentane, and n-Pentane using mass spectrometry analysis at the Woods Hole Oceanographic Institution. The specific procedures for the collection and analysis of these gas samples are being prepared for publication [Leonte et al., 2018]. Parts of this dataset have been converted to more accurately represent values found in the northern Gulf of Mexico. Those converted values can be found here. https://data.gulfresearchinitiative.org/data/R1.x137.000:0028.