Measuring Temporal Variability in Pore-Fluid Chemistry To Assess Gas Hydrate Stability: Development of a Continuous Pore-Fluid Array

Abstract

A specialized pore-fluid array (PFA) sampler was designed to collect and store pore fluids to monitor temporal changes of ions and gases in gas hydrate bearing sediments. We tested the hypothesis that pore-fluid chemistry records hydrate formation or decomposition events and reflects local seismic activity. The PFA is a seafloor probe that consists of an interchangeable instrument package that houses OsmoSamplers, long-term pore-fluid samplers, a specialized low-dead volume fluid coupler, and eight sample ports along a 10 m sediment probe shaft. The PFA was deployed at Mississippi Canyon 118, a Gulf of Mexico hydrate site. A 170 day record was acquired from the overlying water and 1.3 m below seafloor (mbsf). Fluids were measured for dissolved chloride, sulfate, and methane concentrations and dissolved inorganic carbon and methane stable carbon and deuterium isotope ratios. Chloride and sulfate did not change significantly over time, suggesting the absence of gas hydrate formation or decomposition events. Over the temporal record, methane concentrations averaged 4 mM at 1.3 mbsf, and methane was thermogenic in origin (delta13C-CH4 = -32.4 +/- 3.4 per thousand). The timing of an anomalous 14 mM methane spike coincided with a nearby earthquake (Mw = 5.8), consistent with the hypothesis that pore-fluid chemistry reflects seismic events.

Isotopic composition of sinking particles: Oil effects, recovery and baselines in the Gulf of Mexico, 2010–2015

“…The seafloor at this site hosts a large deep-water coral population, and represents a "mineral-prone seep." A mineralprone seep is in the final stages of seep evolution, where the production of authigenic carbonates has blocked conduits and allowed corals to use these carbonate hard grounds for attachment surfaces (Roberts and Carney, 1997;Lapham et al, 2008aLapham et al, , 2008b. This categorization is consistent with the "self-sealing nature of marine seeps" (Hovland, 2002).…”

Detection of Putatively Thermophilic Anaerobic Methanotrophs in Diffuse Hydrothermal Vent Fluids

“…Mississippi Canyon Block 118 (MC118) in the Gulf of Mexico is characterized by methane hydrate deposits and thermogenic hydrocarbonrich fluids (30). It is located offshore of Louisiana at a water depth of ϳ890 m. Samples of sediments covered with a white microbial mat were taken in 2006 using the Johnson-Sea-Link submersible.…”

Microbial response to oil enrichment in Gulf of Mexico sediment measured using a novel long-term benthic lander system

“…These studies were enabled by our development and deployment of a novel oil-enrichment experiment on a benthic "lander" system intended for long-term analysis of in situ sediment microbial oil degradation. This experiment utilized sampling systems that allow for parallel simultaneous collection of samples for biogeochemical and microbiological analyses (Jannasch et al, 2004;Lapham et al, 2008;Orcutt et al, 2010a;Wheat et al, 2011;Robidart et al, 2013) coupled to flow-through sediment reactors deployed at the seafloor for months at a time. We refer to this experiment system as the Microbial Methane Observatory for Seafloor Analysis (MIMOSA).…”

“…The "CH 4 " OsmoSampler collected filtered fluids into gas-tight copper tubing to provide a continuous record of methane and sulfate concentrations and methane stable carbon isotopic composition, based on published designs (Lapham et al, 2008(Lapham et al, , 2013. This sampler included a high-pressure valve that was closed on the seafloor to maintain in situ pressure.…”