To assess the temporal variability in the methane fluxes from marine sediments that overly gas hydrate bearing sediments and the factors that might control its rate, in situ methane concentrations were measured near Bullseye Vent on the Northern Cascadia continental margin. A long-term sampling device collected overlying water and pore-fluid samples from 25 cm above seafloor, at the sediment-water interface, and 7 cmbsf (cm below seafloor) over a 9-month period (August 2009-May 2010).  These samples provide a ~4 day resolution of in situ methane, ethane, propane, sulfate, and chloride concentrations, as well as stable carbon isotope ratios of methane (d13C-CH4) and dissolved inorganic carbon (d13C -DIC).  We show that pore-fluids near the sediment-water interface are saturated or super-saturated with respect to methane (~80 mM) and the methane flux from the seabed is variable over time.  In this Northern Cascadia continental margin setting, possible controls on this variability, such as seismic activity, regional oceanography, and microbial activity are discussed.  No direct correlation between methane flux and earthquake activity (magnitude & distance); typical seasonal storm-patterns and related pressure changes at the seafloor; nor large-scale oceanographic upwelling-cycles on the west coast off Vancouver Island were found.  This study fills a gap in knowledge about which physical factors play a role in methane flux from hydrate-bearing sediments.

Temporal variability of in situ methane concentrations in gas hydrate-bearing sediments near Bullseye Vent

245th National Meeting (2013)

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