Temporal variability of in situ methane concentrations in gas hydrate‐bearing sediments near Bullseye Vent, Northern Cascadia Margin

Abstract

[1] 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 (SWI), and 7 cmbsf (centimeters below seafloor) over a 9 month period (August 2009-May 2010. These samples provide a record at $4 day resolution of in situ methane, ethane, propane, sulfate, and chloride concentrations, as well as stable carbon isotope ratios of methane ( 13 C-CH 4 ) and dissolved inorganic carbon ( 13 C-DIC). We show that pore fluids near the SWI are saturated or supersaturated with respect to methane ($80 mM) and the methane flux from the seabed is variable over time. We hypothesized that regional seismic activity controlled this variable CH 4 flux in the Northern Cascadia continental margin setting. However, we found no direct correlation between earthquakes and CH 4 flux. We also posited alternative controls on CH 4 flux variability, such as storms, regional oceanography and microbial activity. Again, no direct correlation was seen. This study takes first steps toward exploring which physical factors play a role in methane flux from hydrate-bearing sediments.Components: 9,035 words, 7 figures.

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

“…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.…”

Observing methane hydrate dissolution rates under sediment cover

“…A mini Pore Fluid Array system (mPFA, Lapham et al, 2008Lapham et al, , 2013, which incorporated four OsmoSamplers (Jannasch et al, 2004) to collect and store porewater samples from sediments, was deployed at a hydrate outcrop in Barkley Canyon, Northern Cascadia Margin ( Fig. 1) from August 2009 through May 2010.…”

“…Dockside, the sampler package was visually checked for leaks before sample coils were crimped (maintaining their in situ pressure within the copper coil), removed from the valve, and shipped back to Florida State University for analysis. An identical sampler package was used concurrently at a gas seep site (Lapham et al, 2013). Methane saturated water (up to 85 mM) was recovered in that instrument indicating that the mPFA is capable of recovering in situ methane concentrations and that methane is not lost from the coils during transport or storage, at least on the order of 3-4 months (Lapham et al, 2013).…”

“…In order to explore this second mechanism (shielding) and the geochemical controls on natural hydrate stability, we deployed a newly designed mini Pore Fluid Array (mPFA) to collect porewater samples from within a few centimeters of a sediment-covered natural hydrate surface (see Lapham et al, 2013Lapham et al, , 2008 for an earlier prototype). These osmotic-samplers continuously draw pore fluids over time and are capable of storing many months of porewater samples at in situ pressures (Jannasch et al, 2004).…”

“…Conversely, if the geochemistry is being driven by mixing with bottom water (either through advection or by seeping in through cracks in the sediment ('seawater infiltration')), the sulfate concentrations should increase while DIC concentrations decrease. However because bottom water has very low methane concentrations (1.4 × 10 −3 mM, Lapham et al, 2013), seawater infiltration should also lower the total hydrocarbon concentration. Thus, sulfate and DIC could provide valuable clues to understanding the controls on methane concentrations in these sediments.…”