HR: 17:15h
AN: OS52D-06    [PDF]
TI: Thermogenic and Biogenic Gas Hydrates on the Northern Cascadia Margin: A Molecular, Isotopic and Geochemical Comparison
AU: * Pohlman, J W
EM: jp@vims.edu
AF: Virginia Institute of Marine Science, College of William and Mary, P.O. Box 1346, Gloucester Point, VA 23062-1346 United States
AU: * Pohlman, J W
EM: jp@vims.edu
AF: Naval Research Lab, Marine Biogeochemistry Section Code 6115, 4555 Overlook Ave SW, Washington, DC 20375 United States
AU: Chapman, R
EM: chapman@uvic.ca
AF: School of Earth and Ocean Sciences, University of Victoria, P.O. Box 3055, Victoria, BC V8W 2Y2 Canada
AU: Spence, G
EM: gspence@uvic.ca
AF: Pacific Geosciences Center, Geological Survey of Canada, Sydney, BC V8L 4B2 Canada
AU: Whiticar, M
EM: whiticar@uvic.ca
AF: School of Earth and Ocean Sciences, University of Victoria, P.O. Box 3055, Victoria, BC V8W 2Y2 Canada
AU: Knies, D
EM: knies@nrl.navy.mil
AF: Naval Research Lab, Surface Modification Branch Code 6370, 4555 Overlook Ave SW, Washington, DC 20375 United States
AU: Coffin, R B
EM: rcoffin@ccf.nrl.navy.mil
AF: Naval Research Lab, Marine Biogeochemistry Section Code 6115, 4555 Overlook Ave SW, Washington, DC 20375 United States
AB: A seafloor survey of Barkley Canyon, located off the coast of Vancouver Island, was conducted in August 2002. This survey revealed massive outcrops of gas hydrates exposed as large slabs (6-8m in length) and thinly sedimented mounds (2-3m high). Molecular and isotopic analyses of gas hydrates collected at this site suggested a thermogenic gas source, which was corroborated by observation of hydrocarbons seeping from the seafloor. Methane $\delta$$^{13}$C values from disassociated hydrates collected from Barkley canyon during this and a subsequent survey ranged from -43.0 to -56.9 per mil. Additionally, all hydrate samples contained substantial quantities of C$_{2}$-C$_{5}$ hydrocarbons. This hydrocarbon distribution is similar to that of thermogenic hydrates collected in the Gulf of Mexico, which have been classified by Sassen and MacDonald (1994) as Structure II hydrate. Sediment pore water enrichment of 2-methylbutane (isopentane), a hydrocarbon excluded from the lattice of Structure II hydrate, provides further evidence for the occurrence of Structure II hydrate at Barkley canyon. Despite clear evidence of a thermogenic gas source for Barkley Canyon hydrates, relatively light $\delta$$^{13}$C values (e.g., -56.9 per mil) and higher percentages of methane in some samples suggested additional contribution from a biogenic source. Vertical advection of pore water containing high concentrations of biogenic methane is common along the Cascadia Margin accretionary prism (e.g., Hydrate Ridge). Alternatively, localized methanogenesis could be the source of the biogenic methane. To illustrate the spatial heterogeneity of this region region and identify the source of the biogenic methane, gas hydrate composition and geochemical data from pore water collected at a nearby ($\sim$50 km) biogenic site (Bullseye Vent) will be compared to data from Barkley Canyon.
DE: 4806 Carbon cycling
DE: 4825 Geochemistry
DE: 4870 Stable isotopes
SC: Ocean Sciences [OS]
MN: 2003 Fall Meeting