HR: 17:00h
AN: GC14A-04    [Abstracts]
TI: Estimates of Methane Production Rates Based on d13C of the Residual DIC in Pore Fluids from the Cascadia Margin
AU: * Torres, M E
EM: mtorres@coas.oregonstate.edu
AF: COAS-Oregon State University, 104 COAS Admin Bld, Corvallis, OR 97331, United States
AU: Kastner, M
EM: mkastner@ucsd.edu
AF: SIO, University of California San Diego, La Jolla, CA 92093, United States
AU: Wortmann, U G
EM: uli.wortmann@utoronto.ca
AF: Department of Geology, University of Toronto, Toronto, ON M5S 3B1, Canada
AU: Colwell, F
EM: rcolwell@coas.oregonstate.edu
AF: COAS-Oregon State University, 104 COAS Admin Bld, Corvallis, OR 97331, United States
AU: Kim, J
EM: save@rock25t.kigam.re.kr
AF: Petroleum and Marine Resource Division, KIGAM, Daejeon, 305-350, Korea, Republic of
AB: An outstanding question in carbon cycling models, particularly those that involve gas hydrate formation in accretionary margins, is the rate of methane generation in anoxic sediments. This rate is a key parameter in numerical models involving gas hydrate formation, magnitude of the deposits and recharge of the gas hydrate reservoir following a destabilization event. Pore fluid samples recovered from ~100 to 200 mbsf along a transect of sites drilled in the Cascadia convergent margin during ODP Legs 146 and 204 and IODP Leg 311 show a distinct enrichment in the d13C values of the residual dissolved CO2 with progression from a reference site drilled west of the deformation front (Site 888, d13C: -5 ppt) to Site U1329 (d13C: +32 ppt) located ~65 km from the shore, at the eastern limit of gas hydrate occurrence in the Cascadia margin. Assuming that methane production proceeds primarily by carbonate reduction at relatively rapid rates in a closed system (Rayleigh distillation) with a fractionation factor of 1.07, the measured d13C-DIC at Site U1329 corresponds to ~50% removal of the available dissolved carbonate pool that is assumed to be characterized by 10 mM ?CO2, having a d13C of -20 ppt. Although it is hard to know the available DIC pool, 10 mM represents an average value based on alkalinity measurements at these sites. These estimates correspond to an in situ production of 5 mmol/l methane. Sediments from Site U1329 have been dated as late Miocene, ~5.3 Myr old, thus based on the assumption that the methane was generated over this period, in sediments with porosity of 50%, the first order approximation of the calculated rate of methane production is 0.4 mmol/m3yr. Our estimated methane production rate compares well with the published rates for Leg 204, that vary from 10 mmol/m3yr (upper 100 mbsf) to 0.1 mmol/m3yr (deeper than 100 mbsf), based on an alkalinity model. However, estimates using methane production rates for starved methanogens and the numbers of methanogens in the sediments for most of the Leg 204 samples, yielded values of ~0.005 mmol/m3yr, although 25% of these samples yielded estimates that were higher than this value.
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 1050 Marine geochemistry (4835, 4845, 4850)
DE: 3002 Continental shelf and slope processes (4219)
DE: 3004 Gas and hydrate systems
DE: 3060 Subduction zone processes (1031, 3613, 8170, 8413)
SC: Global Environmental Change [GC]
MN: 2007 Fall Meeting