8. MODEL, STABLE ISOTOPE, AND RADIOTRACER CHARACTERIZATION OF ANAEROBIC METHANE OXIDATION IN GAS HYDRATE-BEARING
SEDIMENTS OF THE BLAKE RIDGE¹

Tori M. Hoehler,^{2, 3} Walter S. Borowski,^{4, 5} Marc J. Alperin,³ Nancy M. Rodriguez, ^{4, 5} and Charles K. Paull^{4, 6}

ABSTRACT

Anaerobic methane oxidation (AMO) was characterized in sediment cores from the Blake Ridge collected during Ocean Drilling Program (ODP) Leg 164. Three independent lines of evidence support the occurrence and scale of AMO at Sites 994 and 995.

First, concentration depth profiles of methane from Hole 995B exhibit a region of upward concavity suggestive of methane consumption. Diagenetic modeling of the concentration profile indicates a 1.85-m-thick zone of AMO centered at 21.22 mbsf, with a peak rate of 12.4 nM·d^-1.

Second, subsurface maxima in tracer-based sulfate reduction rates from Holes 994B and 995B were observed at depths that coincide with the model-predicted AMO zone. The subsurface zone of sulfate reduction was 2 m thick and had a depth integrated rate that compared favorably to that of AMO (1.3 vs. 1.1 nmol·cm^-2·d^-1, respectively). These features suggest close coupling of AMO and sulfate reduction in the Blake Ridge sediments.

Third, measured ¹³CH₄ values are lightest at the point of peak model-predicted methane oxidation and become increasingly ¹³C-enriched with decreasing sediment depth, consistent with kinetic isotope fractionation during bacterially mediated methane oxidation. The isotopic data predict a somewhat (60 cm) shallower maximum depth of methane oxidation than do the model and sulfate reduction data.

¹Paull, C.K., Matsumoto, R., Wallace, P.J., and Dillon, W.P. (Eds.), 2000. Proc. ODP, Sci. Results, 164: College Station, TX (Ocean Drilling Program).
²Marine Sciences Department, 12-7 Venable Hall, CB #3300, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, U.S.A.
³Present address: NASA-Ames Research Center, MS 239-4, Moffett Field, CA 94035-1000, U.S.A. thoehler@mail.arc.nasa.gov
⁴Geology Department, University of North Carolina, Chapel Hill, NC 27599-3315, U.S.A.
⁵Present address: Exxon Exploration Company, P.O. Box 4778, Houston, TX 77210-4778, U.S.A.
⁶Present address: Monterey Bay Aquarium Research Institute, 7700 Sandholdt Road, Moss Landing, CA 95039-0628, U.S.A.

Date of initial receipt: 21 April 1998
Date of acceptance: 25 September 1998
Ms 164SR-242