9. SIGNIFICANCE OF ANAEROBIC METHANE OXIDATION IN METHANE-RICH SEDIMENTS OVERLYING THE BLAKE RIDGE GAS HYDRATES1

Walter S. Borowski,2, 3 Tori M. Hoehler,4, 5 Marc J. Alperin,4 Nancy M. Rodriguez,2, 3 and Charles K. Paull2, 6

ABSTRACT

A unique set of geochemical pore-water data, characterizing the sulfate reduction and uppermost methanogenic zones, has been collected at the Blake Ridge (offshore southeastern North America) from Ocean Drilling Program (ODP) Leg 164 cores and piston cores. The 13C values of dissolved CO2 (CO2) are as 13C-depleted as -37.7 PDB (Site 995) at the sulfate-methane interface, reflecting a substantial contribution of isotopically light carbon from methane. Although the geochemical system is complex and difficult to fully quantify, we use two methods to constrain and illustrate the intensity of anaerobic methane oxidation in Blake Ridge sediments. An estimate using a two-component mixing model suggests that ~24% of the carbon residing in the CO2 pool is derived from biogenic methane. Independent diagenetic modeling of a methane concentration profile (Site 995) indicates that peak methane oxidation rates approach 0.005 Ámol cm-3 yr-1, and that anaerobic methane oxidation is responsible for consuming ~35% of the total sulfate flux into the sediments. Thus, anaerobic methane oxidation is a significant biogeochemical sink for sulfate, and must affect interstitial sulfate concentrations and sulfate gradients.

Such high proportions of sulfate depletion because of anaerobic methane oxidation are largely undocumented in continental rise sediments with overlying oxic bottom waters. We infer that the additional amount of sulfate depleted through anaerobic methane oxidation, fueled by methane flux from below, causes steeper sulfate gradients above methane-rich sediments. Similar pore water chemistries should occur at other methane-rich, continental-rise settings associated with gas hydrates.

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).
2Geology Department, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599-3315, U.S.A.
3Present address: Exxon Exploration Company, P.O. Box 4778, Houston, TX 77210-47778, U.S.A. Correspondence author: borowski@lcc.net
4Marine Sciences, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599-3300, U.S.A.
5Present address: NASA Ames Research Center, MS 239-4, Moffett Field, CA 94035-1000, U.S.A. thoehler@mail.arc.nasa.gov 
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: 12 January 1999
Ms 164SR-214

NEXT