Geological Society of America

T105 Biogeochemical Processes at Ancient and Modern Methane Seeps I

Monday, 3 November 2003
1:30 PM - 5:30 PM

THE ANATOMY OF THE SULFATE-METHANE INTERFACE IN A METHANE-RICH CORE FROM THE GULF OF MEXICO

Paper number: T105-5
Program: Technical Sessions
Time: 2:40 PM – 2:55 PM

Authors

Emily Johnson Chair
Elizabeth Windom Chair
William Ussler Presenting Author
Monterey Bay Aquarium Rsch Institute, Moss Landing, CA
Charles K. Paull
Monterey Bay Aquarium Rsch Institute, Moss Landing, CA
Kellie Wall Chair
Jisoo Kim Chair
Steven Hallam
Monterey Bay Aquarium Rsch Institute, Moss Landing, CA
Ed DeLong
Monterey Bay Aquarium Rsch Institute, Moss Landing, CA
Yifeng Chen
Univ of Tokyo, Tokyo, N/A
Ryo Matsumoto
Univ of Tokyo, Tokyo, N/A
Thomas Lorenson
U.S. Geological Survey, Menlo Park, CA
William J. Winters
U.S. Geol Survey, Woods Hole, MA

Abstract

The sulfate-methane interface (SMI) is becoming recognized as a fundamental biogeochemical boundary in methane-rich and gas-hydrate-bearing marine sediments worldwide. The SMI separates sulfate-bearing sediment above from sulfate-depleted, methane-rich sediments below. The SMI is a zone of intense methane oxidation mediated by anaerobic methane oxidizing archaea. During this process methane and sulfate are consumed and dissolved inorganic carbon (DIC) and hydrogen sulfide (HS^-) are produced; excess HS^- and DIC form solid mineral phases.

In 2002 we obtained, using the Marion Dufresne, a 10.5-m giant gravity core over a gas chimney in the Mississippi Canyon area of the Gulf of Mexico. The cross-section of this core was 25 cm x 25 cm and was accessed by removing metal side plates. Upon opening we observed bubble-textured sediment below ~300 cm, indicating methane saturation was achieved during core recovery. We inferred that the SMI was a short distance above and subsampled across the SMI in unprecedented detail by collecting 23 pore water samples at 25-cm increments and 151 sediment samples at 2-cm increments.

Sulfate, methane, DIC, and lithologic data show that the SMI is centered at 300 cm. Overlap between sulfate- and methane-bearing sediment occurs in a 100-cm thick zone. DIC concentrations reach their maximum value (13.5 mM) at 300 cm. DIC in this core is isotopically very light [-63.2‰ (PDB) at 300 cm], suggesting derivation of much of this carbon from microbial methane. Authigenic carbonate nodules are abundant immediately below the SMI to 450 cm. Carbonate d¹³C values (-60.2±0.7 ‰) are like those for DIC suggesting these carbonates formed within the SMI.

Polymerase chain reaction amplification of 16s rDNA sequences from sediment extracts using archaeal-specific primers show that methanogen-related archaea occur throughout the interval sampled, whereas, methanotrophs (e.g, ANME-1 & ANME-2a/b) are more restricted, occurring at and above the SMI where sulfate is readily available.