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

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.