MUD VOLCANOES, GAS HYDRATES, AND CARBONATE FORMATION AT “WARM” SEEPS IN THE NORTHERN GULF OF MEXICO

Seafloor venting of fluidized sediment and hydrocarbons in the northern Gulf of Mexico (GOM) creates a variety of features (gas hydrate mounds, mud volcanoes, and carbonate sediments) that play an important role in the surface geology of the continental slope. The GOM contains extensive gas hydrate accumulations that are composed of both biogenic and thermogenic methane. Due to microbial consumption, little of this methane reserve escapes from sediments surrounding seeps. The process of anaerobic methane oxidation (AMO) in which hydrocarbons are oxidized via the reduction of sulfate, produces pore fluids that display sulfide depletion and simultaneous bicarbonate and hydrogen sulfide enrichments. These microbially-mediated reactions affect dissolved inorganic carbon equilibria and produce calcium-magnesium carbonate byproducts.

Push cores (20-30 cm long, sampled in 2000 with the submersible Alvin) have been analyzed from Green Canyon lease block 272 in the northern GOM. Located at 700 m water depth, the area contains a complex of gas hydrates and mud volcanoes at the seafloor where fluid mud mixed with hydrocarbons is actively being extruded. Pore fluid chemistry from GC 272 sediments indicates that biogenic methane venting drives intense AMO in this area. Pore fluid dissolved inorganic carbon (DIC) has δ¹³C as light as –52 ‰ PDB and radiocarbon measurements of pore fluid DIC indicate the seepage of fossil biogenic methane to these shallow sediments as opposed to biogenic methane derived from in situ organics.

Authigenic carbonates in the sediments form cm-sized nodules composed of dolomite and high-magnesium calcite. GC 272 sediments also contain pseudomorphs of the calcium carbonate hexahydrate mineral ikaite (CaCO₃ + 6H₂O) and represent the first known occurrence of this mineral in the GOM. Carbonate δ¹³C values range from –40 to –13 ‰ PDB, an indication that the carbonates form near the sediment-water interface from a biogenic methane source. Carbonate δ¹⁸O values are particularly varied, ranging from –6.65 to +4.38 ‰ PDB. In the rapid flux setting exemplified by mud volcano formation, considerable heat as well as fluidized sediment and hydrocarbons are transported to the seafloor. Stable isotopes of oxygen imply carbonate nodule formation at temperatures as high as 50˚ C, the first mineral records of “warm” GOM vents.