SREL Reprint #2852

Brine Vents on the Gulf of Mexico Slope: Hydrocarbons, Carbonate-Barite-Uranium Mineralization, Red Beds, and Life in an Extreme Environment

Roger Sassen1, Stephen T. Sweet1, Debra A. DeFreitas1, Nancy L. Eaker1, Harry H. Roberts2, and Chuanlun Zhang3

1Geochemical and Environmental Research Group (GERG), Texas A&M University,
College Station, Texas 77845
2Department of Oceanography and Coastal Studies Institute, Louisiana State University,
Baton Rouge, Louisiana 70803
3Savannah River Ecology Laboratory, Marine Sciences Department, University of Georgia,
Aiken, South Carolina 29802

Abstract: Enormous volumes of middle Jurassic Louann salt dissolve during migration of fluids from depth. At the seafloor, the result is vents, pools, and down-slope flows of brine. At these locations, biogenic methane or C1-C5 thermogenic gases are present in high concentrations. New Δ14C measurements indicate a deep fossil source of this biogenic methane. However, gas hydrate is not found at brine sites because high salinity retards the crystallization process. Microbial hydrocarbon oxidation and sulfate reduction occur in brine and associated sediment that drives massive precipitation of authigenic carbonate rock depleted in 13C. The authigenic carbonate rock is often radioactive and has diagnostic barite-uranium mineralization.
Thick red beds occur as a consequence of mobilization of iron from the subsurface by brines. The flow of these brines over shallow, or exposed, salt in Walker Ridge in a water depth of 2,037 m has deposited red iron hydroxide layers up to 4.6 m thick. Iron comprises 24.1-24.6% of the sediment. Hematite, dolomite crystals, and uranium minerals are minor components of the red beds.
Methane-rich brines favor simple chemosynthetic communities of methane-oxidizing mussels and sulfide-oxidizing bacterial mats, decreasing biologic diversity relative to nearby chemosynthetic commnities not affected by brines. Crude oil is altered by microbial oxidation, concentrating toxic aromatic hydrocarbons in the brine environment along with uranium, radium, and toxic metals. Consequently, brine-related chemosynthetic communities appear to exit in the most extreme environment for life on the seafloor of the Gulf of Mexico slope. The characteristics of hydrocarbon-charged brine explusion features are likely to be preserved during deep burial, allowing dating of major phases of hydrocarbon migration in the geologic past.

SREL Reprint #2852

Sassen, R., S. T. Sweet, D. A. DeFreitas, N. L. Eaker, H. H. Roberts, and C. Zhang. 2004. Brine vents on the Gulf of Mexico slope: hydrocarbons, carbonate-barite-uranium mineralization, red beds, and life in an extreme environment. 2004 Gulf Coast Section Society of Economic Paleontologists and Mineralogists 444-463.

This information was provided by the University of Georgia's Savannah River Ecology Laboratory (srel.uga.edu).