Brine Vents on the Gulf of Miexico Slope: Hydrocarbons, Carbonate-Barite-Uranium Miineralization, Red Beds, and Life in an Extreme Environment
Sassen, Roger
Sweet, Stephen T.
DeFreitas, Debra A.
Eaker, Nancy L.
Geochemical and Environmental Research Group (GERG)
Texas A&M University
College Station, Texas 77845
email: sassen@gerg.tamu.edu
Roberts,
Harry H.
Department of Oceanography and Coastal Studies Institute
Louisiana State University
Baton Rouge, Louisiana 70803
Zhang,
Chuanlun
Savannah 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 14C
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.
