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ABSTRACT: Fluid Circulation in the Western Edge of the Florida Platform: Evidence from Basal Seeps

C.K. Paull, C. S. Martens, A. C. Neumann, P. D. Fullagar, J. P. Chanton

Dense communities discovered via Alvin at the base of the Florida Escarpment depend upon bacterial chemosynthetic primary production. Massive concentrations of sulfide minerals and elevated chlorinities in the pore waters of underlying sediments suggest that sulfides in advecting brines are an important chemosynthetic fuel. Subsequent work has further revealed that the seep fluids are depleted in sulfate and greatly enriched in biogenic methane. This ambient methane contains less than a few percent modern carbon, suggesting a fossil source. The 87Sr/86Sr values in the seep fluids range from seawater values (0.70906) to 0.70781, which requires addition of Sr from the Mesozoic brines that fill the Florida-Bahama Platform. Moreover, values of Cl-, Ca+2, and Sr+2 are consistent with the fluids being a mixture of about 20 parts seawater and one part brine. Wells in the Florida Platform show that Mesozoic brines occupy levels above those of the scarp base. A density driven flow is implied. Because of the lack of sulfate in the seeping fluids, sulfate reduction must occur in the platform within the zone where the recharging seawater and platform brines mix. Methanogenesis may occur where sulfate reduction is complete. Mg+2 concentrations do not rise with the Cl-, Ca+2, and Sr+2 concentrations. The inferred take-up of Mg+2 and concurrent depletion of SO4-2 means that conditions favoring dolomitization occur in the mixing zone on the lanks of this margin. These data require recharging this continental margin circulation system through the exposed flanks of the carbonate platform, underscore the potential of microbial processes within the platform, and suggest that dolomitization in the deep flank setting may be an ongoing process.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990