Seawater circulation through the flank of the Florida platform: evidence and implications

Abstract Dense biological communities at the base of the Florida Escarpment depend on energy provided by reduced compounds for bacterial chemosynthetic production. Sharp gradients in the concentrations of CH 4 , H 2 S, NH 4 + , SO 4 2− , Cl − , Na + , Mg 2+ , Sr 2+ , I − and seabed 226 Ra 2+ exist in the sediments underlying these communities. The compositions of this fluid departs from that of normal seawater, indicating the importance of microbial and diagenetic reactions within the escarpment before it is advected to the seafloor. The 87 Sr/ 86 Sr ratios in the seep fluids range from 0.70920 (∼seawater) to 0.70773, which requires addition of light (Mesozoic) strontium. δ 18 O values (−0.3 to 0.1% SMOW) and ° D values (2–6% SMOV) of these waters are similar to normal seawater values and do not indicate that either meteoric processes or gas hydrates are involved in the genesis of the seeping fluid. Concentrations of Cl − , Ca 2+ and Na + suggest that the fluids which arrive at the seafloor are a mixture of mostly seawater with 4–7% formation brine from within the Florida-Bahama Platform. Mesozoic brines within the Florida Platform occupy levels above the scarp base. A density-driven flow system in implied which involves Mesozoic platform brines that are greatly diluted by seawater entrained through the flanks of the platform. The observed depletion of sulfate in the seeping fluids requires that sulfate reduction must occur in the platform within the zone where the recharging seawater and platform brines mix. In order to produce the high concentrations of biogenic methane observed, methanogenesis is inferred to be occuring within the platform in a zone where sulfate reduction has gone to completion. The zone where the mixing of brine and seawater occurs should be cooled by the flushing seawater and heavily corroded by under saturation occurring as a result of the mixing. The platform interior mixing is non-conservative because of the addition of light (Mesozoic) Sr 2+ and the relative depletion in Mg 2+ with espect to Cl − , Ca 2+ and Na 2+ concentrations. The inferred uptake of Mg 2+ and concurrent depletion of SO 4 2− on the flanks of this margin.