GEOMICROBIOLOGY CONTROL ON DIAGENETIC CALCITE ASSOCIATED WITH PALEOKARST AT THE MISSISSIPPIAN-PENNSYLVANIAN BOUNDARY IN TENNESSEE

Pennington Formation limestone deposits in Tennessee have a complex diagenetic history. The stabilization of metastable components within the Pennington limestone, which lies below a paleokarst surface defining the Mississippian-Pennsylvanian boundary in Tennessee, occurred in a phreatic meteoric setting as supported by covariation of delta 13-C and delta 18-O values from echinoderm grains. Echinoderm stabilization is likely to have been coupled with microbial activity. Paleosols draping the paleokarst surface and coal seams, which were deposited immediately above the Mississippian-Pennsylvanian contact, provided ample organic matter to drive heterotrophic microbial activity. Additionally, paleosols were an ample source of oxidized Fe that likely was used as terminal electron acceptor by an anaerobic microbial community. Paleosols exhibit significant loss of Fe relative to soil parent material. Echinoderm grains immediately below the paleokarst surface have elevated Fe and lower Mg. Therefore, microbial iron reduction likely transferred Fe from the paleosol to meteoric diagenetic calcite during echinoderm stabilization under a paleokarst surface. Fluid-rock interaction of echinoderm grains is modeled using meteoric water as a diagenetic fluid. Gleying of the overlying paleosol likely added significant Fe (1 ppm) to the fluid as well as carbonate alkalinity resulting in a fluid that was undersaturated with respect to calcite, thereby facilitating echinoderm stabilization. Echinoderms proximal to the paleokarst surface exhibit extensive alteration with a cumulative fluid-rock ratio (N) up to 5000. Modeling of both minor element (Mg and Fe) and stable isotopic (delta 13-C and delta 18-O) yields self-consistent cumulative fluid-rock ratio. Additionally, a simple mass balance calculation indicates that paleosols can yield sufficient Fe to account for the elevated Fe concentrations present in echinoderm calcite. Finally, assuming an iron reduction rate that is similar to rates observed in modern anaerobic groundwater systems (0.00001 mmol/ L / yr) constrains stabilization to a timeframe of thousands of years. Significantly, this study provides estimates of the rates of diagenetic processes (echinoderm stabilization) that are comparable to those determined for Quaternary systems.