Mixing-zone diagenesis in the subsurface of Florida and the Bahamas

Three cores from Great Bahama Bank and southern Florida contain examples of mixing-zone diagenesis where typical shallow-water, aragonite-rich sediments were altered to low-Mg calcite. The 40-50 m thick mixing zones are identified on the basis of downcore covariant trends from depleted meteoric δ ¹⁸O = -3.2 ± 0.7‰; δ ¹³C = -1.6 ± 1.8‰) to enriched marine isotopic values (δ ¹⁸O = +1.0 ± 0.3‰; δ ¹³C = +1.9 ± 0.7‰). The age of the mixing zones varies from late Pliocene to Pleistocene, so these cores provide three independent examples of mixing-zone diagenesis. Mixing-zone diagenesis produced a limestone composed of low-Mg calcite with petrographic fabrics including moldic porosity, blocky to dogtooth calcite cementation, and recrystallization of lime mud to micrite and microspar. The mixing-zone diagenetic model predicts alteration in only part of the mixing zone. Because the alteration recorded in these cores shows continuous alteration through the entire mixing zone, we consider the mixing of waters itself to be largely irrelevant. Rather, diagenesis was probably driven by the difference in solubility between aragonite and calcite with some additional dissolution of aragonite driven by the microbial oxidation of buried organic matter. The mixing zone has been proposed as a likely location for dolomitization. With the exception of some clearly sea-floor dolomite in one core (Unda), the mixing zones in these cores lack dolomite despite the sediment clearly having been altered in the mixing zone. The lack of dolomite in these mixing zones, combined with the extensive literature questioning the chemical reasoning for mixing-zone dolomite, supports rejection of the mixing-zone dolomite model.