The modern shallow-platform, calcium-carbonate–dominated sediments of the Florida Keys (Florida Bay and Atlantic reef tract) are diverse in their biological, sedimentological, and geochemical properties. Sites of intense bioturbation and thick seagrass cover are pervasive within Florida Bay and are often characterized by appreciable early diagenetic aragonite dissolution. Additional, less common sites show atypical diagenetic profiles that suggest strong reworking and/or very rapid deposition of the upper sediment layer extending to a depth of at least 20 cm. Diagenesis in these seagrass-free areas is dominated by rapid burial of labile organic matter that would otherwise be degraded aerobically under conditions of slower burial. Correspondingly, these oozy, water-rich sediments display anomalously high rates of microbial decomposition as recorded in 35 S-sulfate reduction rates and patterns of sulfate depletion, high dissolved sulfide concentrations in excess of several millimolar (mM), and elevated alkalinities. Unlike many sites in Florida Bay where solute concentrations suggest volumetrically significant net dissolution of metastable carbonate phases, dramatic increases in carbonate alkalinity from organic matter oxidation during bacterial sulfate reduction support net precipitation of CaCO 3 in the highly reactive surface layer. This early carbonate mineralization is indicated by measured depletions in Ca approaching 4 mM relative to overlying seawater. Geochemical signatures of sediment reworking or rapid sedimentation are corroborated by porosity trends; visual evaluations, including X-radiography; and an interval of essentially constant 210 Pb activity. Rapid burial within the reactive layer gives rise to restricted-system diagenetic behavior that is recorded in the sulfur isotope compositions of dissolved sulfate and sulfide. Nevertheless, despite strong 34 S enrichments in the pore-water sulfate and clear evidence for diagenetic calcium carbonate precipitation, carbonate-associated sulfate (CAS) trapped within the muds (at concentrations from ∼2400 to 4200 ppm) preserves the original 34 S/ 32 S ratio of the overlying seawater. Such preservation of the δ 34 S of seawater sulfate in bulk lime mud samples—even in the presence of appreciable diagenetic overprinting—confirms the broad utility of the CAS approach in reconstructing ancient ocean chemistry.

Abstract Organic-rich Late Devonian Antrim and New Albany Shales are important hydrocarbon source rocks in the Michigan and Illinois Basins. These shales have been investigated using STEM/AEM and SEM techniques to clarify textures and mutual genetic relations between the low to moderately mature organic matter (R 0 = 0.45–0.6 %) and authigenic illite-rich clays. The Antrim and New Albany Shales contain up to 15 wt % TOC dominated by the marine algae Tasmanites. Organic matter forms an interconnected network within the clay-rich matrix with no detectable intergranular pore space even at the STEM scale. The clay-rich matrix is principally illite-rich mixed-layer I/S. Crystal sizes, compositions, defect states and presence of smectite interlayers are consistent with authigenesis of illite from precursor smectite with subsequent preservation of that immature, diagenetic illite after neoformation. Textural relations verify that gas transport from the Antrim Shale source occurs via desorption from organic matter and diffusion through the interconnected organic and authigenic illite matrix to open fractures. In both the Antrim and New Albany Shales, plastic deformation of organic material entrains illite crystals, suggesting that illite formation preceded or was concurrent with thermal maturation of organic matter. Based on existing thermal maturity data, illite authigenesis occurred prior to Late Pennsylvanian/Early Permian time at depths less than ˜2000 m and temperatures less than ˜120°C. In contrast to the Gulf Coast Tertiary sequence, fluids derived from smectite dehydration were probably not a significant driving force for primary hydrocarbon migration from these intracratonic basin shales as a result of the presence of relatively thin shale sequences and normal pressures during burial compaction.