LA-ICPMS STUDY REVEALS LARGE VARIATIONS IN THE REDOX AND DIAGENESIS SENSITIVE TRACE METALS IN SEDIMENTARY CARBONATES AT ~30-MICRON SCALE

Sedimentary carbonates are one of the primary sources of information used to reconstruct the biological and geochemical history of Earth’s past oceans. The stratigraphic variation in the concentration of major and trace metals in the carbonate lattice is used to track variations in the redox conditions of paleo-seawater. A key challenge, however, has been to identify and control for the impact of post-depositional diagenetic processes on bulk carbonate chemistry from the pristine geochemical. To quantify the potential influences of primary versus secondary processes on paleo redox indicators in sedimentary carbonates within individual samples, we mapped the geochemistry of different carbonate components using LA-ICP-MS with a 30 µm spot size to determine spatial variation in geochemical composition across different carbonate components within shallow-marine carbonate samples from a lower Cretaceous stratigraphic section. The micron-scale analysis revealed large variations in major (Sr, Fe, Mg, and Mn) and trace (U, Mo) metal concentrations within and across petrographic components (micrite, oncoid, skeletal grain, cement). Micrite exhibits the largest variations in the concentration of redox-sensitive elements such as uranium (0-3 ppm) and molybdenum (0-0.5 ppm). The micritic components also contain the highest concentrations of redox-sensitive trace metals. We interpret differences in the concentrations of redox sensitive trace elements (U, Mo, Fe) across petrographic phases within single rock samples to result from microbially mediated redox reactions at and below the sediment-water interface. In general, we expect that the micritic phases, being more porous and permeable, are more prone to sediment-fluid interaction than the skeletal and cement phases. The fluid-buffered alteration of the micritic phase would allow it to record the reducing pore-water redox conditions below the sediment-water interface due to microbial metabolic processes like manganese, iron and sulphate reduction. The quantification of geochemical heterogeneity of sedimentary carbonates at the micron scale and of the relationship between petrographic phases and geochemical properties holds promise for improving the fidelity of carbonate-based proxies and for unraveling the impact of microbial metabolic processes on sediment and porewater chemistry.