Determining Effects of Diagenesis on Geochemical Dating of Plio-Pleistocene Shallow Marine Fauna

Abstract:

Accurate dating of fossil shorelines is essential for establishing chronology related to paleo sea level changes, providing insight into past climates, paleo ice volumes and local dynamic topography. Biogenic carbonate is a particularly useful tool for dating marine terraces, as they record chemical signatures of seawater at the time of formation. However, the primary geochemical methods (marine Sr isotopes, U-series disequilibrium, and U-Pb dating) used to date shallow water fauna older than the late Pleistocene are often subject to open system effects, such as diagenesis (recrystallization and meteoric weathering). This is especially problematic for Sr isotope dating during the Mid-Pliocene, when the ⁸⁷Sr/⁸⁶Sr seawater calibration curve shows small changes over time, and small variations in ⁸⁷Sr/⁸⁶Sr ratios correspond to large uncertainties in ages. To identify pristine carbonate material that generates reliable seawater Sr isotope ratios, we examined different species of Plio-Pleistocene bivalves and corals from various geographic locations and environments by combining optical images, scanning electron microscope (SEM), cathodoluminescence and x-ray diffraction (XRD) with elemental and Sr isotope analyses of micro-drilled calcite and aragonite layers within individual organisms. The results from this study will help to evaluate the effect of secondary diagenesis and recrystallization on trace element abundances and Sr isotope ratios. Using these methods, we show that reliable ages can be achieved using the Sr isotope seawater curve.