INVESTIGATING PALEOENVIRONMENTAL AND DIAGENETIC CONTROLS ON INTRABASINAL VARIABILITY OF STABLE ISOTOPE RECORDS IN ANCIENT LIMESTONES

Stable isotope data from carbonate sedimentary rocks, particularly δ¹³C and δ¹⁸O, are key to understanding Earth’s environmental conditions in deep time. However, divergent, coeval stable isotope records within and between ancient sedimentary basins are not uncommon. Post-depositional processes have been shown to alter geochemical proxies on multiple spatial scales in many modern environments; such variability might be predicted in the geologic record. We use Ordovician-Silurian limestones from Anticosti Island, Québec to test the hypothesis that intrabasinal geochemical variability in ancient carbonate rocks may be due to a) syndepositional processes such as sediment remobilization operating in different depositional environments within the basin and b) differing diagenetic regimes at across the basin. We examine two suites of samples: one time series from a single section, and another set of coeval samples (as constrained by published bio- and lithostratigraphy) from three localities in the basin. Transmitted light and cathodoluminescence petrography are used to characterize the diagenetic history and microfacies traits of the samples and to determine paleoenvironment. The variability of δ¹³C, δ¹⁸O, and trace elements (Sr, Mn, Fe) in both primary and diagenetic textures is analyzed to constrain the effect of each on the values of the bulk sample. Statistical tests quantify the extent to which particular microfacies and paleoenvironments are associated with depletion/enrichment in geochemical proxies. The results will inform the interpretation of basin-scale variability in the stable isotope record through space and time.