QUANTIFYING EARLY MARINE DIAGENESIS IN SHALLOW-WATER CARBONATE SEDIMENTS DURING THE LATEST ORDOVICIAN GLACIATION (Invited Presentation)

Shallow-water carbonate sediments constitute one of the most abundant and widely used archives of Earth's surface evolution. One of the main limitations of this archive is the susceptibility of the chemistry of carbonate sediments to diagenetic alteration. Here we present a numerical model of marine carbonate diagenesis that tracks the elemental and isotopic composition of Ca, Mg, C, O, and Sr during dissolution of primary carbonates and re-precipitation of secondary carbonate minerals. The model is validated using measurements of geochemical proxies from sites on and adjacent to the modern Bahamas platform and authigenic carbonates in the organic-rich deep marine Monterey Formation. Model results demonstrate that covariation between Ca isotopes and Sr concentrations can provide a semi-quantitative estimate of the extent and style (fluid-buffered vs. sediment-buffered) of early marine diagenesis. Further, we apply the model to geochemical records of Upper Ordovician carbonate rocks deposited during the Late Ordovician glaciation. Covariation between Ca isotope values and Sr concentrations differentiates diagenesis in fluid-buffered versus rock-buffered pore-waters. Relatively enriched Ca isotope values and low Sr concentrations are recorded in limestones with baseline d¹³C values deposited during sea-level highstands, suggesting open-system fluid-buffered diagenesis. In contrast, the positive Hirnantian carbon-isotope excursion is carried in limestones with depleted Ca isotope values and high Sr concentrations deposited during peak glacial conditions, indicating that the primary d¹³C signature is preserved through rock-buffered diagenesis of former aragonite. A possible interpretation of the covariation between the positive d¹³C values and negative Ca isotope values is that the widespread Hirnantian d¹³C excursion largely reflects changes in the d¹³C values of dissolved inorganic carbon in shallow-water aragonite-producing environments (and the style of subsequent diagenetic alteration) and not changes in the d¹³C values of global seawater.