BASIN-SCALE CA ISOTOPIC MAPPING REVEALS MAGNITUDE AND SCOPE OF CARBONATE DIAGENESIS IN EPEIRIC MARINE CARBONATES

Marine carbonates record geochemical information on the state of Earth’s surface environment in the geological past, but recrystallization often makes the records difficult to interpret. We report shore to basin gradients in δ⁴⁴Ca values in burrow dolomite (–2.04‰ to –0.66‰) and matrix limestone (–1.31‰ to –0.47‰) across a lithostratigraphic time-slice of Late Ordovician carbonate in the Williston Basin, North America. Both gradients have diagenetic origins with limestone δ⁴⁴Ca values developed in a bathymetrically controlled sediment to fluid buffered diagenetic system operating between center and edge of the basin: Two dolomitizing events formed the gradient in dolomite. The sediment buffered limestone comes closest to preserving the δ⁴⁴Ca value of the original carbonate mud in the basin center, which reflects contemporaneous seawater offset by a kinetic isotope fractionation factor. Fluid buffered limestone near the basin edge comes closest to preserving the contemporaneous δ⁴⁴Ca value of unfractionated seawater, because (1) drivers of benthic Ca exchange—a form of submarine groundwater discharge—are more intense in shallow water, (2) shallow nearshore deposits exchange more Ca with seawater during recrystallization than deep offshore deposits, and (3) diagenetic calcite forms with negligible fractionation under equilibrium conditions. The minimum fractionation factor of the originally produced sediment was –0.84‰, which is close to the nominal value of ~ –0.90‰ for primary calcite in modern marine settings. Limestones located midpoint along the length of the gradient preserve mixed signatures of fluid and sediment buffered diagenesis that reflect cycles of shoreline progradation. The dolomite δ⁴⁴Ca value is fractionated relative to calcite at equilibrium with an estimated value of –0.41‰ at ambient temperature. Applying this to basin center dolomite yields a value of –1.6‰ for Ca in the hydrothermal fluid, which passed into the structural center of the basin from the underlying crust during a younger anomalous thermal event. Geochemical records of carbonates are often used to reconstruct secular variation in the geochemistry of seawater in the geological past. We show that lithostratigraphic time slices provide snapshots of similar information in a depositional/diagenetic framework that improves proxy reliability.