The role of authigenic carbonate in Neoproterozoic carbon isotope excursions

Sedimentary carbonates from the Neoproterozoic Era host some of the largest carbon isotope excursions in the geological record, in some cases as large as 12 parts per thousand to 15 parts per thousand. The origin of these signals remains a matter of debate - including whether they reflect local diagenetic effects, or record changes in the dissolved inorganic carbon (DIC) of global seawater that are indicative of shifts in the larger carbon cycle. Producing such changes through oxidation of organic carbon or methane requires unreasonably large fluxes that imply massive perturbations to carbon and oxygen budgets that are not observed. We present a plausible way to drive large changes in the delta C-13 of the global ocean via large-scale precipitation of isotopically fractionated authigenic carbonates distributed globally at modest concentrations in terrigenous sediments, particularly if the isotopic fractionation of authigenic carbonate shifts rapidly relative to average marine carbonate. We suggest that changes in the fractionation associated with authigenic carbonate precipitation could therefore be responsible for at least some of the variability in isotopic records drawn from primary marine carbonate. Using a diffusion-reaction model, we find that oscillations in sulfate concentrations are one mechanism for altering the isotopic composition of authigenic carbonates, thus forcing abrupt changes in seawater delta C-13. This mechanism is particularly effective under the geochemical conditions that likely prevailed during the Neoproterozoic. A sulfate-driven, authigenic origin for Neoproterozoic carbon isotope excursions makes testable predictions for the expression of the anomaly over depth transects, and for the evolution of delta S-34 through such events. (C) 2020 Elsevier B.V. All rights reserved.