DO MARINE OOID SIZES RECORD PHANEROZOIC SEAWATER CARBONATE CHEMISTRY?

Seawater carbonate chemistry links global climate and the carbon cycle through the production and preservation of carbonate sediments. Carbon cycle models and abundance records of specific carbonate facies have generated testable hypotheses about trajectories of seawater carbonate chemistry across Phanerozoic time, with particular attention to key events in the evolutionary history of carbonate-biomineralizing organisms. In theory, pCO₂ reconstructions could be combined with a proxy for a second unknown in the carbonate system in order to solve for the remaining unknowns and test these hypotheses. However, such tests have remained elusive due to a lack of proxies for seawater carbonate chemistry that are widely applicable across Phanerozoic strata. Here, we combine a novel proxy for the carbonate mineral saturation state (Ω) of ancient seawater based on the diameters of ooids—concentrically-coated carbonate sand grains—with reconstructions of pCO₂, tropical seawater surface temperature, and seawater ion chemistry to estimate Ω, total dissolved inorganic carbon (DIC), alkalinity, and pH of seawater from a suite of 140 formations spanning Phanerozoic time. Our data reveal that Ω values were dynamic but elevated until ~120 Ma, after which Ω dropped to lower and more stable values; we interpret this shift as the fingerprint of the Mid-Mesozoic Revolution. We also report a trend of initially high, but decreasing, Ω values across Ordovician time, suggesting a possible causal relationship with the Great Ordovician Biodiversification Event. Finally, trends in DIC and alkalinity—high but decreasing across Paleozoic time, with a return to higher values during Mesozoic time—align with some carbon cycle model predictions. Our results demonstrate that ooid sizes reliably track Phanerozoic seawater chemistry and are a useful tool for reconstructing Earth’s ancient carbon cycle.