Marine Ooid Sizes Record Phanerozoic Seawater Carbonate Chemistry.

Seawater carbonate chemistry links Earth's climate and carbon cycle through the production and preservation of carbonate sediments. Models and carbonate facies abundance records have generated hypotheses about trajectories of seawater carbonate chemistry, including responses to key events in the evolutionary history of carbonate biomineralizers. However, tests of these hypotheses have remained elusive. We applied a novel proxy for the carbonate mineral saturation state (Ω) of seawater based on the diameters of ooids—concentrically‐coated carbonate sand grains—to estimate Ω, dissolved inorganic carbon, alkalinity, and pH of seawater spanning Phanerozoic time. Reconstructed Ω values decreased sharply around ∼120 Ma, which we interpret as the fingerprint of the Mid‐Mesozoic Revolution of planktic calcifiers. Shifts in Ω across Ordovician time also suggest a possible causal relationship with the Great Ordovician Biodiversification Event. Our results demonstrate that ooid sizes are a useful tool for reconstructing Earth's ancient carbon cycle. Plain Language Summary: Earth's oceans play an important role in removing carbon from Earth's surface environments. One of the ways this happens is through the production and burial of calcium carbonate sediments, which include shells and calcium carbonate minerals formed in other ways. The chemistry of the oceans, including pH and the concentrations of carbonate ions, affect how easy it is for carbonate minerals to form, and whether they can survive long enough to be permanently buried on the seafloor. Tracking these aspects of the chemistry of ancient oceans can enable scientists to better understand the balance of carbon entering and exiting Earth's surface environments in the past. Until recently, we have not had the right tools to extract this information from sedimentary rocks. Here, we used a recently developed method that uses the sizes of ooids—sand grains that grow by accumulating concentric layers of calcium carbonate—to track ancient ocean carbonate chemistry. We compared the results of our new approach with previous efforts and found that our approach works well. Our work demonstrates that ooid size measurements can improve our understanding of ancient oceans. Key Points: We estimated seawater calcite saturation states spanning Phanerozoic time using a proxy based on ooid sizeWe combined our calcite saturation state values with pCO2 and seawater chemistry proxy data to calculate seawater carbonate chemistryDissolved inorganic carbon, alkalinity, and pH estimates based on the ooid size proxy agree with predictions from carbon cycle models and carbonate facies records [ABSTRACT FROM AUTHOR]