The where, when, and how of ooid formation: What ooids tell us about ancient seawater chemistry

Ooids are a common constituent of carbonate rock throughout the last 3.2 billion years. The chemical and physical characteristics of ooids record essential details of the carbonate chemistry and hydrodynamic conditions of their depositional environment. As a result, ooids may be a powerful tool for reconstructing Earth's ancient climates and environments. However, lack of constraints on fundamental aspects of ooid formation, such as the role of abrasion, underscores the potential for additional discoveries utilizing ooids as paleoenvironmental proxies. Here, we use three-dimensional models built by serially grinding and imaging ooids to show how ooid shape changes during growth. We find that ooid abrasion plays only a small role in the formation of ooids, even in giant ooids, where the mass should cause abrasion to be most intense. A lack of abrasion during ooid formation suggests a greater sensitivity of ooid distribution and size to the chemistry of depositional environments. This insight enables us to harmonize our understanding of ooid formation with longstanding questions regarding their spatial and temporal distribution, as well as size variations. With a new database of ooid occurrences, we analyze the influence of seawater chemistry, carbonate shelf area, and evolutionary developments on ooid abundance in the Modern, Last Glacial Period, and throughout the Phanerozoic. Finally, we leverage our new understanding of ooid growth to show how ooid size, including giant ooids, can shed light on relative changes in alkalinity throughout the Phanerozoic.