WHY DO OOIDS GROW SO SLOWLY?

Ooids are a significant component of carbonate sequences across Earth history, commonly occurring in well-sorted, cross-stratified ooid grainstones. The internal textures and size characteristics of ooids record a history of the chemical, biological, and physical conditions that each grain has experienced. Radiocarbon data from geologically modern ooids have provided some insight into ooid histories and stepwise dating of ooid cortices illustrates that the ooids grow exceptionally slowly (Beaupre et al. 2015). This net growth rate data, however, yield a conundrum: both empirical carbonate precipitation rate estimates and experimental growth of “proto-ooids” predict that ooids should grow four to five orders of magnitude faster that the radiocarbon net growth rate. Why, then, do ooids grow so slowly? Drawing on the hypothesis that abrasion during sediment transport might play an important role in limiting ooid growth, we used abrasion mills to experimentally determine net abrasion and precipitation rates of ooids as a function of grain size and transport stage. The experimental data illustrate that net abrasion and net precipitation rates for typical sand-sized marine and lacustrine ooids are three to five orders of magnitude greater than the radiocarbon net growth rate estimates. Slow net growth could therefore reflect the small net difference between two much faster rates—precipitation and abrasion—acting in opposition, with the corollary that ooids should rapidly approach a single critical size that represents a dynamic equilibrium between precipitation and abrasion. Under the dynamic equilibrium hypothesis, properties that can be readily measured or estimated from the rock record—ooid size and transport stage—can be used to invert for precipitation rate, and ultimately seawater carbonate saturation state. Furthermore, our experimental abrasion and precipitation rate data suggest that chemical and physical processes bear significant influence on ooid growth rates, sizes, and textures; these represent background conditions upon which biological influences and textures are superimposed.