Evidence for surface reaction-controlled growth of carbonate concretions in shales

Estimates for the rate of concretionary growth in shales are based on models which assume that growth is diffusion-controlled. However, laboratory and field studies of CaCO₃ precipitation in organic carbon-rich sediments indicate that surface reactions control growth, due to inhibition by various dissolved species. The spatial distribution of carbonate concretions in the Jet Rock (Lower Jurassic, England) is also inconsistent with diffusion-controlled precipitation of CaCO³ into concretions, and growth must have been at least partly surface reaction-controlled.