Carbon Isotopes and Limestone Cement

Most marine limestones have about the same δC¹³ as marine carbonate sediments, about 0 per mil versus the PDB isotopic standard. Because the limestones are half cement, the cement too must have δC¹³ of about 0 per mil. In contrast, limestones lithified by contact with vadose or phreatic meteoric waters, as in Bermuda, have strongly negative δC¹³. This suggests that most limestones in the geologic record are cemented, at least mainly, by a different process that takes place during burial diagenesis. Pressure solution is important in transferring CaCO₃ from “donor” sites along stylolites and microstylolites to “receptor” sites in pores. Shales may also be important donors for limestone cements.