Covariance of phosphate content of carbonate rocks and inferred paleosalinity was studied from a core interval characterized by cyclic carbonate rock alternating with anhydrite. Phosphate content of limestone was found to have an inverse relationship to inferred salinity. Inferred paleosalinity was based upon the following: fossil sequences within the carbonates, containing Texagryphaea, established normal marine salinities; anhydrite (originally gypsum) and thin-bedded laminated limestone and dolomite indicated a saline environment. A total of thirteen cyclic carbonate rock alternations with anhydrite were studied. The core, from the Lower McKnight Formation, Comanchean (Cretaceous), South Texas, records periodic influxes of seawater into the marginal evaporitic McKnight basin where dilution of the brine induced an increase in pH. This enhanced the dissociation of HPO 4 (super -2) to PO 4 (super -3) resulting in elevation of phosphate uptake by calcium carbonate and an increase in total phosphate measured in limestone.