Uranium-series dating of Pleistocene molluscan shells from southern California-An open system model

Pleistocene molluscan shells from the marine deposits of southern California were dated by the uranium-series disequilibrium method. The closed system ages were calculated from the measured Th230/U234 and Pa231/U235 activity ratios. Because of discordances found between the Th230 and Pa231 ages of many of the shell samples analyzed, it was assumed that shells do not usually form an ideal closed system. A mathematical solution for an open system model that can compensate for the deviation of a sample from the closed system model is proposed. The average open system ages of the first terrace at San Pedro of the Palos Verdes Hills and the Dume terrace and terrace C at Point Dume were calculated to be near 85,000, 105,000, and 130,000 years, respectively.