Precise measurements of 786 marine carbonate, evaporite, and phosphate samples of known age provide a curve of seawater 87Sr/86Sr versus geologic time through the Phanerozoic. Many episodes of increasing and decreasing values of 87Sr/86Sr of seawater have occurred through the Phanerozoic. The Late Cambrian–Early Ordovician seawater ratios are approximately equal to the modern ratio of 0.70907. The lowest ratios, ∼0.7068, occurred during the Jurassic and Late Permian. The configuration of the curve appears to be strongly influenced by the history of both plate interactions and seafloor spreading throughout the Phanerozoic. The curve provides a basis for dating many marine carbonate, evaporite, and phosphate samples. Furthermore, diagenetic modifications of original marine 87Sr/86Sr values are often interpretable. Analysis of 87Sr/86Sr data, therefore, may provide useful information on regional diagenetic patterns and processes.
All of the Cenozoic samples and some of the Cretaceous samples are from Deep Sea Drilling Project (DSDP) cores. With the exception of the DSDP samples, the curve was constructed only from samples containing at least 200 ppm Sr and not more than 10% dilute acid insoluble material. All measurements are made by comparison with standard SrCO3 (NBS SRM 987) for which a 87Sr/86Sr of 0.71014 is assumed. Precision is estimated to be ± 0.00005 at the 95% confidence level. Measured ratios of 42 modern marine samples average 0.70907, with a standard deviation of 0.00004.