Seawater Sr isotope variations through time: A procedure for constructing a reference curve to date and correlate marine sedimentary rocks

A new curve of seawater ⁸⁷Sr/⁸⁶Sr variation through the past ∼450 m.y. enables dating and correlation of marine sedimentary rocks with quantifiable uncertainties. We discuss the method by which this curve was constructed. Strictly standardized ⁸⁷Sr/⁸⁶Sr ratios and biostrat igraphic data for ∼1300 ⁸⁷Sr/⁸⁶Sr analyses of marine minerals were input into a data base. Objective criteria were used to assign each data point a reliability score. These scores were used as initial weights in a robust nonparametric regression (LOWESS) to obtain a curve of seawater ⁸⁷Sr/⁸⁶Sr variation with rigorous 95% confidence intervals for prediction of age uncertainty. Precise dating and stratigraphic correlation are feasible for much of the Phanerozoic. The main problem with future Sr isotope stratigraphy is likely to be sample quality; we discuss guidelines for sample selection.