This method provides a numerical age related to a global scale based on the ⁸⁷Sr/⁸⁶Sr ratio of past seawaters. The amount of radiogenic strontium in the world’s oceans is principally controlled by contributions from continental runoff, exhalations at oceanic ridges and meteoric waters. In terms of geological time oceanic mixing it is instantaneous. The assumption that the ⁸⁷Sr/⁸⁶Sr ratio of the world’s oceans has fluctuated uniformly through time has enabled a Global Standard Strontium Curve (GSSC) to be established (Howarth and McArthur, 1997).

Since Sr is taken up by aquatic organisms in equilibrium with the surrounding water it is possible to use their fossilised remains as a proxy for oceanic seawater. Suitable material includes molluscs, foraminifera, conodonts, fish teeth and calcareous algae. whole rock samples (carbonates, dolomitised carbonates and evaporites) for analysis providing any diagenesis was early.

Non-oceanic environments will of course give rise to atypical Sr ratios which will differ from the global seawater strontium standard. Closeness to a major river system is one such environment where one would expect Sr ratios to vary from the coeval oceanic values.

Data from individual samples are compared to a database which includes all published and many unpublished Sr ratios which have first been subjected to a vetting procedure and weighted for stratigraphical, biostratigraphical and chemical accuracy. The figure opposite illustrates the global seawater Strontium curve.

The strontium isotope ratio analyses are carried out by IsoBar Science Ltd. For more information go to https://isobarscience.com/strontium-chronology.