shell or indeed a clay artefact is directly comparable to the source rocks upon which it was formed or upon which the organism lived. Thus, by measuring the current-day 87Sr/86Sr ratio (and often the 143Nd-144Nd ratios as well) the geological fingerprint of an object or skeleton can be measured, allowing migration patterns to be determined. Strontium isotope stratigraphy Strontium isotope stratigraphy relies on recognised variations in the 87Sr/86Sr ratio of seawater over time. The application of Sr isotope stratigraphy is generally limited to carbonate samples for which the Sr seawater curve is well defined. This is well known for the Cenozoic time-scale but, due to poorer preservation of carbonate sequences in the Mesozoic and earlier, it is not completely understood for older sequences. In older sequences diagenetic alteration combined with greater uncertainties in estimating absolute ages due to lack of overlap between other geochronometers (for example U–Th) leads to greater uncertainties in the exact shape of the Sr isotope seawater curve. References External links CSIRO Petroleum - Global Sr Seawater Isotope Evolution Radiometric dating Rubidium Strontium