Strontium isotope stratigraphy relies on recognised variations in the Sr 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.
During the centuries preceding the Spanish conquest of the Aztec Empire, the Aztecs had expanded to incorporate a large part of central Mexico, and its influence caused the variety of Nahuatl spoken by the residents of Tenochtitlan to become a prestige language in Mesoamerica.
The Rb-Sr dating method has been used extensively in dating terrestrial and lunar rocks, and meteorites.
If the initial amount of Sr is known or can be extrapolated, the age can be determined by measurement of the Rb and Sr concentrations and the Sr ratio.
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.