How Are Strontium Isotopes Used To Trace Geographic Origins?

 

Strontium is incorporated into animal tissues from the food and water we consume. The bedrock of geologically-different regions has distinct levels of strontium, so different geological zones have unique strontium signatures. Assuming people were eating mostly local foods (this would never work today when our apples come from Chile, beef comes from Mexico, etc.), we can match up the Sr signature from a person's tooth enamel and bone to that unique geological signature, and that tells us where they were living during the time of tissue formation. Dental enamel is formed during childhood and early teen years, while bone is remodeled throughout life. This means that strontium values from enamel tells us where individuals lived in childhood while strontium values tell us where they lived during the last years of their life. If either of those signatures is different from the signature where the person was buried, that tells us they migrated to the burial zone.

In some parts of Peru this analysis doesn't work very well. For example, the Nasca region is a broad desert area where even far-away areas have similar strontium signatures. In my research area, though, we have baseline data from archaeological samples and modern fauna that eat local food, and we know the bedrock is geologically unique enough in the Majes Valley for these methods to work. We are also excited to try out lead isotopes as a second tracer for geographic origins with this sample.

If you want to read more about how strontium isotopes have been used to trace place of residence and migration, check out the "References" lab note at the bottom of the lab notes page.