Heavy rain marks the spot where you were born

Here’s another story about isotopes, this time in the service of archaeology. Imagine you have a tooth of someone who lived 1000 years ago or more. How could you tell where they came from?

One answer to this question uses the facts that there are two distinct stable isotopes of oxygen (O¹⁶ and O¹⁸), and likewise of two of strontium. It turns out that O¹⁸, having two extra neutrons, is roughly 12% heavier than O¹⁶ and so a water molecule H₂O¹⁸ is likewise heaver than H₂O¹⁶. When rain clouds drive in from the sea, the heavier rain falls first, causing a decreasing gradient in the proportion of O¹⁸ in fresh water and plants as you move inland, and a corresponding gradient in the take-up of the two oxygen isotopes in the teeth of someone growing up there.

A second parameter that distinguishes your region of origin is the proportion of two isotopes of the element strontium. They have 48 and 49 neutrons respectively and their relative proportions in the underlying rocks are mirrored in the top-soil and in the plants that grow there. Ultimately the humans ingest the strontium in the same proportions through the food chain and this can again be measured by the ratio of the two isotopes deposited in their teeth.

Together, the values of these two isotope ratios, for oxygen and strontium, are sensitive enough to locate where the owner of the teeth was raised, on the assumption that most people don’t move far from where they were born until they reach adulthood. This technique was used by Douglas Price and his team at the University of Wisconsin in Madison to shed light on the geographical origins of sacrificial victims cast into the Sacred Cenote, a sink hole in the limestone of the Yucatán peninsula in Mexico believed by local Mayans to be the entrance to the underworld. They analysed 40 ancient human teeth recovered from the lake and concluded that half of them were locals, around a quarter had come from somewhat farther afield, and the remainder from places hundreds of kilometres away, in what are now western Honduras and Mexico’s central highlands. (Taken from a report in The Economist, 3rd August 2019.)