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How can fluorine analysis show that bones from one layer were buried at different times?

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Archaeology Archaeologists mapping their finds at Pachacamac, Peru, an Indigenous town occupied from approximately 200 bce to 1532 ce, when it was sacked by conquistadors under the command of Francisco Pizarro.

Fluorine analysis is a method of relative dating used to determine if bones found within the same archaeological or geological layer are contemporary.1

The process relies on the fact that when bones are buried in soil, the hydroxyapatite in the bone gradually undergoes a chemical change, absorbing fluorine from groundwater to become fluorapatite. Because this accumulation occurs over time, bones that have been buried longer generally contain higher concentrations of fluorine.23

Archaeologists use this technique to identify "intrusions," or instances where remains were introduced to a layer at a different time than the surrounding materials. If bones discovered in the same level exhibit significantly different fluorine levels, it indicates that they were not deposited simultaneously. This method has been instrumental in detecting faked remains and later burials, such as in the case of the Piltdown Man hoax.12

Because factors like soil permeability, temperature, and groundwater composition vary by location, fluorine content cannot serve as an absolute chronometer; it is most reliable when used in a restricted locality and supplemented by measurements of uranium and nitrogen to ensure accurate chronological conclusions.2