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Lead isotopes have frequently been used in archaeology to determine the provenance of metal objects. More recently, they have also been helping to assess mobility in humans and other animals. This form of analysis is still in its infancy, however, and the accuracy of using lead as a proxy for migration is still up for debate. A recent study has taken lead-isotope analysis in combination with the more tried-and-tested analysis of strontium isotopes to help explore this question – and in this month’s ‘Science Notes’ we will examine its results, which were recently published in the Journal of Archaeological Science (https://doi.org/10.1016/j.jas.2025.106269).
At the heart of the study was a cattle molar (below) from a jawbone that was discovered in 1924 at the bottom of the ditch surrounding Stonehenge, which is believed to date from the monument’s first stage, c.2995-2900 BC. The mandible was found within the southern entrance through the ditch, and another had been placed at the bottom of the west terminal. It is believed that both jawbones were between 55- and 270- years-old when they were deposited, probably representing curated artefacts that had held some sort of meaning for the community that left them there.
Since the molar dates to the Neolithic period, before the advent of the human mining of lead, it made an excellent candidate to explore the relationship between lead and strontium isotopes in living things. The research team, led by Jane Evans from the British Geological Survey, took cross-samples down the molar before conducting a series of investigations including oxygen (O), carbon (C), strontium (Sr), and lead (Pb) isotope analyses, as well as peptide analysis (see ‘Science Notes’ in CA 337). This last technique showed that the animal was probably female, while the isotope analyses were able to speak to the animal’s food and water consumption, as well as migration patterns.
The team found that the oxygen data represent about half a year’s worth of enamel growth, most likely between winter (January) and summer (July). As this particular molar develops between 10 and 24 months of age, the sample was found to reflect six months of mineralisation during the second year of the cow’s life. The carbon- and strontium-isotope profiles, which indicate the plants that the animal consumed, revealed that there had been a slow shift in its diet during this six-month period, either indicating that the animal had moved from forest to more open grassland, or suggesting that its winter food had primarily been harvested from forest sources such as leaves, nuts, and twigs.
In terms of using lead as a proxy for migration, the researchers found that the lead values did not mirror the strontium ones, as had been expected, but were instead defined by a marked peak indicating that the animal had been accessing a substantially different Pb-isotope source sometime in the spring.
The team suggest that the most likely reason for the differing lead and strontium values is due to the different way in which the animal accessed these two elements. While strontium was derived from the animal’s diet, the lead must have come from somewhere else. Lead can sometimes be remobilised from the skeleton in humans and animals during times of stress, particularly around times of pregnancy and lactation; in order to promote embryo bone growth and milk production, the body releases calcium stored in bones, and this process allows the release of lead at the same time. Once in the bloodstream, the lead can then be redeposited in tooth enamel during mineralisation. The team suggest that this peak in lead isotopes seen in the molar sample may be picking up an incidence of pregnancy and/or lactation – an argument strengthened by the fact that the peak seems to have happened in the spring, when calving usually occurs.
While the study found that lead and strontium, in this case, did not provide the same mode and timescale for deposition in tooth enamel, it did reveal that lead isotopes might be able to be used to identify periods of physiological stress in an individual, particularly instances of pregnancy and lactation. In the absence of such stresses or anthropogenic pollution, however, strontium and lead isotope analysis may still be expected to reflect the uptake of these two elements via dietary sources.
Text: Kathryn Krakowka / Images: reproduced with permission from the British Geological Survey © UKRI 2025. All Rights Reserved; Udit Kapoor, CC BY-SA 4.0
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