Pb Isotopes and Human Mobility: Natural, Cultural, or Diagenetic Signal?

Abstract

Strontium (87Sr/86Sr) and oxygen (δ18O) isotopes are widely used in the archaeological sciences to decipher origin and mobility patterns of humans and animals. Similarly, lead (206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb) isotopes can also provide origin and mobility information for archaeological remains. Furthermore, the Pb isotope system has the potential to provide insights about cultural and economic patterns of ancient societies. However, regardless of its potential in archaeology and related fields, there are a number of complications associated with the application of Pb isotopes to bioarchaeological research. Pb concentration in prehistoric human tissue is very low, and it is a challenge to obtain reliable Pb isotopes from preserved tissue, principally tooth enamel. Pb is typically present in sub-ppm levels or in some cases even below detection limits for individuals not exposed to anthropogenic sources of Pb. In addition, the inherently low Pb content in bioarchaeological tissues makes the Pb isotope system much more susceptible to taphonomic effects during diagenesis. It is recommended that targeted tooth enamel samples are analyzed first for Pb, V, Mn, Fe, REE, U, and Th concentrations in order to evaluate their preservation status. Pb isotope ratios in samples with higher than the acceptable in vivo concentrations for one or more of the above trace elements may be compromised by diagenetic processes. The Pb isotopic information can be utilized for bioarchaeological studies only if samples exhibit none or minimal diagenetic alteration. Another complication is that modern soils, plants, animals, and humans virtually everywhere on Earth contain various amounts of industrial anthropogenic Pb. Therefore, archaeological soils and/or pre-industrial sediment and bedrock samples must be used to estimate local Pb baselines and/or reconstruct Pb isoscapes for archaeological research. Once the natural Pb background is established, then a comparison of archaeological Pb isotope data with isotope variations across the landscape can provide information about human or animal origin and mobility patterns. In certain regions, Pb isotopes from ore samples also can be useful for estimating regional isotopic variations applicable to bioarchaeological studies. Pb has been mined and used by humans for millennia, and as a result, historical soils and sediments in a number of regions show evidence for Pb pollution in pre-industrial times. Due to ancient Pb mining and usage, archaeological remains from certain regions show isotopic evidence for significant exposure to anthropogenic Pb, which also provides information about cultural and economic practices of ancient societies. Therefore, Pb isotopes in tooth enamel samples, free of diagenetic alteration, can provide important archaeological information that is not readily accessible from other isotope systems.