Human skeletons provide unique insights into the past. Besides studying structures and DNA, archaeologists use isotope signatures in teeth and bones to reveal information of ancient people. Strontium isotopes are especially powerful. Why? Strontium isotope signatures in bones and teeth reflect the local geology and vegetation. The amounts of 87Sr and 86Sr isotopes in rocks vary according to the local geology. Different minerals carry different Sr isotope signatures and rocks of different ages have unique 87Sr/86Sr isotope compositions.
In very general terms, one can say that older rocks have accumulated more 87Sr than 86Sr. Rocks are prone to weathering, thereby releasing strontium and other elements into the environment. In this way, strontium gets incorporated into the surrounding soil and groundwater, which in turn is taken up by plants. The plants then carry the isotope signature of the initial rocks. When humans consume plants and groundwater, they will absorb strontium and create a chemical signature of the local geology within their bodies (i.e. in teeth and bones). By measuring the 87Sr/86Sr ratio within these parts of the skeleton, archaeologists can find out many things about an individual, such as what they ate and how the environment was where they grew up in.
As tooth enamel and bones are formed at different stages of a person’s life, their strontium isotope signatures give insights into migration patterns. Whereas tooth enamel forms in a child’s first few years and remains unchanged during the rest of their life, strontium in bones gradually changes over a seven- to ten-year period. Hence, the 87Sr/86Sr isotope ratio of tooth enamel matches the geology in the area where a person spent his or her childhood, and strontium in bones reveals the region where people spend the last decade of their lives. By matching the 87Sr/86Sr isotope ratios in teeth and bones to those in specific geographic regions (with a unique geology), an archaeologist is able to tell whether a person migrated between childhood and death, and sometimes can even identify where the person was born.
There are numerous archeologic studies applying strontium isotope analysis of human remains from various epochs during the last 2,000 years of human civilization. I want to highlight a few:
Hughes et al. (2018) have applied strontium isotope analysis to study residential mobility of 19 individuals from an early Anglo-Saxon cemetery on the southern English coast (5th – 7th century AD), in order to shed some light on the Adventus Saxonum event. This describes the transition from Roman Britain to early Anglo-Saxon England and has traditionally been associated with a large-scale invasion by Germanic peoples. The study showed that this hypothesis does not hold true. Most of the individuals seem to have local roots. Some of them are likely continental immigrants and immigrated from nearby communities. Only a few could have originated from Western Europe or other parts of England. This pattern, together with the fact that most graves showed a lack of wealth, is not consistent with simple models of mass invasion of elite takeover. Would you like to read more? Here is the link to the publication:
Hughes et al. (2018) Isotopic analysis of burials from the early Anglo-Saxon cemetery at Eastbourne, Sussex, U.K. Journal of Archaeological Science: Reports V19, pp513–525, https://doi.org/10.1016/j.jasrep.2018.03.004
A study by Wong et al. (2018) focused on the mobility of Roman (1st – 7th century AD) and Byzantine (9th–13th century AD) individuals buried at the UNESCO World heritage site of Hierapolis, Turkey. The majority of the population buried at this site have local strontium isotope values, suggesting local origin. Some individuals however showed strontium isotope values falling outside the local range. Especially noteworthy were some male outliers suggesting increased migration or pilgrimage into Hierapolis during the Byzantine period. More details can be found in the article:
Wong et al. 2018 Pursuing pilgrims: Isotopic investigations of Roman and Byzantine mobility at Hierapolis, Turkey. Journal of Archaeological Science Reports, V17, pp520–528, https://doi.org/10.1016/j.jasrep.2017.12.005
Since strontium isotope variations in nature are small, high precise mass spectrometry techniques are mandatory in order to determine variability. The above studies carried out strontium isotope analysis using the Thermo Scientific™ Neptune Plus™ Multicollector ICP-MS and Thermo Scientific™ Triton Plus™ Thermal Ionization Mass Spectrometer, both enabling high precise bulk and in-situ analysis of Sr isotopes.
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