Strontium isotope analysis
Description
Strontium isotope analysis determines the proportions of the stable strontium isotopes 87Sr to 86Sr.
Most applications at CEZA are part of studies in bioarchaeometry and use human or animal teeth to investigate questions regarding mobility. In this respect, strontium isotope analysis is today a standard procedure and often used in combination with oxygen isotope analysis.
The foundation of the method is strontium as a trace element in rocks, which occurs in different isotope compositions depending on the petrological properties and age of geological units.
Application
Strontium isotope analysis has a broad field of applications. 87Sr/86Sr isotope ratios are determined during rubidium strontium dating of rocks, used in hydrological studies, or to verify the origin of food.
In bioarchaeometry, strontium isotope ratios help in identifying people who did not spend their first years of life at the place where their skeletal remains were found. This knowledge provides the basis for addressing further questions regarding the residential changes of single persons or of larger groups of people. Such aspects include the identification of historical personalities, investigations of residential rules, or evalutations of coherences between a non-local origin of an individual, or foreign grave goods or grave architecture.
The investigation of animal husbandry strategies is a further field of application, as 87Sr/86Sr ratios in the enamel of animal teeth may indicate the use of pastures with different geological conditions. For high-crowned teeth (e.g. of cattle, sheep, goats or horses) the combination with oxygen isotope analysis is recommended. In such cases, samples are taken in sequences of horizontally alligned grooves that are distributed equally along the crown heights. The minima and maxima of the seasonality curves established by oxygen isotope analysis represent enamel that formed during summer or winter, respectively. Different strontium isotope ratios of these samples reflect seasonally differentiated pastures and thus indicate mobile animal husbandry strategies, e.g. transhumance.
Basics
The foundation for strontium isotope analysis is the trace element strontium in rocks, which has four stable isotopes (84Sr, 86Sr, 87Sr and 88Sr). Because the radiogenic isotope 87Sr is produced by radioactive decay of 87Rb (rubidium), its share in total strontium – expressed as the 87Sr/86Sr ratio – varies depending on the original content of rubidium and the age of the rock.
Due to weathering, strontium becomes biologically available and is passed on to animals and humans via the food chain. It replaces calcium in the hydroxyapatite of teeth and bones. After its mineralization during the first years of life, enamel remains unchanged and, as the hardest tissue of the human and animal body, it remains very stable even during burial in the ground. Therefore, even thousands of years after the death of a person, the remaining enamel reflects the isotope composition of the food consumed during childhood.
The identification of non-native individuals requires archaeological or modern comparative samples that reflect the closer surroundings of the site. Moreover, the data distribution within larger sample sets from a burial site may reveal outliers that represent non-local individuals or those with similar values who may correspond to groups of people with a shared origin.
Modern analytical techniques allow a highly precise and exact determination of the isotope composition of the samples. Regular and systematic analyses of control standards allow an assessment of the correctness and precision of the results. Quality standards materials have certified or long-term averaged isotope compositions, which are analysed in the same way as the samples. For an optimal traceability of the analytical quality, the data transfer includes the analytical values of the standardized materials.
Limitations
Data of samples from analysis runs that fulfill the quality criteria need to be interpreted with regard to the archaeological question posed.
In order to assess whether the samples represent local or non-local individuals or material, the data are compared to local isotope ranges of the respective sites. Limitations arise from the quality of the local isotope data ranges, as well as from diagenetic alteration during burial, which particularly affects bones. Local isotope baseline ranges are, for example, based on strontium isotope ratios of archaeological animal teeth of species with a potentially limited catchment area from the same site or from as close as possible, or on modern vegetation or water samples. For samples from archaeological contexts, an influence of strontium from modern land use is unlikely. Domestic animals, however, depend on husbandry strategies and may, in cases, not be representative for the respective site. In contrast, while modern samples can be collected from specifically selected locations, their isotope composition may diverge from ancient values due to modern anthropogenic overprint. In general, the representativeness of local isotope ranges increases with the number and variety of comparative samples that contribute to their estimation.
One of the greatest challenges of interpreting Sr isotope data is the determination of possible areas of origin of individuals that have been recognized as non-local. The isotope composition of the biologically available elements can vary considerably within a small area, while the questions posed to the investigations often aim to distinguish larger regions.
On the other hand, due to similar geological conditions, it is sometimes impossible to differentiate Sr isotope ratios sufficiently despite considerable distances between regions. Due to these redundancies, both non-local individuals may remain unrecognised, and there may be several possible areas of origin of samples with an isotope ratio outside the local isotope range.
The interpretation of Sr isotope data benefits from proposing precise research questions and hypotheses as well as from their integration into interdisciplinary studies that consider different strands of data.
Sample properties
Tooth enamel is the prefered sample material for strontium isotope analysis in bioarchaeometry. The optimum sample size for pre-treatment and analysis is approx. 12 mg of enamel or bone powder.
We recommend submitting complete teeth along with information on their anatomical position in the jaw. The required amount of sample is separated and remaining material may be returned upon request
Human teeth
In the case of sample series, e.g. from a cemetery, teeth should be selected as uniformly as possible. For the analysis itself, the type of tooth is irrelevant if the enamel is generally well preserved. However, depending on the research question, the period of enamel mineralization during childhood may be of relevance.
In the case of a combination with oxygen isotope data and sampling of one tooth per individual, we recommend selecting enamel that is mineralised after the end of the breastfeeding period, approx. from the age of 3 years. Suitable teeth include second molars, premolars, or wisdom teeth (3rd molars). Teeth that form earlier in childhood (1st molars, front teeth, or decidous teeth) can have slightly higher δ18O values. Breast milk is enriched in the heavy isotope 18O compared to drinking water and can lead to misinterpretations regarding a possible non-local origin of the respective individual. Early forming teeth should therefore only be selected for combined analyses if later forming teeth are not available.
If changes of residence during childhood are of interest, we recommend sampling enamel of both early and later forming teeth (e.g. 1st molar and wisdom tooth) of the same individual. Similar isotope ratios would indicate growing up in the same location, whereas significant differences may indicate changes in the origin of the food and thus a possible change of residence within the period covered by the analyses.
Animal teeth
Animal teeth should be submitted as complete as possible, along with information on the animal species and anatomical position of the tooth.
A piece of enamel suitable for the analyses will be separated from low-crowned teeth.
For serial analyses, enamel powder is milled out in horizontally alligned grooves along the crown of the tooth at intervals of a few millimetres using a dental drill. Such samples allow tracing changes in the isotope composition across the crown height, which indicate habitat changes during the period of enamel mineralization. Serial sampling can be combined with oxygen isotope analysis. Oxygen isotope data allows for the detection of enamel that formed in different seasons and enables the selection of Sr isotope samples that represent specific times within a year.
For sheep, goat or cattle molars, two Sr isotope samples may be taken at a distance of approx. 2 cm from each other. Remarkable differences between their Sr isotope ratios indicate differences in the grazing habitats during opposite seasons. The addition of oxygen isotope data for a series of samples from the same tooth helps in identifying the seasons represented in the Sr isotope samples and of the use of different pastures.
In general
Tooth and bone samples must be sent individually packaged along with information on species and anatomical position.