Spatially resolved LA-MC-ICPMS strontium isotope microanalysis of archaeological fauna

The analysis of the strontium isotope composition of archaeological materials can provide important information about the mobility of a range of mammals, including humans. The basis of this method is that, prior to any postburial diagenesis, the Sr87/Sr86 ratio of bone and teeth reflects the geological environment from which food and water were sourced while these biominerals were forming. Teeth are particularly amenable to tracing the geographic origins of humans as they mineralize during the first 12–13 years of life (White and Folkens, 2005) and do not subsequently change strontium composition after this time (Schweissing and Grupe, 2003). Strontium isotope analysis can be used to determine if individuals are local or nonlocal by comparison to the isotopic composition in and around their burial location (i.e., Schweissing and Grupe, 2000; Bentley et al., 2007; Conlee et al., 2009). In order to quantify the extent of faunal mobility, the strontium isotope composition of biominerals from fossil samples needs to be compared with a regional map of values obtained either from local faunal material (Price et al., 2002) or from analysis of the bioavailable component of strontium from plants, regolith, or bedrock (Capo et al., 1998).Strontium isotope analysis has been extensively applied to the determination of archaeological mobility, as reviewed by Price et al. (2002), Budd et al. (2004), Bentley (2006), and Montgomery (2010). Although studies of Paleolithic or older archeological material are rare, some have been undertaken on hominins (Sillen et al., 1995, 1998; Richards et al., 2008; Copeland et al., 2011) and faunal material (Horn et al., 1994; Copeland et al., 2010; Britton et al., 2011).Enamel has been shown to be the most favorable material for strontium isotope analysis based on its ability to resist postburial alteration far better than dentine, cement, or bone (Trickett et al., 2003). Enamel of some species grows over extended periods and, when analyzed with microprofiling or laser ablation analysis, can be used to reconstruct comprehensive life histories. This approach demonstrated the seasonal mobility of cattle in Iron Age Britain (Horstwood et al., 2008), Neolithic Germany (Bentley and Knipper, 2005), and England (Viner et al., 2010).

Source of strontium in archaeological mobility studies—marine diet contribution to the isotopic composition

The strontium isotope composition of human tissues is widely used in archaeological mobility studies. However, little attention is paid to the relative contributions of terrestrial versus marine sources of strontium in these studies. There is some debate over the role of a solid diet versus drinking water as the most important source of strontium for the human body, with related possibilities of misinterpretation of the archaeological record if only strontium isotope compositions of the biosphere are studied. However, there is a third component, marine strontium, which is commonly not assumed to contribute towards the strontium isotope composition of archaeological skeletal remains, especially in locations that are not directly coastal. To illustrate the potentially obfuscating effects of mixed Sr sources in a human population, we present a case study of twelve individuals from the medieval Finnish site Iin Hamina with a known dietary history. Our study shows that marine consumption is a significant factor explaining the strontium isotope composition of the Iin Hamina human remains, with implication of erroneous conclusions about immigration without prior knowledge of diet composition. Thus, future studies should always incorporate a rigorous analysis of dietary history, with special regard to potential consumption of aquatic resources, when strontium isotope analysis is used as a method in the study of palaeomobility.