Strontium isotope identification of possible rural immigrants in 17th century mass graves at St. Gertrude Church cemetery in Riga, Latvia

The aims of this study were to explore the origins of 19 children buried in two mass graves and the general cemetery at the post-medieval St Gertrude Church cemetery in Riga, Latvia, using strontium isotope analysis (⁸⁷Sr/⁸⁶Sr), and to establish local soil Sr biosphere ranges from faunal samples from two areas of Latvia. The results confirmed the presence of one clear outlier in the population and one child who may have had originated from the region of Vidzeme. The lack of significant differences in ⁸⁷Sr/⁸⁶Sr between the other individuals analysed suggested that they were representative of one population. A strong correlation between ⁸⁷Sr/⁸⁶Sr ratios and previously obtained mean δ¹³C and δ¹⁵N values in children from mass graves suggested possible short-term dietary changes with increased proportion of marine resources and food sources subject to sea spray. The study has yielded the first comparative ⁸⁷Sr/⁸⁶Sr data from archaeological skeletons in Latvia, which will be essential for future research addressing comparative mobility studies in the region.