Spatial variation in bioavailable strontium isotope ratios (⁸⁷Sr/⁸⁶Sr) in Kenya and northern Tanzania: Implications for ecology, paleoanthropology, and archaeology

Eastern Africa is a key region for studying archaeological, palaeontological, and ecological movements. This region hosts critical developments in hominin and human evolution, the dispersal of food-producing populations across the continent, and some of the largest known contemporary mammalian migrations on the planet. Strontium isotope analysis of biominerals such as tooth enamel, eggshell, and other tissues in modern animals have been used to reconstruct migration, residential mobility, and provenience. The diverse geologies of Kenya and Tanzania, ranging from the Archaean Basement System rocks of the Tanzania Craton to the recent volcanics of rift valleys, make it a highly promising area for mobility and provenience studies using strontium isotope ratios (⁸⁷Sr/⁸⁶Sr). Nevertheless, the application of strontium isotope analysis to reconstruct migration and individual mobility has been limited in the region due to the lack of a map predicting biologically available (bioavailable) ⁸⁷Sr/⁸⁶Sr. We present bioavailable ⁸⁷Sr/⁸⁶Sr data from a variety of modern and archaeological materials throughout Kenya and northern Tanzania. We show that ⁸⁷Sr/⁸⁶Sr of living organisms in the study area range from as low as 0.70439 for samples collected from Neogene volcanics to 0.72796 for samples collected from Precambrian Basement System rocks. We also present an ⁸⁷Sr/⁸⁶Sr map (isoscape) of Kenya and Tanzania developed using a machine-learning framework and a compilation of bioavailable ⁸⁷Sr/⁸⁶Sr data from Africa. This map provides the first predictions of bioavailable ⁸⁷Sr/⁸⁶Sr for East Africa, and represents a crucial resource for future work on ancient and modern animal and human mobility.