Bioavailable Strontium, Human Paleogeography, and Migrations in the Southern Andes: A Machine Learning and GIS Approach

BARBERENA, RAMIRO; CARDILLO, MARCELO; LUCERO, GUSTAVO; LE ROUX, PETRUS J.; TESSONE, AUGUSTO; LLANO, CARINA; GASCO, ALEJANDRA; MARSH, ERIK J.; NUEVO-DELAUNAY, AMALIA; NOVELLINO, PAULA; FRIGOLÉ, CECILIA; WINOCUR, DIEGO; BENÍTEZ, ANAHÍ; CORNEJO, LUIS; FALABELLA, FERNANDA; SANHUEZA, LORENA; SANTANA SAGREDO, FRANCISCA; TRONCOSO, ANDRÉS; CORTEGOSO, VALERIA; DURÁN, VÍCTOR A.; MÉNDEZ, CÉSAR

Frontiers in Ecology and Evolution

Frontiers

Año: 2021 vol. 9

Strontium isotopes (87Sr/86Sr) in human teeth and bone remains average the biologically available strontium in the landscape(s) inhabited in the past (Price et al., 2002; Bentley, 2006), providing a proxy of the geographic place of origin of an individual and the spatial scale of the territories occupied (Price et al., 2000; Montgomery, 2010; Knudson et al., 2014a; Laffoon et al., 2017). Since first applied in archeology (Ericson, 1985), strontium isotopes (87Sr/86Sr) have widened the scope of archeological enquiry across analytical scales, from small-scale aspects of human life-history and identity construction (Kutschera and Müller, 2003; Montgomery, 2010; Andrushko et al., 2011; Knudson et al., 2012; Torres-Rouff and Knudson, 2017) to large-scale group migration processes (Borić and Price, 2013; Yu et al., 2020). In addition, convergent interdisciplinary efforts from geology, ecology, paleoecology, forensics, and archeology have resulted in increasingly more sophisticated approaches to assess the mobility of the organisms under study and, building on this, to identify non-local residence and migration on different spatial and social scales (Price et al., 2002; Bentley, 2006; Montgomery et al., 2007; Bataille et al., 2014, 2020; Kootker et al., 2016; Tipple et al., 2018; Willmes et al., 2018; Scaffidi and Knudson, 2020; Snoeck et al., 2020). Thus, resulting from the combination of growing frames of scientific enquiry, robust methodological approaches, and extensive databases, we consider it is fair to describe the present time as a ?golden age? of strontium isotopes to geographically source materials across space and time. However, this advance is highly heterogeneous in the different continents (Bataille et al., 2020). In South America, strontium isotopic research has been largely focused in the Central and South-Central Andes encompassing Peru, western Bolivia and northern Chile (Knudson et al., 2014a,b; Dantas and Knudson, 2016; Chala-Aldana et al., 2018; Mader et al., 2018; Slovak et al., 2018; Standen et al., 2018; Takigami et al., 2019; Santana-Sagredo et al., 2021). We have recently started a project with the goal of extending the bioavailable strontium framework to the southern Andes in Central Argentina and Chile (Barberena et al., 2017, 2019, 2020; Durán V. et al., 2018). From a comparative perspective, this project allows incorporating Andean regions with different archeological trajectories of socio-economic change, complexity, and possibly diverse incidence of migrations compared to the more systematically studied Central Andes.The general objective of this project is to develop a multidisciplinary approach to human scale of mobility, role of migration, and trans-Andean interaction since the initial human peopling in the Pleistocene-Holocene transition (García, 2003), with a focus on the period witnessing the evolution of agropastoral economies in the last 2,000 years (Gil et al., 2014; Falabella et al., 2016; Llano et al., 2017; Durán V. et al., 2018; Gasco, 2018). In this agenda, the specific goal of this paper is to present the strontium isotope (87Sr/86Sr) results for plant and rodent samples from a trans-Andean transect encompassing from the Pacific Ocean in Central Chile to the eastern lowlands in Argentina (31°?34°S) (Figure 1). The ?Southern Andean Strontium Transect? (SAST, from now onward) samples the main geological provinces across the Andes that span the last ca. 450 my, between the lower Paleozoic and the late Quaternary. From a methodological perspective, we apply an innovative and robust approach based on Random Forest regression designed to assess which biophysical variables predict the observed strontium isotope variation with the highest resolution (Bataille et al., 2014, 2018, 2020; see also Serna et al., 2020). Our results demonstrate that the southern Andes provide an ideal geological setting for the development of this approach, since there is an impressive geological variation in rock age and composition that produces distinctive isotopic signatures for each biogeographical region of archeological relevance. Finally, we apply this geostatistical framework to a recently published archeological case-study from the Uspallata montane Valley in northwestern Mendoza, Argentina (Figure 1). The specific archeological goal consists in assessing the spatial scale of mobility of human remains buried in five archeological cemeteries spanning the key period between the consolidation of agropastoral economies and the regional conquest by the Inka Empire (AD 800?1400) (Barberena et al., 2020). Building on this, we assess the incidence of migration in the conformation of these human groups through time.