Biogeochemical reconstructions of life histories as a method to assess regional interactions: Stable oxygen and radiogenic strontium isotopes and Late Intermediate Period mobility on the Central Peruvian Coast

Biogeochemical reconstructions of life histories of mobility offer a means to obtain nuanced information about regional interactions in the past. We test this method using the Late Intermediate Period Ychsma society on the central Peruvian coast as a case study. Archaeological and ethnohistoric evidence indicates that the Rimac and Lurín Valleys inhabited by the Ychsma served as a key regional hub for the religious and administrative activities of the Inca Empire and the Spanish Viceroyalty. The nature of regional interactions prior to Inca imperial influence, however, remains unclear. Well-known historical narratives describe populations from the adjacent Huarochirí highlands defeating coastal Ychsma populations for agricultural land, but archaeological evidence concerning the timing and extent of coastal-highland interactions is debated. Here, we assess the potential for radiogenic strontium and stable oxygen isotopic reconstruction of mobility over the life course to shed light on the regional interactions of coastal Ychsma groups during the Late Intermediate Period. We present ⁸⁷Sr/⁸⁶Sr and δ¹⁸O results for 61 regional environmental baseline samples and archaeological human tooth enamel and bone samples from 64 individuals buried at Armatambo and Rinconada Alta, in the Rimac Valley. Results reveal a broad isotopic range for baseline and archaeological samples likely due to diversity in water and bedrock sources. Nevertheless, one individual presents a non-local strontium value indicating mobility to an outside region. We discuss the advantages of a life history approach, the complications of using isotopes to assess mobility in the Central Andes, and suggested directions for future research.