Examining surface water δ18O and δ2H values in the western Central Andes: A watershed moment for anthropological mobility studies

Oxygen isotopes are commonly applied to study archaeological human and animal mobility among the vertical ecological zones of the Central Andes in South America. Such research assumes that oxygen and hydrogen isotopes in meteoric waters demonstrate an inverse relationship with elevation. However, because the primary source of precipitation in the Central Andes is the Atlantic Ocean, this expectation is likely complicated by surface-level processes on the western Andean slope. We evaluate the spatial patterning of stable isotope values in surface waters along a coast-highland transect in southern Peru (∼15-17°S). Surface water δ18O and δ2H values in the study area are consistent with regional and global meteoric waters. However, lowland and highland surface waters demonstrate wide variability and overlapping ranges of surface water isotope values. Therefore, it is challenging to discern the origin of surface waters based on elevation alone. Rather, surface water δ18O and δ2H values appear to reflect hydrologic processes including seasonality, stream order, catchment size, and distance from the source. We identify the “Watershed Effect,” which precludes the use of δ18O and δ2H in Andean bioarchaeological studies of inter-zonal mobility. Moreover, changing hydroclimate over the Holocene and present sampling precision for biological analytes confound existing interpretations of δ18O derived from archaeological bioapatite. Given the regional complexities of δ18Owater presented here, isotopic assessments of human paleomobility require better baseline data than those currently available for the Central Andes. We contend that previous archaeological datasets using δ18Obioapatite to assess mobility between high and low elevations should be re-evaluated. Further, future studies should provide adequate baseline data to justify archaeological analyses and support subsequent interpretations.