Comprehensive mapping and compositional analysis of the Alca obsidian source, Peru

Abstract

The Alca obsidian source in southern Peru is one of the largest and most geochemically complex sources of volcanic glass in South America. Hunter-gatherers first discovered and used Alca obsidian for stone tools at the end of the Pleistocene. Alca later became one of the three most economically important and widely distributed sources of obsidian in the Central Andean region. Systematic mapping and geochemical characterization efforts spanning 20+ years have revealed an extensive high-elevation source region composed of six geographically and compositionally distinct sub-sources. Here we synthesize research documenting the 2000 km2 spatial extent of the Alca obsidian source, and we present expanded geochemical datasets for six Alca sub-sources (n = 238 geologic samples) obtained using neutron activation analysis (NAA), laboratory x-ray fluorescence (XRF), and portable (p)XRF. Results for Alca and for six other obsidian sources in the Peruvian Andes illustrate the efficacy of these techniques to discriminate Peruvian obsidian sources, including Alca sub-sources. Comprehensive compositional data from the Alca source area, examined against accumulating obsidian artifact datasets from throughout Peru, reveal past human use of various Alca sub-sources. These cases contribute fine-grained behavioral information, made possible by a complex obsidian source with geographically patterned geochemical variation and a >12,000-year sequence of human interaction with this geologic resource.