Splitting obsidian: Assessing a multiproxy approach for sourcing obsidian artifacts in British Columbia

Abstract

Archaeology and geology inherently intersect when attempting to determine the geographic origin of lithic materials manufactured into artifacts by ancient people. Obsidian, or volcanic glass, is frequently found in archaeological sites, and researchers commonly use the trace element characteristics of obsidian artifacts to identify their source. However, the trace element concentrations of obsidian flows are not necessarily unique, potentially leading to inaccurate conclusions when used in isolation, especially if all possible sources have not been identified and systematically characterized. In this pilot study, we assess variations among the trace element, structural, and isotopic characteristics of obsidian sources in British Columbia and describe a multiproxy approach for provenancing obsidian artifacts. Our approach combines the survey of an artifact assemblage using non-destructive techniques with further analysis of a pre-selected subset of representative artifacts using minimally-invasive analytical methods. The initial non-destructive survey step includes determination of trace element concentrations by portable X-ray fluorescence and structural characteristics by Raman spectroscopy. The minimally-invasive step includes the measurement of lead isotope ratios and more precise trace element concentrations with split stream-laser ablation-inductively coupled plasma-mass spectrometry (SS-LA-ICP-MS) on only a select few representative artifacts. We tested this approach on 14 obsidian artifacts from an archaeological site in British Columbia. Our results provide multiple lines of evidence which indicate that two of the analysed artifacts likely originated from Mount Edziza, ~360 km north of the site, and all others likely originated from Anahim Peak, ~275 km south of the site. The combined use of multiple proxies significantly enhances confidence during sourcing studies and highlights the strength of multidisciplinary and multivariate geochemical approaches to archaeological challenges.