SOURCING PREHISTORIC OBSIDIAN TOOLS IN THE ISLANDS OF FOUR MOUNTAINS, ALASKA

Obsidian possesses homogeneous compositions amenable to analysis by portable X-Ray Fluorescence (pXRF). Obsidian distribution within prehistoric Aleutian sites indicates its value although other commonly found tools are made of aphyric lavas and jasper/opal. In the Islands of the Four Mountains (IFM), obsidian artifacts are found within cultural layers rich in datable organic matter and interbedded with distinctive tephra deposits. A comparison of the compositions of 121 artifact fragments reveals lithic resource use among six sites on three of the Islands of Four Mountains. The isolated, windswept islands of Chuginadak, Carlisle, and Herbert lie in the transition from the eastern to the central Aleutian arc. Although currently uninhabited, ¹⁴C dates of Unangax (Aleut) occupation by the IFM research team span as far back as circa 4000 years BP. In 2014 and 2015, obsidian artifacts especially debitage (debris from making stone tools) were recovered from three prehistoric sites on Chuginadak Island and two prehistoric sites on Carlisle Island. In Unit 4, within a Carlisle Island site, the stratigraphy revealed many tools or tool fragments within multiple occupation horizons. Replicate analyses of each sample and interspersed analytical standards produced accurate and precise measurements of multiple elements. Trace element ratios, such as Zr/Nb, Fe/Mn, and Ba/Rb, show most IFM obsidian artifacts cluster at consistent values and show that prehistoric IFM occupants primarily utilized one obsidian source. The discovery of a single lithic source suggests information sharing and cultural interaction among different IFM villages and islands. However, one deeply buried artifact within a site on Carlisle Island matches Okmok obsidian elemental signature. Additionally, artifacts collected from Carlisle Island Unit 4 show that the same obsidian type was used consistently among stratigraphically-related different cultural horizons, i.e., through time.