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Sourcing prehistoric obsidian artifacts in the Islands of Four Mountains, Alaska
Sam H. Sheffer
May 11, 2016
Unlike lavas and jaspers, obsidian possesses more homogeneous compositions amenable to analysis by portable X-Ray Fluorescence (pXRF). Fractured obsidian produces thin, sharp edges ideal for knives and projectiles, though prehistoric Aleutians sites also include tools made of conchoidally fracturing, aphyric lavas, of jasper, and of 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. Because it offers rapid non-destructive analysis, pXRF is used to characterize and compare the chemical signatures of possible volcanic source materials available for tool-making and the tools themselves. A comparison of the compositions of 121 artifact fragments reveals similarities and differences for 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 these islands are currently uninhabited, the Unangaˆx, also known as Aleuts, lived here for millennia. Dates of occupation measured with ¹⁴C dating by the IFM research team span as far back as 3632-3724 calibrated years BP (100% probability, 2σ). In 2014 and 2015, obsidian artifacts especially tiny flakes or 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. This investigation uses calibrated chemical element data to identify a unique signature for most IFM obsidian artifacts. Three replicate analyses of each sample and interspersed analytical standards produced accurate and precise measurements of elements Zr, Zn, Ca, Pb, K, and Nb. Use of the obsidian standards SPHM-1 and GBO plus powdered GSP-2, also provided calibration curves to correct the accuracy for precise elements Sr, Rb, Th, Fe, Mn, Ti, Ba, Al, and Si. Trace element ratios show most IFM obsidian artifacts cluster at consistent values such as Zr/Nb (24-26 ppm), Fe/Mn (27-32 ppm), and Ba/Rb (12-20 ppm) shows that prehistoric IFM occupants primarily utilized one obsidian source. The reliance upon a single lithic source suggests information sharing and cultural interaction among different IFM villages and islands. Zr/Nb, Fe/Mn and Ba/Rb clusters are distinct from previously characterized obsidian sources on Okmok and Akutan. 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.