PROVENANCE OF OBSIDIAN FRAGMENTS RECOVERED FROM ADAK ISLAND, CENTRAL ALEUTIAN ISLANDS: EVIDENCE FOR LONG-DISTANCE TRANSPORT

In the central Aleutian Islands, archaeological excavation has recovered small obsidian flakes from two prehistoric sites on Adak Island, Zeto Point and nearby Clam Lagoon, yet obsidian does not outcrop on the island. Two possible Aleutian obsidian sources are quite distant east of Adak. A debris flow deposit with obsidian clasts outcrops on Akutan Island, ~ 800 km east. A second obsidian deposit from Okmok volcano, Umnak Island is slightly closer to Adak (650 km east). Inductively Coupled Plasma Mass Spectrometry obtained abundances of 38 elements from four obsidian flakes and a dacitic fragment from the Adak sites, and five comparative samples from the Akutan obsidian exposure. The standard deviation of the compositions (35 of 38 trace elements) for the excavated Adak obsidian flakes is less than the error in the analytical accuracy for these elements; essentially the compositions of the analyzed obsidian flakes are the same though they were excavated from two different sites and from different occupation layers. Furthermore, the compositional match between the Adak obsidian flakes and the Okmok obsidian source is very compelling with only Ba abundances differing significantly among the samples (Ba=1160-1217 ppm) and the possible source (Ba=1545 ppm). Principal Component Analysis (PCA) supports the hypothesis that the Adak obsidian came originally from Okmok volcano. The difference in Ba is possibly explained 1) inhomogeneity in the obsidian, 2) leaching of Ba from the obsidian flakes within the archaeological sites, 3) an unknown source of obsidian was used for the Adak lithic source. Akutan Island obsidian is not as close a compositional match as the Okmok source. Based on the PCA, the closest known potential source for the dacitic fragment is a lava flow from Atka Island, located ~160 km east of Adak. These results indicate that Unangan peoples transported high quality, lithic materials large distances whether through trade, direct transport or raiding.