The significance of obsidian hydration dating in assessing the integrity of Holocene midden, Hokkaido, northern Japan

Abstract

Since its novel invention in 1960, obsidian hydration dating is now recognized as the chronometric method to give dates of archaeological sites, based on measurements of hydration rim thickness. Contrary to the increased awareness of various factors that affect hydration rates and reliability in measurements, the question whether and the extent to which the validity in the rim thickness measurements corresponds to the interested event has been less discussed. Here, validity of obsidian hydration dates that designate event of human site occupations is discussed in terms of site integrity of the Early Jomon midden in the Holocene site of Ocharasenai (OCH), Hokkaido, northern Japan. Expected integrity of the midden evidently left by prehistoric hunter–gatherers is tested through a comparison of microscopically-measured average rim thicknesses between the primarily extracted specimens and those chosen after assessing the validity. No statistical difference in estimated hydration dates between these two groups supports high integrity of the midden. The estimated date for the Early Jomon occupation at the OCH is 4388–4942 BP, fallen into the end of Middle Holocene. Results of the present examination indicate that obsidian hydration dating will aid to understand site formation processes.