A Possibility Of OSL-Dating As An Indictor Of Transportation History Of Turbidites

Turbidite is induced by various geologic events such as earthquake, flood, tsunami, and so on. Especially, seismo-turbidite (e.g. Inouchi et al., 1996) and flood induced turbidite (hyperpycnite: e.g. Mulder et al., 2003) occupy majority of reported turbidites, and are often used as indicators of earthquake and catastrophic river flow, respectively. These researchers revealed such turbidite origins correlating age of turbidites with historical records or taking account of topographic setting, and then attempted to discover inherent descriptive features of seismo-turbidite/hyperpycnite. However, we cannot distinguish one from another at a glance, now. It should be necessary to seek not only usual descriptive clues but also another independent clue for distinguishing seismo-turbidite and hyperpycnite. We attempted to utilize optically stimulated luminescence (OSL) dating method as a new distinguishing method to specify turbidite origins. An OSL signal intensity implies the amount of natural dose accumulated in mineral grains. One can estimate an OSL age dividing the natural accumulated dose by the annual dose. It is important that OSL signal is canceled by sunlight stimulation (bleaching). So, the OSL age generally implies the burial age of sediment after transportation. Hyperpycnal flow should flow directly from river mouth to deep marine environment. Whereas, collapse-induced turbidite, its majority is considered as seismo-turbidite, is formed as a result of collapse of sediments at shallower marine environments. On the assumption that mineral grains are bleached during discharge from land to marine environments through river with a certain probability, some grains of a hyperpycnite should show OSL ages as much as the hyperpycnite depositional age estimated from radiocarbon age of planktonic foraminifers in hemipelagite. On the other hand, OSL age of seismo-turbidite grains must be obviously older than depositional age of the turbidite. Preliminary OSL experiments using KT98-18-P1 and -P2 cores sampled from the off Tokai, central Japan are carried out. Potassium feldspar (K-F) grains with 0.25-0.18 mm in diameter are extracted from five turbidite layers for OSL measurement and the single grain dose distribution of each layer is obtained. Dose distributions of K-F grains sampled from the present flood-induced fluvial deposits are also measured as references. We show detail of the methods and results of the preliminary OSL dating.