Description and geochemical composition of tephra layers from ODP Hole 186-1150A and Site 186-1151

The sediments collected at Sites 1150 and 1151 during Leg 186 included many tephra layers and volcaniclastic detritus. In order to identify these tephras, the major oxide compositions of individual glass shards were determined by electron probe microanalyzer. The uppermost four tephras in sediments from Hole 1150A are correlated with the Towada-Hachinohe tephra (To-H; Tohoku district), Shikotsu Daiichi (1st) tephra (Spfa-1; Hokkaido district), Narugo-Yanagisawa tephra (Nr-Y; Tohoku district), and Aso-4 tephra (Kyushu district), respectively. The uppermost tephra in Hole 1151C is correlated with To-H tephra. To-H, Spfa-1, and Aso-4 tephras are also present in piston core KH94-3, LM-8, collected between Sites 1150 and 1151. Eruptive ages of To-H and Spfa-1 estimated from the oxygen isotopic Stages of core KH94-3, LM-8 are between 14.9-15.3 and 39.5-40.1 ka.

Distribution, depositional form and geochemical composition of tephra horizons in ODP Sites 186-1150 and 186-1151

Major element geochemical composition was established for 59 tephra horizons from Ocean Drilling Program Sites 1150 and 1151, located in the Japan forearc. These data, encompassing typically between 15 and 30 individual shard analyses per tephra horizon, were used to investigate the degree to which sediment reworking, postdepositional geochemical alteration, and geochemical uniqueness of individual eruptives facilitate or impede the potential for establishing a tephrostratigraphical framework for the Japan Trench, as well as usage of the tephra record to document arc evolution. Evidence was found that hydration (termed phase 1 alteration) of glass shards increases with age in the Pliocene-Pleistocene, but there is no indication that element leaching (phase 2 alteration) has occurred. Post- or syn-depositional differences in preservational style are shown to have no significant bearing on tephrogeochemical homogeneity and suitability for tephrostratigraphical analysis. Overall, therefore, the volcaniclastic record is suitable for investigating medium- to long-term changes in arc geochemistry and, provided consideration is given to the potential for nonunique geochemical signatures, is suitable for erecting tephrochronological frameworks. A limited number of Pleistocene tephra correlations are suggested in furtherance of this framework goal.