Holocene tephra deposits in the northern Okinawa Trough

摘要

The mineralogical and geochemical characteristics of sediments of Core CSH1, which was collected from the northern Okinawa Trough, indicate that large amounts of volcanic materials have deposited in the northern Okinawa Trough during the Holocene. On the basis of down-core variations in mineral and element contents of sediments, two layers in the uppermost section of Core CSH1 charac- terized by high quartz, Na2O, MnO, K2O, uranium contents and low contents of clay minerals, volatiles, Fe2O3, MgO, CaO and stron- tium, have been identified as the tephra deposits. Systematic grain-size measurements also suggest that sediments from the northern Okinawa Trough are made up of terrigenous materials and volcanic ashes with different proportion during the Holocene. The sedi- ments of tephra layers in Core CSH1 show bi-modal patterns in grain-size distribution with modal grain-sizes of 74.3 and 7.81 μm, respectively. According to the radiocarbon dating on shells of zooplankton foraminifera, two tephra layers in Core CSH1, formed at 7 250 and 10 870 a BP (cal), approximately correspond to the K-Ah tephra [7 300 a BP (cal)] and the eruption of Kuju Volcano (12￣10 ka BP), respectively.