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[HQR15-04] Identification of Nantaisan Shichihon-sakura/Imaichi tephra (Nt-S/I) in the sediment core MD01-2421 collected off the Kashima coast, Japan
Keywords:Nantaisan Shichihon-sakura/Imaichi tephra, tephrostratigraphy, oxygen isotopic stratigraphy, IMAGES, marine core, off Kashima
Tephra 1 (925.6-933.1 cm) are characterized by coarse and bad selected volcanic sands consisting of white to pale brown pumice and black scoria which grain size is limited to 3 mm. Its thickness is 7.5 cm disturbed in liquid mud. The deposited age of this tephra is estimated at 17.7±1.38 ka based on the oxygen isotopic stratigraphy of the benthic foraminifera and the calendar ages of foraminiferal fossils calculated from the accelerator mass spectrometry (AMS) 14C ages. Pumices are cross-foaming and include many microlites. Refractive index of volcanic glass shards exhibits bimodal as 1.498-1.503 and 503-1.509. Aoki et al. (2008) pointed out that 63-125 µm size particles consist of weathering minerals besides volcanic materials and geochemistry of 125-250 µm size volcanic glass shards indicated heterogeneity, hence there is a strong possibility that particles limited to 250 µm in this tephra bed included contaminant besides essential volcanic materials.
To address this problem, we hand-picked 2-3mm size pumice grains, and crushed them for EPMA analysis. Consequently, it becomes clear that geochemistry of pumices are rhyolitic and rich in alkaline component elements relatively (SiO2; 78.3wt%,NaO2; 3.18wt%, K2O; 3.79wt%). Next, geochemistry of Nantaisan Shichihon-sakura/Imaichi tephra (Nt-S/I), in some tephra erupted around 17.7+-1.38 ka,has slightly bimodal, and the group which K2O content is rather little (SiO2; 77.7wt%,NaO2; 3.23wt%,K2O; 3.97wt%) is similar to the geochemistry of pumices in Tephra1.
Nt-S/I is originated from Nantaisan, which belong to the Nikko volcanic area, west Tochigi Prefecture and consisting of Shichihon-sakura pumice unit (SP; Kanto Roam Kenkyu Group, 1965) as upper part and Imaichi pumice unit (IP; Kanto Roam Kenkyu Group, 1965) as lower part.These fall-out pumice units involving scoria distribute toward east from the source mainly, and its correlatives are found at the Kinu River Lowland area, around Mito and furthermore the Jyoban Coast (Machida and Arai, 2003; Suzuki, 2011).As Nt-S/I correlatives in Tephra 1 are almost pumice grains and there are much contaminated grains in fine materials of Tephra 1,it seems less possibly that Nt-S/I tephra fell out at this site off Kashima. We suppose, as a highly probable transportation process, that Nt-S/I pumice grains falling out at sea off Mito, or carried out to the Pacific by the Kinu River and the Naka River are transported off Kashima by marine current.
Then, Suzuki (2011) reported that Asama Itahana Yellow tephra (As-YP; Machida and Arai, 2003) found just under Nt-S/I around Nikko area. Twenty three tephra beds in this core Aoki (2008) reported are visible tephra beds, so As-YP would be bearing under Tephra 1 that correlated to Nt-S/I.