Improving tephrostratigraphy and cryptotephrostratigraphy since 1 Ma of Hole U1437B in the Izu-Bonin arc, NW Pacific: Differentiation of widespread tephras with similar shard chemistries

We investigated the deep-sea sedimentary sequence of Hole U1437B, corresponding to the last 1 Ma, in the Izu-Bonin arc, NW Pacific Ocean, and refined its tephrostratigraphy. We correlated many tephras and cryptotephras with terrestrial and marine tephras on the basis of the major- and trace-element compositions of their glass shards by referring to the reported tephrostratigraphy of marine sediments in core MD012422 (since 0.35 Ma) off Shikoku Island, in Holes ODP1151C (0.2–0.8 Ma) and C9001C (since ca. 0.8 Ma) off northern and northernmost Honshu, respectively, and on the Boso Peninsula (since 1 Ma). Many widespread tephras and cryptotephras were detected in the hole such as those originated from the Kyushu volcanic zone (from north to south, Yufu-Tsurumi, Shishimuta, Aso, Kakuto, Kobayashi, Aira, Ata, and Kikai volcanoes/calderas), Hakkoda-Kokumoto tephra originated from Hakkoda caldera, northernmost Honshu, and some tephras probably originated from source(s) in northern Honshu. Among the widespread tephras, those with similar shard chemistries occurred at different horizons. Kasamori 5 tephra (Ks5) was newly detected by this study in a position above the previously reported "Ks5" horizon in Hole U1437B. The newly detected Ks5 tephra occurs in the marine isotopic stage (MIS) 12 segment of the δ18O stratigraphy, consistent with the reported stratigraphic age of Ks5 (MIS 12b–c) in Hole C9001C. Although the reported "Ks5" horizon also occurs in the MIS 12 segment in Hole U1437B, it is actually a different unidentified tephra, as indicated by its slightly different shard chemistry. Further, we correlated Kasamori 10 tephra (Ks10: mid-MIS 13) in Hole U1437B to subspike G19.3 in Hole C9001C but not to subspike G4.4 in Hole ODP1151C. Although subspike G4.4 in Hole ODP1151C was previously estimated on the basis of its shard major-element composition to correspond to subspike G19.3 in Hole C9001C, their shard trace-element compositions differ. Instead, on the basis of their shard chemistries, we correlated subspike G4.4 in Hole ODP1151C to spike G90 in Hole U1437B (MIS 14), which occurs below the Ks10. This tephra has not been reported in the marine Kasamori Formation on the Boso Peninsula. Further, spike G7 in Hole ODP1151C, which is below subspike G4.4 and above spike G9 (a correlative of Ks22 and Kaisho-Kamitakara tephra, MIS 16), is here correlated to Ks18 in the MIS 15 horizon of Hole U1437B on the basis of their shard chemistries. The stratigraphic ages of Ks5, Ks10, the spike G90 tephra, and Ks18 at most (but not all) locations are roughly consistent with the age of a globally correlated biohorizon, the last appearance datum of Pseudoemiliania lacunosa (ca. 0.45 Ma). These findings emphasize that Hole U1437B sediments are as good as or better than the marine sediments on the Boso Peninsula as archives of the regional tephrostratigraphy. Thus, they are useful for improving the Middle Pleistocene tephrostratigraphy in the NW Pacific region.