Late Quaternary tephrostratigraphy and cryptotephrostratigraphy of deep-sea sequences (Chikyu C9001C cores) as tools for marine terrace chronology in NE Japan

摘要

We investigated late Quaternary deep-sea sequences of the C9001C cores (D/V Chikyu shakedown cruise) to refine the tephrostratigraphy and reconsider the marine terrace chronology along the Pacific coast of northern Honshu, Japan. We identified 16 concentrations (spikes) of glass shards (numbered G1-G16 from top to bottom), which correspond to 12 visible tephras and 4 non-visible tephras (cryptotephras). We correlated several spikes with known dated tephras: G1 with Towada-Ofudo (To-Of: last MIS 3, ≥32 ka), G2 with Shikotsu-1 (Spfa-1: mid MIS 3, ≥42-44 ka), G3 with Aso-4 (MIS 5b, 87 ka), G4 with Toya (MIS 5d, 106 ka), G9 with White Pumice (WP: mid MIS 7, 220 ka), G12 or G13 with ODP794 2H-2- 120 (mid MIS 8, 274-283 ka), and G14 with Shiobara-Otawara (So-OT: mid MIS 9, 320 ka). We classified spike G6 as a turbidite. The two lowest of these tephras, ODP794 2H-2-120 and So-OT, were assigned to MIS 7 in a previous age model on the basis of the first occurrence of Emiliania huxleyi. However, our tephra-based age model is consistent with the last occurrence of Proboscia curvirostris, and we assigned the core sediment corresponding to MIS 7 from a previously reported segment 55 m long to a younger segment 22 m long, and then verified that the sedimentation rate was constant through the core. We used the newly obtained age for the WP tephra, along with outcrop and borehole core evidence, to reconsider the marine terrace chronology in the Kamikita Plain. A marine terrace that was previously assigned to MIS 7 with a possible vertical fault offset was reinterpreted as two separate terraces, one correlated with MIS 7e or 7c and the other with MIS 9. Regional uplift rates based on the revised terrace chronology and shoreline angle elevations appear to have been fairly steady at 0.18-0.36 m/ky during the late Quaternary.