[S-GL44_2PM1] The Lower-Middle Pleistocene boundary of the Kazusa Group

[SGL44-06] Identification of Pleistocene tephra layers in marine sediment core C9001C, offshore Shimokita Peninsula, NE Japan

*Takeshi HASEGAWA¹, Manami SUGAYA¹, Makoto OKADA¹, Nobutatsu MOCHIZUKI², Satomu FUJII², Hidetoshi SHIBUYA² (1.Faculty of Science, Ibaraki University, 2.Kumamoto University)

Keywords:Shimokita Peninsula, marine sediment core, Pleistocene, tephra, CHIKYU, glass chemistry

Correlations for Pleistocene tephra layers in marine sediment core C9001C was investigated. The core was obtained from near the Shimokita Peninsula, Japan by the CK06-06 D/V CHIKYU Shakedown Cruise in 2006. The stratigraphy of the Hole C9001C (365 m long) has been well established based on the correlation of the benthic d18O curve with the LR04 stack (Domitsu et al., 2011). Tephra layers, a few millimeters to centimeters in thickness, can be often recognized in this sediment core that is mainly composed of dark olive-gray, diatomaceous silty clay. Two tephra layers at 30.3 mbsf and 54.3 mbsf were already correlated with the Spfa-1 and Aso-4, respectively. In this study, we focus on relatively thick and coarse tephra samples from 20 mbsf to 150 mbsf (30 - 240 ka, LR04 age). As the result, seven tephra layers were newly identified on the basis of tephro-stratigraphy and petrology, such as glass chemistry and mineralogy. We identified the To-Of (BP1) at 19.6 mbsf, To-GP at 24.8 mbsf, Ko-i at 25.5 mbsf, Toya at 61.4 mbsf, Aso-3 at 73.9 mbsf, Mb-1 at 115.6 mbsf and Tn-C at 145.9 mbsf based on tephra databases (e.g. Okumura, 1991; Machida and Arai, 2003; Aoki and Machida, 2005). Descriptions of each tephra layer are as follows: The tephra layer at 19.6 mbsf is 6 cm thick, medium-sand sized, crystal vitric ash, including Cpx and Opx crystals. Chemical composition of glass shards is SiO2=77.5%, K2O=1.2% (100% normalized). The tephra layer at 24.6 mbsf is 3 cm thick, medium-sand sized, vitric crystal (Cpx, Opx) ash, showing Low-K glass composition (SiO2=75.4%, K2O=1.1%). The tephra at 25.5 mbsf is patchy (5 mm in maximum thickness), fine-sand sized, vitric ash, showing the Medium-K composition (SiO2=76.2%, K2O=2.1%). The tephra at 61.4 mbsf is 1.5 m thick, fine-sand sized, vitric ash, containing trace amount of Opx. Glass shards have Medium-K composition (SiO2=79.0%, K2O=2.7%). The tephra layer at 73.9 mbsf is 2 cm thick, medium-sand sized, vitric crystal (Cpx, Opx) ash, characteristically showing High-K glass composition (SiO2=70.3%, K2O=4.6%). The tephra layer at 115.6 mbsf is 4 cm thick, medium-sand sized, vitric crystal ash, characteristically including Bt, Hb crystals in addition to pyroxenes. The glass chemistry is: SiO2=78.1%, K2O=3.9%. The tephra layer at 145.9 mbsf is 20 cm thick, medium to coarse-sand sized, vitric crystal (Cpx, Opx) ash, showing relatively Low-K glass composition (SiO2=78.4%, K2O=1.5%). We can re-examine the correlations for tephras with marine isotope stages (MIS) based on LR04 age. The To-Of tephra from Towada volcano can be newly correlated with early MIS 2 (<29 ka). The Aso-3 can be correlated with late MIS 6. It is also needed to revise the estimations of eruption volume and distribution of Aso-3. The Tn-C tephra from Osore volcano can be correlated with MIS 7 (<240 ka). Detailed identification of these seven tephras and further correlations for other tephra samples are now in progress, and will be presented elsewhere.