Orbital-scale stratigraphy and paleoenvironmental changes in the Japan Sea during the last 230 kyr based on oxygen and carbon isotopes of benthic foraminifera

Paleoceanographic conditions in the Japan Sea changed drastically during the late Quaternary in association with orbital and suborbital cycles and glacio-eustatic sea level changes. However, few studies had been examined long-term sediment records older than last intergracial period because of the lack of long cores which contains continuous paleoclimatic records and difficulties reconstructing high-resolution age models. Especially disappearance of benthic foraminifers by euxinic bottom water conditions during glacial maxima makes it difficult to understand specific bottom water environments. In this study, we reconstructed benthic foraminiferal stable isotopic stratigraphy of the Japan Sea core with the resolution in 2 kyr. and compared the global stack of benthic foraminfieral δ18O curve (LR04, Lisiecki & Raymo, 2005). The investigated core MD01-2408, was recovered from 806 m water depth, off Akita Prefecture during IMAGES VII cruise. It had recorded continuous paleoenvironmental histories during the last 230 kyr through the core. This core was mainly consisted of alternating layers of homogeneous light colored silty clay and laminated or bioturbated dark colored silty clay, occasionally intercalating volcanic ash layers. The most abundant benthic foraminiferal species are Uvigerina akitaensis, Angulogerina kokozuraensis and Islandiella norcrossi. Abundance of these species fluctuated drastically through the core. No benthic foraminifera is found in two dark layer intervals 4 ~ 5 mbsf and 25 ~ 25.5 mbsf, corresponding 15 ~ 25 kyr and 149 ~ 152 kyr, respectively, probably representing super anoxic bottom water conditions(Oba et al 1984 1991;Tada et al., 1999). δ18O of U. akitaensis and A. kokozuraensis showed a similar values with a precision of ± 0.27‰ and it was inferred that both species had similar habitat. δ18O of both species in core MD01-2408 fluctuated with the range of 2‰ over 230 kyr. Although benthic δ18O stack of LR04 reached to 5 ‰ during the glacial maxima, δ18O of core MD01-2408 were 2.6 ‰ and 2.7 ‰ during MIS 2a and MIS 6, respectively. Differences of bottom water δ18O between global average and the Japan Sea were 2.3 ~ 2.4 ‰. This reason of negative excursions was considered as downward diffusion of low saline surface water to the bottom (Oba et al 1984 1991). However, the interglacial stages during MIS3 to 5, benthic foraminiferal δ18O was rather heavier (4.0 ~ 4.5‰) than MIS2 and 6. Furthermore, benthic δ18O values had became ligher from MIS5a to 2 gradually. These δ18O shift should be attributed to bottom water salinity change rather than temperature. It is considered that the thickness of less saline surface water had increased and strongly affected to the bottom water properties in the glacial maximum stages through the last 230 kyr.