Abstract
A 350.2-m-long sediment core, the Shobu core (GS-SB-1), was recovered from the northern edge of the Omiya Upland, Saitama Prefecture, Central Japan, where the inner part of paleo-Tokyo Bay developed intermittently during the interglacial periods of the middle Pleistocene. Of the nine marine deposits in the core (M1-M9, in descending stratigraphic order), we focus on the M4 and M3 deposits, which are assigned to marine isotope stages (MIS) 11 and 9, respectively, with the aim of inferring the paleoenvironment based on an analysis of fossil ostracodes. Seventy-four ostracode species were identified in these deposits. The M4 deposit contains abundant muddy, enclosed bay species such as Bicornucythere sp. and Nipponocythere bicarinata, and sandy subtidal to sublittoral species such as Loxoconcha optima and Loxoconcha tamakazura. The M4 deposit was divided into lower and upper parts based on ostracode faunal composition and lithology. The paleoenvironment of the lower part was basically subtidal or sublittoral open bay during a transgression. The lower part records at least three cycles of paleoenvironmental change from sandy bay to muddy bay. The paleoenvironment of the upper part was enclosed middle to inner bay during a highstand in sea level. Paleoenvironmental variations were found in the upper part, reflecting changes in the inflow of organic material from the inland areas and changes in the proximity of the source area. The horizon of maximum water depth (20-50 m) is tentatively assigned to the boundary between the lower and upper parts. Neomonoceratina delicata and Bicornucythere bisanensis, the former of which was not found in the M4 deposit, are abundant in the M3 deposit. The paleoenvironment of the M3 deposit was the inner to middle part of an enclosed bay via an open sandy bay. In summary, the paleoenvironment of paleo-Tokyo Bay differed between MIS 11 and MIS 9, as did the ostracode biogeography.
