Vertical Changes of Holocene Ostracodes in Bore Hole Cores from off Kobe, Related to the Opening of Straits and Relative Sea-Level Change in Western Japan.

Abstract

Fossil ostracodes were studied in two bore hole cores (OB-1 and OB-2) from Osaka Bay off Kobe in western Japan to examine vertical changes in ostracode faunas. Fifty-two species were identified in 56 sediment samples. Based on numerical analyses of these ostracodes, the samples examined are grouped into five biofacies: BM (brackish mud), SM (shallow mud), SS (shallow sandy mud), DS (deep sandy mud) and DM (deep mud). These biofacies are distributed in upward sequence in both cores. From ca. 11, 000-10, 000cal yrs BP, the predominant species living were those dominant in enclosed inner brackish bays at water depths of 2-7m. Inner to middle, enclosed shallow muddy bay species increased at the study sites after the opening of the Akashi Strait (ca. 9, 700cal yrs BP). Ostracode assemblages changed ca. 8, 000cal yrs BP, when Osaka and Harima-nada Bays were connected with Mizushima-nada Bay. Tidal currents from the Akashi Strait have since been strong. As a result, specimens of intertidal to shoreface species were transported with sandy sediments into deeper Osaka Bay off Kobe. Thus, their individual numbers increased rapidly. A peak in the number of intertidal to shoreface species and deep mud-dwellers occurred around 5, 500cal yrs BP, suggesting that tidal currents from the Akashi Strait and paleo-water depth reached their maxima at this time (ca. 33m). These species then decreased with time, with the start of a regression. After ca. 2, 000-1, 600cal yrs BP, mud-dwellers inhabiting enclosed, organic matter-rich bays were abundant. Their numbers increased further as coarse materials from the Akashi Strait decreased and the Yodo River began to bring flood clays into the study sites. Both “form A” and “form M” of Bicornucythere bisanensis are present in both core samples. “Form M” first invaded the study sites through the Kitan Strait to the south. “Form A” came to Osaka Bay off Kobe, ca. 8, 350-8, 000cal yrs BP, and has since become the dominant of the two forms.