Holocene Ostracods from the Borehole Core at Oppama Park, Yokosuka City, Kanagawa Prefecture, Central Japan: Paleoenvironmental Analysis and the Discovery of a Fossil Ostracod with Three-Dimensionally Preserved Soft Parts

Abstract

Abstract. Forty-seven sediment samples were taken from a borehole core at Oppama Park, Yokosuka City, Kanagawa Prefecture, central Japan. Fifty-eight ostracod species, representing 32 genera, were identified in the 32 samples. Three ostracod biofacies (I–III) were identified using Q-mode cluster analysis. On the basis of ostracod biofacies, the following paleoenvironmental changes were recognized in the study area: land mudflat (no ostracods) → muddy bottom environment of inner bay (biofacies III) → sandy bottom environment of bay near rocky shore (biofacies II) → shallow mudflat (biofacies I). Modern analog technique (MAT) showed that the paleoenvironment of the study area was a warm (maximum bottom temperature in summer around 30°C; minimum bottom temperature in winter around 7°C) and shallow bay, similar to that around Honshu Island today. An ostracod specimen, Parakrithella pseudadonta, with exceptionally well preserved soft parts was discovered in one of the horizons, at a core depth of 20.4–20.5 m. ...