The calcareous nannofossil biostratigraphy from the Sites 1150 and 1151 in the Japan Trench of the western Pacific Ocean was studied in detail. Although the abundance and species diversity of the calcareous nannofossil flora was poor, it is still possible to show that the sedimentary sequences range from Quaternary to Miocene in age (nannofossil Zones CN15 to CN3).
A sedimentary section from Holes 1150A and 1150B recovered a total thickness of 1181.6 m of middle Miocene to Holocene sediment. From Site 1151, the sediment section from 0 to 1113 mbsf was cored in three holes (1151A, 1151C, and 1151D). Nannofossil biostratigraphy suggests that the sequence represents a record from the middle Miocene to Pleistocene.
The abundance of C. pelagicus was studied in sediments younger than 3.83 Ma from both Holes 1150A and 1151A in order to elucidate past climate conditions. Between 3.83 and 2.82 Ma, C. pelagicus was less abundant than it was subsequently. This pattern supports the observations and conclusions by Sato et al. (1998), based on their study of eight DSDP/ODP sites around the globe, two of which are located in the high and equatorial latitudes of the western Pacific Ocean. My study area lies conveniently between the latter two sites, thereby filling a gap in the existing coverage. In agreement with their study, the change in the C. pelagicus abundance pattern observed here appears to be related to a change in the current system around the western Pacific Ocean and eastern Atlantic Ocean that occurred in response to the final elevation of the Isthmus of Panama.