The scientific objective of Ocean Drilling Program (ODP) Leg 186 is to investigate past and present tectonic and paleoceanographic conditions of the Japan Trench region (Sacks, Suyehiro, Acton, et al., 2000). For this objective, integration of various stratigraphic data, including biostratigraphy, magnetostratigraphy, and volcanic ash stratigraphy, is required.
Previous studies of foraminiferal biostratigraphy based on deep-sea drilling sites in the Japan Trench region were conducted at six sites (Sites 434, 435, and 436 of Leg 56 and Sites 438, 439, and 440 of Leg 57) of the Deep Sea Drilling Project (DSDP) (Thompson, 1980; Keller, 1980). The foraminiferal records of those sites were discontinuous probably owing to carbonate dissolution beneath the carbonate compensation depth (CCD) (Keller, 1980). Despite such discontinuities, a number of foraminiferal biohorizons have been recognized at those sites and some of these biohorizons, namely, the first and last occurrences of Globoquadrina asanoi (= Neogloboquadrina asanoi) and the evolutional lineage of Globorotalia inflata group, have been correlated with Japanese onland sections (Keller, 1980).
Sites 1150 and 1151 of Leg 186 are located on the deep-sea terrace along the Japan Trench (Site 1150: 39°11´N, 143°20´E, water depth = 2680 m; Site 1151: 38°45´N, 143°20´E, water depth = 2180 m) (Fig. F1). These sites are located in an active plate subduction zone where the oldest part of the Pacific plate (>100 Ma) is being subducted (Sacks, Suyehiro, Acton, et al., 2000). In addition, these sites are well located for monitoring the paleoceanographic change of the North Pacific because of their position relative to both warm Kuroshio and cold Oyashio Current systems (Fig. F1). At Site 1150, ocean-bottom sediments down to a subbottom depth of 1182 meters below seafloor (mbsf) were obtained. The cored sequence reached to the upper Miocene with an average recovery rate of 70%. At Site 1151, the core penetrated to 1113 mbsf and reached to the middle Miocene with an average recovery rate of 68%.
We report a preliminary planktonic foraminiferal biostratigraphy based on Neogene sediments from Sites 1150 and 1151 during ODP Leg 186. We then correlated our results with diatom biostratigraphic data, which we consider to be the best biostratigraphic scheme in the northwest Pacific realm, to estimate the reliability of each planktonic foraminiferal biohorizon.