Abundance of stratigraphically important planktic foraminifera and biohorizons of ODP Site 186-1150 and Hole 186-1151A

The planktonic foraminiferal biostratigraphy of Sites 1150 and 1151 drilled in the Japan Trench region during Ocean Drilling Program Leg 186 has been examined. At these drill sites, more than 1100 m of sediment, reaching to the middle Miocene, were obtained with good core recovery. Although planktonic foraminiferal records of the two sites are discontinuous, probably because of carbonate dissolution beneath the carbonate compensation depth, 16 stratigraphically important taxa of planktonic foraminifers were detected. Eight biohorizons were recognized in same stratigraphic order between these two sites. With respect to the diatom data, these eight planktonic foraminiferal biohorizons are possibly synchronous for correlating marine strata from the upper Miocene to Pleistocene in the Japan Trench region. The last occurrence of Neogloboquadrina asanoi falls within Olduvai Subchron. Therefore, the Pliocene/Pleistocene boundary of both sites is situated a little above this biohorizon.