The Japan Trench area is probably the best investigated island arc-trench system on the planet, where two borehole geophysical observatories were installed at ~1100 meters below seafloor (mbsf) on the deep-sea terrace during Ocean Drilling Program (ODP) Leg 186. The borehole geophysical observatories were planned to monitor the ongoing tectonic processes at two sites (Sites 1150 and 1151) ~25 km north and south of 39°N (Fig. F1). A forearc basin developed in the deep-sea terrace and trench upper slope, which extends from the coast of Hokkaido >600 km to the south and is filled with Cretaceous and Cenozoic sediments as much as 5 km thick. Sites 1150 and 1151 on the deep-sea terrace, located ~100 km west of the Japan Trench, are on the eastern edge of the forearc basin, where the Neogene section is ~1.5 km thick (Sacks, Suyehiro, Acton, et al., 2000).
Previous drilling in the forearc area took place during Deep Sea Drilling Project (DSDP) Legs 56, 57, and 87, which transected the Japan Trench at ~39.8°N-40.7°N. During DSDP Legs 56 and 57, five sites were drilled along across the trench at 39°45´N. Sites 438 and 439 during Leg 57 were drilled across the trench at 40°40´N, where Site 584 was also drilled during DSDP Leg 87 (Shipboard Scientific Party, 1980; Kagami, Karig, Coulbourn, et al., 1986). These legs focused on the study of the mechanism and dynamics of plate convergence and their effects on sedimentation because the Japan Trench has the dynamic properties of one of the world's most active plate subduction zones, where the oldest oceanic plate (>100 Ma) is subducting at a high rate (~90 km/m.y.). It is presently widely accepted that little tectonic accretion is occurring. A surprise finding from Site 439 was the unexpected presence of andesitic volcanic rocks only 90 km west from the trench axis (Shipboard Scientific Party, 2000a).
These legs established not only the concept of tectonic erosion along a subduction zone but also provided a biostratigraphic standard applicable to the middle to high latitudes in the North Pacific Ocean. The sedimentation rates between Sites 1150 and 1151 are broadly similar after 8 Ma and are consistent with those reported at DSDP Leg 57 Site 438 and Leg 87 Site 584, but differ before 8 Ma (Sacks, Suyehiro, Acton, et al., 2000). Extensional tectonics continued from the middle Miocene until the early Pliocene. Numerous ash layer records from all the DSDP and ODP sites suggest that onshore volcanic activity increased during the latest Miocene and continued through the early Pliocene (Shipboard Scientific Party, 1980; Kagami, Karig, Coulbourn, et al., 1986).
Sites 1150 (39°11´N, 143°20´E) and 1151 (38°45´N, 143°20´E) are located in areas with contrasting seismic characteristics at water depths of 2681 and 2182 m, respectively. The Neogene subsidence history of the forearc was documented, and numerous ash records were obtained that span the past 9 m.y. The ages of the recovered sediments are 10-0 Ma at Site 1150 and 16-0 Ma at Site 1151 (Shipboard Scientific Party, 2000a). Biostratigraphic research objectives for the forearc basin-floor drilling focus on a few major threads. These are (1) documenting the succession of surface ocean floras and faunas in the Japan Trench area, (2) investigating the longer-term linkage between the oceanographic evolution of the North Pacific and continental records of mainly volcanic activities, and (3) defining age frameworks for deep-sea sediment sequences.
The sediments collected during Leg 186 provide a unique opportunity to enhance current biostratigraphies and to improve the temporal biochronologic framework for refined paleoceanographic analysis. The authors had to do their diatom work within 4 months before the second postcruise meeting because they were unable to join the shore-based scientific party until the late spring of 2001.
In the near future it is expected that drilled sediments shall contribute to the understanding the oceanographic and climate histories along the North Pacific rim, along with drill sites from the eastern and western equatorial Pacific (Legs 138 and 130, respectively), through the eastern and western somewhat temperate mid-latitude margins (Legs 167 and 186, respectively), and toward the high-latitude North Pacific (Leg 145). Scientific goals must be to link the evolution of the North Pacific climate and the development of the tectonic events in the Northern Hemisphere and, finally, to resolve the teleconnections responsible for the Miocene through Pleistocene diatom records (Barron, 1989, 1992b, 1995, 1998).
The purpose of this paper is (1) to document the diatom stratigraphy at Sites 1150 and 1151, (2) to test the reliability of diatom datum levels and zones for integration with radiolarian biostratigraphy and magnetostratigraphy, and (3) to reexamine a diatom zonation associated with the Miocene/Pliocene boundary. The emphasis of this paper is to substantiate the practical age assignment of the North Pacific Miocene through Pleistocene diatom datums given that most of these events have been previously calibrated to paleomagnetic stratigraphy (Koizumi and Tanimura, 1985; Barron and Gladenkov, 1995; Yanagisawa and Akiba, 1998).