Site 1149 was drilled in the northwestern Pacific during Ocean Drilling Program (ODP) Leg 185; it is located at 31.3°N, 143.3°E, on the Pacific plate in the Nadezhda Basin southeast of Japan in a water depth of ~5800 m. It resides among low abyssal hills on a slight high ~100 km east of the Izu-Bonin Trench, where the Pacific plate is flexed upward prior to its entry into the subduction zone (Fig. F1).
One of the objectives of drilling at Site 1149 was to obtain a reference site for the Cretaceous Pacific oceanic crust and its sedimentary cover being transported into the Izu-Bonin subduction zone, in order to compare the geochemical fluxes into the Izu-Bonin arc system with those of the Mariana arc, which show significant differences in key elements. Other goals of the drilling were to compare basement alteration characteristics to those in Hole 801C, to provide constraints on the Early Cretaceous paleomagnetic timescale (Plank, Ludden, Escutia, et al., 2000), and to provide constraints on mid-Cretaceous carbonate compensation depth (CCD) and equatorial circulation fluctuations in the western Pacific.
Besides the major goals of the leg, this study will provide new data on silicoflagellate biostratigraphy and paleoceanography in the late Neogene Pacific Ocean and will help to constrain some silicoflagellate bioevents with the geochronologic timescale of Berggren et al. (1995).
The sedimentary section recovered at Site 1149 ranges from Lower Cretaceous to Pleistocene. The top of the section, recovered in Hole 1149A, consists of carbonate-free clay with common and discreet ash layers and dispersed ashes, containing relatively abundant and well-preserved assemblages of diatoms, radiolarians, silicoflagellates, ebridians, and sponge spicules (lithologic Unit I; 0-118.2 meters below seafloor [mbsf]) (Plank, Ludden, Escutia et al., 2000), and dark brown pelagic clays (Unit II; 118.2-179.1 mbsf) (Plank, Ludden, Escutia et al., 2000). Unit II clays are barren of any siliceous or calcareous microfossils but contain ichthyoliths. According to shipboard magnetostratigraphic data, Unit I in Hole 1149A ranges from late Miocene to Pleistocene age (Plank, Ludden, Escutia, et al., 2000); Unit II is still undated (ichthyolith biostratigraphy is in progress; F. Lozar, pers. comm., 2001).
This study deals with the biostratigraphic and paleoceanographic analysis of silicoflagellate assemblages recovered from Unit I in Hole 1149A. The silicoflagellates are a minor component of the marine phytoplankton, but their potential is good for biostratigraphic and paleoceanographic correlation in marine sediments deposited below the calcite compensation depth (CCD) and under upwelling conditions or high-nutrient water masses (Bukry, 1995).