4. Silicoflagellate Biostratigraphy, Hole 1149A (ODP Leg 185, Nadezhda Basin, Northwestern Pacific)Francesca Lozar and Massimo MussaABSTRACTINTRODUCTIONMATERIAL AND METHODSBIOSTRATIGRAPHYDictyocha fibula Zone (Martini, 1971) Bukry, 1977Dictyocha stapedia stapedia Zone (Bukry, 1977)Dictyocha perlaevis ornata Subzone (Bukry, 1981a).Dictyocha perlaevis delicata Subzone (Bukry, 1976)Mesocena quadrangula Zone (Bukry and Foster, 1973; emended this work)Dictyocha aculeata aculeata Zone (Bukry, 1979)Main Biostratigraphic and Paleoclimatic Events Recorded at Site 1149SUMMARYACKNOWLEDGMENTSREFERENCESTAXONOMIC APPENDIXFIGURES: ThumbnailF1. Location of Site 1149, p. 14.F2. Stratigraphy of upper Cenozoic Unit I, p. 15.FIGURES: Full pageFigure F1. Location of Site 1149, drilled during Leg 185; the bathymetric map shows the relative position with respect to the Izu-Bonin arc system (from Plank, Ludden, Escutia, et al., 2000).Figure F2. Stratigraphy of the upper Cenozoic Unit I recovered from Hole 1149A. Silicoflagellate bioevents are indicated and cor...TABLE: ThumbnailT1. Silicoflagellate distribution, p. 16.TABLE: Full pageTable T1. Silicoflagellate distribution in selected samples, Hole 1149A.PLATES: ThumbnailP1. Silicoflagellates, p. 17.P2. Silicoflagellates, p. 18.PLATES: Full pagePlate P1. All pictures are 800X. 1, 2. Dictyocha aculeata aculeata (Sample 185-1149A-4H-3, 131-141 cm); (1) high focus, (2) lPlate P2. 1, 2. Distephanus boliviensis (Sample 185-1149A-10H-5, 135-141 cm); (1) low focus, (2) high focus. 3, 4. Distephanus s...Ludden, J.N., Plank, T., and Escutia, C. (Eds.)Proceedings of the Ocean Drilling Program, Scientific Results Volume 1854. SILICOFLAGELLATE BIOSTRATIGRAPHY,HOLE 1149A (ODP LEG 185, NADEZHDA BASIN, NORTHWESTERN PACIFIC)1Francesca Lozar2,3 and Massimo Mussa2ABSTRACTSilicoflagellate assemblages have been studied from Hole 1149A(northwestern Pacific, Ocean Drilling Program Leg 185) in a continuousrecord of pelagic clay from lithologic Unit I. The biostratigraphic frame-work of D. Bukry, adapted in this work, is calibrated to tropical and sub-tropical fossil floras. Four biozones and three subzones have beenrecognized (from the lower Pliocene Dictyocha fibula Zone to the Qua-ternary Dictyocha aculeata aculeata Zone), and their zonal boundarieshave been calibrated to the magnetostratigraphic scale. In particular,the boundary between the Dictyocha perlaevis delicata and the Dictyochaperlaevis ornata Subzones approximates the Pliocene/Pleistocene bound-ary.The identification of three closely spaced climatic events (the asper-oid/fibuloid reversal at 75.11 meters below seafloor [mbsf], the sharpdecrease in abundance of Distephanus sp. at 70.05 mbsf, and the last oc-currence of Distephanus boliviensis at 68.46 mbsf) indicates the increasedstrength of the warm Kuroshio Current in the interval between 3.3 and2.8 Ma, due to the final closure of the Central American Seaway andleading to the present-day hydrodynamic regime.The beginning of the acme of Mesocena quadrangula at 37.01 mbsf(~1.1 Ma) also shows a sharp warming in surface waters.1Lozar, F., and Mussa, M., 2003. Silicoflagellate biostratigraphy, Hole 1149A (ODP Leg 185, Nadezhda Basin, northwestern Pacific). In Ludden, J.N., Plank, T., and Escutia, C. (Eds.), Proc. ODP, Sci. Results, 185, 1–18 [Online]. Available from World Wide Web: <http://www-odp.tamu.edu/publications/185_SR/VOLUME/CHAPTERS/009.PDF>. [Cited YYYY-MM-DD]2Dipartimento di Scienze della Terra, Università degli Studi di Torino, via Accademia delle Scienze 5, I-10123 Torino, Italy. Correspondence author: [email protected] Istituto di Geoscienze e Georisorse, via Accademia delle Scienze 5, I-10123 Torino, Italy.Initial receipt: 6 November 2001Acceptance: 20 November 2002Web publication: 31 March 2003Ms 185SR-009F. LOZAR AND M. MUSSASILICOFLAGELLATE BIOSTRATIGRAPHY 2INTRODUCTIONSite 1149 was drilled in the northwestern Pacific during Ocean Drill-ing Program (ODP) Leg 185; it is located at 31.3°N, 143.3°E, on the Pa-cific 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 kmeast of the Izu-Bonin Trench, where the Pacific plate is flexed upwardprior to its entry into the subduction zone (Fig. F1).One of the objectives of drilling at Site 1149 was to obtain a referencesite for the Cretaceous Pacific oceanic crust and its sedimentary coverbeing transported into the Izu-Bonin subduction zone, in order to com-pare the geochemical fluxes into the Izu-Bonin arc system with those ofthe Mariana arc, which show significant differences in key elements.Other goals of the drilling were to compare basement alteration charac-teristics to those in Hole 801C, to provide constraints on the Early Cre-taceous paleomagnetic timescale (Plank, Ludden, Escutia, et al., 2000),and to provide constraints on mid-Cretaceous carbonate compensationdepth (CCD) and equatorial circulation fluctuations in the western Pa-cific.Besides the major goals of the leg, this study will provide new dataon silicoflagellate biostratigraphy and paleoceanography in the lateNeogene Pacific Ocean and will help to constrain some silicoflagellatebioevents with the geochronologic timescale of Berggren et al. (1995).The sedimentary section recovered at Site 1149 ranges from LowerCretaceous to Pleistocene. The top of the section, recovered in Hole1149A, consists of carbonate-free clay with common and discreet ashlayers and dispersed ashes, containing relatively abundant and well-pre-served 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 pelagicclays (Unit II; 118.2–179.1 mbsf) (Plank, Ludden, Escutia et al., 2000).Unit II clays are barren of any siliceous or calcareous microfossils butcontain ichthyoliths. According to shipboard magnetostratigraphicdata, Unit I in Hole 1149A ranges from late Miocene to Pleistocene age(Plank, Ludden, Escutia, et al., 2000); Unit II is still undated (ichthy-olith biostratigraphy is in progress; F. Lozar, pers. comm., 2001). This study deals with the biostratigraphic and paleoceanographicanalysis of silicoflagellate assemblages recovered from Unit I in Hole1149A. The silicoflagellates are a minor component of the marine phy-toplankton, but their potential is good for biostratigraphic and pale-oceanographic correlation in marine