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Calcareous nannofossils were encountered at only one of the sites (435) drilled during DSDP Leg 56. Cores from Hole 435A yield fairly diverse early and late Pliocene assemblages. The section shows considerable reworking, however. Three to five biostratigraphic datum events provide a reasonable biochronology. The datums range from about 3.3 Ma in Core 11 to about 1.8 Ma in Core 3. Paleobiogeographic data indicate relatively stable and warm climatic conditions in this area in the early Pliocene, becoming more unstable in the late Pliocene when the cosmopolitan species become dominant. INTRODUCTION AND OBJECTIVES During Leg 56 of the Deep Sea Drilling Project, first of the two-leg Japan Trench transect, the D/V Glomar Challenger drilled three holes in September and in October 1977. Site 434 was drilled on the lower trench slope, Site 435 on the upper slope of the trench wall, and Site 436 on the crest of the outer rise (Figure 1). The Japan Trench transect was the first of several active margin transects in the Pacific Ocean chosen to study the major trench and volcanic belt systems. The main objectives of Leg 56 were to determine the type and degree of tectonic activity related to the overthrust wall of the actively subducting Japan Trench. These objectives dictated that sites be either where water depth was too great for calcite preservation or where the area was tectonically active and thus too highly disturbed for detailed biostratigraphic studies of calcareous nannoplankton. For example, Sites 434 and 436 were both drilled at water depths in excess of 5000 meters and yielded practically no calcareous nannofossils (with the exception of a thin layer in Hole 434-12-1, 73-75 cm, in which poorly preserved and rare specimens of small coccoliths, Dictyococcites minutus and Sphenolithus neoabies, were encountered, possibly as contaminants). Only Site 435, on the upper slope of the trench, drilled in water depth of about 3400 meters, contained a relatively better preserved and fairly diverse Pliocene nannoflora. The purpose of this chapter is to report the calcareous nannofossil assemblages recorded at Site 435. The sediments at this site consist of muddy diatomaceous ooze drilled to a total sub-bottom depth of 244.5 meters. The recovered section also contains thin, intermittent layers of volcanic ash and minor amounts of terrigenous sand. 140° 145° 150° Figure 1. Location of DSDP Leg 56 sites. NANNOFOSSILS AT SITE 435 The nannofossil species recorded at Site 435 are presented in Table 1. Although all sections of Cores 1 through 11 of Hole 435A were examined for nannoflora, barren samples (e.g, Cores 4, 7, and 8) are excluded from the table and only those samples with at least rare amounts of nannofossils are listed. Samples in which frequent nannofossils were encountered in the

Calcareous Nannofossils from the Japan Trench Upper Slope, Leg 56, Deep Sea Drilling Project