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Three sites (Sites 305, 310, and 313) drilled during DSDP Leg 32 in the central North Pacific recovered Neogene calcareous sediments. These sites, now located between 2900 and 3500 meters water depth, lay below the lysocline throughout the time of deposition. Site 313, within a basin in the Mid-Pacific Mountains, is at a locale subject to slumping and has little biostratigraphic value. Sites 305 and 310, located on Shatsky Rise and Hess Rise, respectively, provide a valuable record of late Neogene planktonic foraminiferal and paleooceanographic events. The occurrence of calcareous and siliceous microfossils in these two sections permits the direct comparison and correlation of zonation schemes based on the two microfossil types. The positions of the Plio-Pleistocene and MioPliocene boundaries, placed at the oldest occurrence of Globorotalia truncatulinoides and G. tumida, respectively, are in good agreement with the boundaries based on other microfossil zonations. Robust, temperate-water forms of Globorotalia that make up the G. conoidea to G. inflata bioseries are consistent elements of the Pliocene and Pleistocene assemblages. The lowest appearances of G. conoidea at the Miocene/Pliocene boundary and Neogloboquadrina pachyderma in the uppermost Miocene result from improved preservation of calcareous material above those levels. The disappearance of Globoquadrina dehiscens in this area is associated with a late Miocene cooling. This species disappears earlier at the higher latitude Site 310 than at Site 305. Globorotalia puncticulata and G. crassaformis first appear at about 4.0 to 4.2 m.y. B.P. simultaneously with the radiolarian species Lamprocyclas heteroporos. This level, which approximates the first occurrence of Sphaeroidinella dehiscens in temperate latitudes of the North Pacific and has been taken as the Mio-Pliocene boundary in marine sections of the northeastern Pacific margin, is approximately 1 m.y. younger than the type MioPliocene boundary in Italy. The disappearance of keeled forms of the G. conoidea-G. inflata group (represented by G. conoidea s.l.) at approximately 2.7-2.8 m.y. B.P. is followed very shortly by the first appearance of G. inflata, which apparently migrated into the North Pacific slightly later than elsewhere. Late Neogene sediments younger than approximately 6.3 m.y. were deposited with an average rate of accumulation of 7.6 to 10.8 m/m.y., with an increased rate in late Pliocene and a 1-m.y. interval of nondeposition in the late early Pliocene. Neogene sediments older than 6.3 m.y. accumulated at a much lower average rate, resulting in a very condensed series lying unconformably on sediments of Oligocene age. Coiling changes in populations of N. pachyderma reflect surfacetemperature variations. The substantial late Miocene cooling observed in marine sections throughout the margins of the North Pacific resulted in the southward shift of the 10°C surface isotherm over Hess Rise. A sharp decrease in calcium-carbonate dissolution, reflected in both sequences by a change in dissolution facies, occurs near the Mio-Pliocene boundary and is probably correlated with the abrupt drop of the CCD at that time in equatorial regions of the Pacific. INTRODUCTION and Hess rises, respectively. These rises are broad, irregularly shaped plateaus, broken into blocks by norCalcareous Neogene sections were recovered at three mal faulting and elevated above the surrounding abyssal of the drilling sites (Table 1) during DSDP Leg 32 in the sea floor. Site 313 lies within a basin among the North Pacific. Sites 305 and 310 are located on Shatsky seamounts of the northeastern Mid-Pacific Mountains.

Neogene Planktonic Foraminifera from the Central North Pacific, Leg 32, Deep Sea Drilling Project