C. pelagicus is interpreted as being a cold-water species throughout the Pliocene and Quaternary (Roth, 1994). The geographical distribution of this species indicates that a change in the current system around the western Pacific and eastern Atlantic Oceans occurred at 2.82 Ma that was related to the final elevation of the Isthmus of Panama (Sato et al., 1998). ODP core coverage in the western Pacific Ocean consisted only of two sites, one equatorial and one high latitude (Sites 806 and 883; 0° and 51°N, respectively). Therefore, data for the North Pacific Ocean are badly needed around the latitude of Japan in order to determine the trend of C. pelagicus in this part of the globe.
Following the methodology of Sato et al. (1998) (see also "Methods"), 500 nannofossil specimens per slide were counted from sediment younger than 3.83 Ma (the base of Zone CN12) from Holes 1150A and 1151A to determine the ratio of C. pelagicus against the total count. The results are shown in Figure F5.
For Hole 1150A, between 3.83 and 2.82 Ma, except for the two samples near the 2.82-Ma boundary that contain abundant C. pelagicus, 27 samples contain no more than 10 specimens of C. pelagicus, an average 4.6 C. pelagicus per 500 specimens. The abundance of C. pelagicus shows a peak near the 2.82-Ma boundary (52 C. pelagicus in 500 specimens from Sample 186-1151A-28X-3, 10-11 cm). From 2.82 to 1.65 Ma, the abundance of C. pelagicus increases significantly and varies greatly, an average of 15 per 500 specimens. In the sediment younger than 1.65 Ma, the average is 21 C. pelagicus per 500 specimens.
From Hole 1151A, the same pattern was observed (Fig. F5). Between 3.83 and 2.82 Ma, C. pelagicus averaged 9 per 500 specimens. From 2.82 to 1.65 Ma, the abundance of C. pelagicus increases significantly, to an average of 24.4 per 500 specimens.
The relative abundance of selected nannofossils was also determined for Hole 1150A (Fig. F6) in order to provide a comparison with a similar study by Sato et al. (1998) for Hole 883C. In Hole 1150A, E. huxleyi dominates the first three samples counted. Below that, small Reticulofenestra dominate the assemblages. Small Gephyrocapsa peak at 254 mbsf (Sample 186-1150A-9H-1, 76-77 cm), whereas P. lacunosa increases in abundance before its extinction point. R. asanoi was an important component during its short life span.
For Hole 1150A, the abundance of C. pelagicus shows a peak at 2.82 Ma (Fig. F5), the same pattern seen in Holes 722A, 999, 606, and 608 located in the Indian Ocean, Caribbean Sea, and the Atlantic Ocean (Sato et al., 1998, fig. 7). Sato et al. (1998) suggested that this event was related to glaciation of the Northern Hemisphere during the late Pliocene.
The C. pelagicus abundance patterns shown here support the conclusions of Sato et al. (1998). Between 3.83 and 2.82 Ma, C. pelagicus was less abundant than it was later. This was prior to the final elevation of the Isthmus of Panama when warm currents flowed from the Atlantic Ocean to the Caribbean Sea and on into the Pacific Ocean (Fig. F7). The elevation of the isthmus closed the Central American Seaway and cut off the warm-current communication between the equatorial Atlantic and the Pacific Oceans, which caused the abundance of the C. pelagicus in the study area to increase (Fig. F8).