RESULTS

The oxygen isotopic record from Hole 994C is shown in Table 1 and Table 2 and Figure 1. The ¹⁸O values of both species, G. sacculifer and G. ruber, mostly fluctuate between +0.5 and -1.7 (Fig. 1). This range is nearly same as, or slightly smaller than, those of previous reports (Keigwin and Jones, 1989; Haskell et al., 1991). The mean ¹⁸O value of G. ruber is about 0.2 lighter than that of G. sacculifer, because the former species lives in slightly shallower water depths than the latter species (Hemleben et al., 1989, fig. 10.6). In Figure 1, the average ¹⁸O values between G. sacculifer and G. ruber are connected with a solid line, except at a few levels where ¹⁸O values of only one species were measured. When the ¹⁸O curve of Hole 994C is compared with that of ODP Site 659, which is located in the eastern equatorial Atlantic, each of the oxygen isotopic peaks from Stage 1 to Stage 21 can be tentatively correlated between the two sites (Fig. 1). On the other hand, biostratigraphic events in Hole 994C are found between 8.05 and 9.05 mbsf for the first appearance of Emiliania huxleyi at 0.26 Ma and between 19.0 and 20.0 mbsf for the last occurrence of Pseudoemiliania lacunosa at 0.46 Ma. The paleomagnetic Brunhes/Matuyama boundary is determined to be at 37.8 mbsf in Hole 994C. These events are consistent with the ¹⁸O correlation between Hole 994C and Site 659 as shown in Figure 1. The correlation indicates that sedimentation rates at Hole 994C varied through time. For example, the sedimentation was very slow and/or temporarily ceased during oxygen isotopic Stages 5₁-5₄, 6, 13-15, and 20, whereas the rate was high in Stages 1-2, 10-11, 12-13, and 15-16. Furthermore, it is very difficult to find out any relationship between the sedimentation rate and glacial-interglacial climatic change (Fig. 1). Such a high variation in sedimentation rates is most likely due to the mixture of reworked foraminifer specimens transported by the contour current. Nevertheless, the oxygen isotopic curve contains all the stages from Stages 1 to 21, and coccolith events, as well as the Brunhes/Matuyama boundary, are also consistent with the oxygen isotopic stratigraphy. This suggests that the pelagic sediments from Hole 994C, including at least these two species, continuously accumulated during each time period without redeposition of old sediments, which can be used to construct a valid chronostratigraphy for this area.