As a preliminary step, a total of 38 selected samples out of 108 samples were examined to check biostratigraphy, and 25 of them contained sufficient numbers of calcareous nannofossils to study the floral composition (Table T1).
The latest "standard" nannofossil event is the first occurrence (FO) of E. huxleyi at 0.268 Ma (Thierstein et al., 1977). E. huxleyi is present in Sample 186-1150A-3H-7, 0-4 cm (25.52 meters below seafloor [mbsf]), which is the lowest sample studied here (Table T1). The Shipboard Scientific Party (2000a) estimated the FO of E. huxleyi at 46.33-55.73 mbsf. Therefore, the entire part of the studied sections are younger than 0.268 Ma. The base of the E. huxleyi acme Zone is a diachronous event and was reported between 85 ka in low latitudes and 73 ka in transitional waters (Thierstein et al., 1977). This event was identified by the reversal in abundance of G. caribbeanica/E. huxleyi (e.g., Thierstein et al., 1977) or by reversal in abundance of Gephyrocapsa muellerae/E. huxleyi (Flores et al., 2000). E. huxleyi is more abundant at the lowest sample studied here than G. caribbeanica and G. muellerae from Core 186-1150A-1H up through Core 3H. Therefore, the entire range of studied samples belong to the E. huxleyi acme Zone (Fig. F2).
The barren intervals (Fig. F2) likely indicate the weakening of the Kuroshio Current and may correspond to glacial-stadial intervals, but without oxygen isotope data the hypothesis is difficult to test.
Seventy-five samples collected from Cores 186-1151C-1H through 3H were examined, and 52 of them contained sufficient numbers of calcareous nannofossils to study the floral composition (Table T2). Reworked specimens of Pseudoemiliania lacunosa are commonly present in the two upper cores (186-1151C-1H and 2H) but are scarce in the lower core (3H) (Table T2; Fig. F3). The preliminary study observed the last occurrence (LO) of P. lacunosa (0.408 Ma) in Sample 186-1151C-5H-3, 98 cm (Shipboard Scientific Party, 2000b). Therefore, the entire range of studied sections is younger than 0.408 Ma. Weaver and Thomson (1993) reported an abrupt decrease of G. caribbeanica at the boundary between marine oxygen isotope Stages (MISs) 7 and 8 (0.245 Ma). The abrupt decrease of G. caribbeanica observed between 9.99 and 8.85 mbsf in Core 186-1151C-3H is likely to correspond to this event. (Table T2; Fig. F3). Based on the LO of P. lacunosa (0.408 Ma) (Shipboard Scientific Party, 200b) and the abrupt decrease of G. caribbeanica (0.245 Ma), the lowest sample studied here can be estimated as 0.36 Ma in age (horizontal axis of Fig. F3). In this study, the FO of E. huxleyi was observed in Sample 186-1151C-2H-5, 65-67 cm (8.56 mbsf); however, the event has been reported between Samples 2H-CC (12.10 mbsf) and 3H-CC (21.44 mbsf) (Shipboard Scientific Party, 2000b). Because of the vulnerability to dissolution, the FO of E. huxleyi is often difficult to identify in a poorly preserved assemblage. Therefore, the true FO of this species may be lower than the identified FO of E. huxleyi in this study. E. huxleyi is very abundant in the entire Core 186-1151C-1H, except the for Sample 186-1151C-1H-CC. Sections 186-1151C-1H-1 and 1H-2, therefore, are assigned to the E. huxleyi acme Zone.
Calcidiscus leptoporus prefers tropical to transitional waters; on the other hand, Coccolithus pelagicus prefers Arctic to subarctic waters (e.g., Winter et al., 1994). In the central Pacific Ocean, Florisphaera profunda is abundant in the lower photic zone of the tropical to transitional waters and is barren in the subarctic Oyashio Extension water (Okada and Honjo, 1973a). In the studied samples at this site, C. leptoporus, C. pelagicus, and F. profunda are the major species (Table T2; Fig. F3). Therefore, it is clear that the surface water at Site 1151 has been affected by both warm Kuroshio and cold Oyashio Extensions during the last 0.38 m.y.
Stratigraphic variation of the lower photic zone species including F. profunda is useful as a paleoceanographic indicator, and a lower abundance of lower photic zone species can be interpreted as an indicator of surface water mixing that triggers a shallower nutricline and higher primary productivity (Molfino and McIntyre, 1990a, 1990b; Okada and Matsuoka, 1996; Beaufort et al., 1997). Studies of living calcareous nannoplankton revealed that the relative abundance of lower photic zone species within the water column is controlled by the absolute abundance of the upper photic zone species (Hagino et al., 2000), and small placolith-bearing species flourish in the upper photic zone of the eutrophic equatorial Pacific (Okada and Honjo, 1973b; Hagino, 1999; Hagino and Okada, 2001). Although Gephyrocapsa (small) and Gephyrocapsa (very small) are only minor components of the flora in the uppermost 2 mbsf, their abundances increased significantly between 2 and 10 mbsf, showing a concordant stratigraphic trend to that of F. profunda. (Fig. F3). In the lower core (10-21 mbsf), Gephyrocapsa (small) and Gephyrocapsa (very small) are minor components and Gephyrocapsa caribbeanica (small) becomes a major component, showing an opposite trend to that of F. profunda (Fig. F3). According to Molfino and McIntyre (1990b) and Okada (2000), F. profunda and small to very small Gephyrocapsa are good indicators of stratified and mixed photic layer conditions, respectively. The stratigraphic trend of G. caribbeanica (small) is concordant with this theory, but the trends of Gephyrocapsa (small) and Gephyrocapsa (very small) are contrary to this theory.
The inavailability of oxygen isotope data and the presence of many barren intervals limits what can be learned about (Fig. F3) the paleoceanography from the nannoflora in this area.
Sedimentation rate was calculated based on the depth of the nannofossil datums (Fig. F4). The base of the E. huxleyi acme Zone has been reported from 0.073 to 0.085 Ma (e.g., Thierstein et al., 1977). The only datum information in Cores 186-1150A-1H through 3H is that the deepest studied sample (Sample 186-1150A-3H-7, 0-4 cm; 25.52 mbsf), is younger than 0.085 Ma. This result indicates that the sedimentation rate between Cores 186-1150A-1H and 3H (upper 25.52 mbsf) is >300 m/m.y. The Shipboard Scientific Party (2000a) estimated that the sedimentation rate for the upper 50.72 mbsf is 204 m/m.y., based on the FO of E. huxleyi. Therefore, the sedimentation rate between 25.52 and 50.72 mbsf would be <94 m/m.y. (Fig. F4).
In Hole 1151C, the base of the E. huxleyi acme Zone (0.076-0.085 Ma) lies within Samples 186-1151C-1H-2, 35-37 cm (1.92 mbsf), through 1H-CC (2.08 mbsf); therefore, the sedimentation rate is >23.5 m/m.y. and <26.3 m/m.y in the upper 2.0 mbsf. An abrupt decrease of G. carribeanica (0.245 Ma) observed between Samples 186-1151C-2H-5, 95-97 cm (8.85 mbsf), and 2H-6, 65-67 cm (9.99 mbsf), indicates that the sedimentation rate is >43.9 m/m.y. and <46.4 m/m.y. (between 2.00 and 9.42 mbsf). The preliminary study reported the LO of P. lacunosa (0.408 Ma) in Samples 186-1151C-5H-3, 98 cm (23.69 mbsf), and 4H-2, 98 cm (34.68 mbsf) (Shipboard Scientific Party, 2000b). On the basis of an abrupt decrease of G. carribeanica and the LO of P. lacunosa (0.408 Ma), the sedimentation rate between 9.42 and 29.18 mbsf can be estimated as 121.1 m/m.y.