Standard nannofossil datum events observed at the three Blake Ridge sites are summarized in Table 1. Practically all nannofossil zones and subzones are present at the three sites studied, but because of minor reworking, boundaries for some zones and subzones were difficult to identify at Sites 995 and 997, (Fig. 3). The zonal correlation at Hole 994C (Table 2, PDF format only, oversize material, this volume) is in agreement with the magnetostratigraphy (Hiroki, Chap. 39, this volume).
Age-depth plot graph of zonal boundaries shows the downward trend of generally increasing sedimentation rate (Fig. 4). The sedimentation rate in Hole 994C increases from an average of 5.9 cm/k.y. in the Quaternary to 26.2 cm/k.y. in the Miocene. The sedimentation curves indicate unconformities or condensed sections in Holes 995A, 997A, and 997B (Fig. 3, Fig. 4). The short unconformity or condensed section detected between 570 and 574 mbsf in Hole 995A, as well as the reduced sedimentation rates observed at the similar depths in Holes 994C and 997B, correspond to a major condensed section (~5.7 Ma) in the Gulf of Mexico (Gartner and Shyu, 1996). Moreover, the other unconformity or condensed section at 310-340 mbsf (4.6-3.7 Ma) in Holes 995A and 997A roughly corresponds to a condensed interval in the Gulf reported by Gartner and Shyu (1996). Thus, there is a possibility that the unconformity or condensed sections observed at the Blake Ridge sites correlate with the sequence stratigraphy in the Gulf of Mexico.
As is common in hemipelagic settings, ceratoliths are rare in all intervals, and some datum events based on the phylogeny of Ceratolithaceae were difficult to identify in the upper Miocene to lower Pliocene sequence. However, the last occurrence (LO) of Amaurolithus amplificus (top of Subzone CN9c), the first occurrence (FO) of Ceratolithus acutus (base of Subzone CN10b), and the FO of Ceratolithus rugosus (base of Subzone CN10c) were easy to identify. But the general paucity of Amaurolithus primus makes the placement of the CN10/CN11 boundary difficult. The LO of Amaurolithus delicatus lies at 4.42 Ma. This is the youngest occurrence of Amaurolithus spp. in Hole 994C, which is concordant with the observation at Site 905 off the New Jersey coast (Gartner and Shyu, 1996). As Young et al. (1994) reported, specimens of Amaurolithus tricorniculatus exhibiting weak to moderate birefringence are fairly common in Subzone CN11a.
The LO of Triquetrorhabdulus rugosus and the FO of C. acutus are reported to correlate closely (Berggren, et al., 1985; Raffi and Flores, 1995), but in Hole 994C, the former event lies below the latter (Fig. 5), a similar relationship has been observed at Site 905 (Gartner and Shyu, 1996). Zonal assignment of Subzones CN11b to CN12d was straightforward. Although it is not a zonal marker, the FO of the acme of Discoaster triradiatus (2.18 Ma) is a commonly used biostratigraphic event (Berggren et al., 1995a). Because of the general paucity of this taxon, however, this event was not detectable in Hole 994C. Determination of the Quaternary datum events posed no problem, except for the base of Zone CN13 as noted below. Emiliania huxleyi is only common in the upper two sections of Core 164-994C-2H, but it's abundance increased (>50%) throughout Core 164-994C-1H (Table 2, PDF format only, oversize material, this volume). Therefore, the entire Core 164-994C-1H is younger than 0.085 Ma, and a fairly large coring gap is suspected below this core. The oxygen isotope data (Oba et al., Chap. 18, this volume) indicates an identical interpretation.
Various species or morphotypes of the genus Gephyrocapsa have been used in Quaternary biostratigraphy. The following Gephyrocapsa events are currently used: (1) the FO of medium-sized Gephyrocapsa in the earliest Pleistocene, (2) the FO and LO of large Gephyrocapsa within the early Pleistocene, and (3) the first common occurrence of medium Gephyrocapsa after the "small Gephyrocapsa Zone" of Gartner (1977, 1988) in the early Pleistocene (Fig. 2). Thus while the sporadic occurrence of medium to large Gephyrocapsa within the Pleistocene is a well-known fact, this study revealed a similar pattern of occurrence in the small to medium Gephyrocapsa in the Pliocene.
Samtleben (1980) reported FO of Gephyrocapsa species, his Gephyrocapsa sp. 1, within Zone NN15 of Martini (1971) (Subzone CN11b), but the FO of this taxon was observed at ~4.3 Ma, within Subzone CN11a or Zone NN13 in the sites studied here (Table 2, PDF format only, oversize material, this volume). The small Gephyrocapsa (2.0-3.0 µm) became an abundant to common component of the middle Pliocene flora followed by an abrupt decrease in abundance in the 3.3- to 2.9-Ma interval. This taxon became absent between ~2.9 and 2.5 Ma (Fig. 5). The small Gephyrocapsa re-emerged in Sample 164-994C-16H-6, 77-79 cm, and occurs continuously to the top of core. Although it was included in the very small placolith category, the very small forms of Gephyrocapsa (<2.0 µm) show an identical stratigraphic distribution as the small Gephyrocapsa.
Because many researchers have ignored the small and very small Gephyrocapsa in biostratigraphic studies, it is not clear whether the temporary disappearance in the upper Pliocene in Hole 994C is a global event. Because age-diagnostic discoasters are often absent in hemipelagic sequences, the disappearance and reappearance of common to abundant small Gephyrocapsa in the upper Pliocene provide a useful biostratigraphic marker.
The FO of Gephyrocapsa caribbeanica was originally used to define the base of Subzone CN13b (Bukry, 1973; Okada and Bukry, 1980). Because of the taxonomic confusion of Gephyrocapsa species, however, this boundary event has recently been revised as the FO of medium Gephyrocapsa (Berggren et al., 1995a). In Hole 994C, early specimens of G. caribbeanica occurs with Gephyrocapsa oceanica (medium; Table 2, PDF format only, oversize material, this volume). Actually, these two taxa co-occur discontinuously in the uppermost Pliocene (Fig. 5). Most of the medium Gephyrocapsa observed in the upper Pliocene range from ~3.0 to 4.0 µm in coccolith size, but very rare specimens, slightly larger than 4.0 µm, were also observed. Above the two temporal disappearances in the upper Pliocene, G. caribbeanica and G. oceanica (medium) occur continuously in the lower Pleistocene upward from Sample 164-994C-12H-1, 64-66 cm (Table 2, PDF format only, oversize material, this volume; Fig. 6), and specimens of G. oceanica larger than 4.0 µm are common. The base of Subzone CN13b, therefore, should be defined by the base of the continuous occurrence of the larger medium-sized Gephyrocapsa (larger than 4.0 µm).
Thus, the sporadic occurrence is a common feature for all sizes of Gephyrocapsa, and the "small Gephyrocapsa Zone" of Gartner (1977) represents just one example of the temporary disappearances of medium to large Gephyrocapsa (Fig. 5). As Matsuoka and Okada (1989, 1990) demonstrated, the younger forms emerging after the temporal disappearances are likely to be new morphotypes that evolved from the base stock of small forms.
Umbilicosphaera aequiscutum is a major component of the nannoflora in the upper Miocene and Pliocene in all the Blake Ridge sites studied, and this taxon is particularly abundant in the upper Pliocene (Fig. 6). This species also occurs commonly in the lower upper Miocene (Zones CN 7 and CN8) of Carolina Ridge Sites 992 and 993. Because its occurrence is so prevalent, the fact that this taxon has not been recorded in previous studies of ODP cores is surprising. The species was first described from the Pliocene of Jamaica (Gartner, 1967), and Aubry (1993b) reported its early occurrence in the lower middle Miocene Zone CN4 (NN5) in the Gulf of Mexico. This species appears to have been ignored in the previous studies because of its small size. However, because U. aequiscutum is so abundant and easily recognizable under a cross-polarized microscope, it should not be overlooked in future calcareous nannofossil studies. Moreover, the sharp decline in the abundance of the taxon in the middle part of the uppermost Pliocene Subzone CN12d is obviously its LO, and rare to few specimens found above Sample 164-994C-15H-5, 45-47 cm, are likely to be reworked. Because Subzone CN12d has a long duration (~0.5 m.y.) and the only biostratigraphic event in this subzone, the base of the acme of Discoaster triradiatus (2.18 Ma) is hard to detect in hemipelagic sediments; the LO of U. aequiscutum at ~2.3 Ma is a useful biostratigraphic event.