Sediments recovered from the two holes cored at Site 898 provide a discontinuous record for the Pleistocene through the upper Oligocene. Calcareous nannofossils generally are abundant to very abundant in all of the sedimentary sequence. Planktonic and benthic foraminifers are abundant to common in the top 157 m of Hole 898A, but are mostly less common below that depth.
Site 898 is located in the eastern part of the Iberia Abyssal Plain (water depth of 5278 m). Hole 898A was cored to a depth of 341.5 mbsf, with a total recovery of 289.22 m of sediment. On the basis of the calcareous nannofossils, we defined two stratigraphic successions, one from the upper Pleistocene (Zone NN21 of Martini, 1971) to the uppermost Pliocene (Zone NN19) and the other from the middle Miocene (Zone NN7) to the upper Oligocene (Zone NP24). A major hiatus representing most of the Pliocene to the upper Miocene lies within Core 149-898A-18X; this also corresponds to the transition from lithostratigraphic Unit I to Subunit IIA (Section 149-898A-18X-4 at 75 cm; see "Lithostratigraphy" section, this chapter). Most of the material in Unit I was deposited by turbidity currents. To eliminate the influence of reworked specimens, biostratigraphic age assignments were determined by one or more samples from the pelagic intervals in every core. The nannofossil record indicates that the Pliocene/Pleistocene boundary is at the top of Core 149-898-A-17X.
Calcareous nannofossils generally are very abundant and well-preserved in the Pleistocene through the latest Pliocene age interval (0-162.34 mbsf), but become less abundant and moderately to well-preserved in the middle Miocene through the upper Oligocene age interval. The biostratigraphy for Site 898 is summarized in Figure 16.
Sample 149-898A-1H-1,7 cm, contains rare specimens of Emiliania huxleyi and Emiliania pujosae, but no Pseudoemiliania lacunosa and has been assigned to Zone NN21 from the uppermost Pleistocene. This sample is older than 85 k.y., as indicated by the dominance of small Gephyrocapsa spp. (>2.5 µm) having an open central area (morphogroup 1 of Gard and Backmann, 1990). Samples 149-898A-1R-1, 80 cm, and -2R-6, 106 cm, belong to Zone NN20, as indicated by the absence of Emiliania huxleyi and Pseudoemiliania lacunosa. Sample 149-898A-1R-1, 80 cm, has abundant Gephyrocapsa spp. (>2.5 µm) having an open central area; abundant Gephyrocapsa spp. (<2.5 µm; morphogroups 1 and 2 of Gard and Backmann, 1990); and few Gephyrocapsa spp. (>2.5 µm) having a closed central area (morphogroup 3). Sample 149-898A-2R-6, 106 cm, contains abundant Gephyrocapsa spp. (>2.5 µm) having both open and closed central areas (morphogroups 1 and 3). The interval from Sample 149-898A-2H-CC to -18X-4, 75 cm, was assigned to Zone NN19.
To improve the zonal scheme for Site 898, the subzones defined by Rio et al. (1990) were integrated into Figure 16 (Subzones NN19A-NN19F). Very abundant small Gephyrocapsa spp. (>2.5 µm) having an open central area, common small Gephyrocapsa spp. (>2.5 µm) having a closed central area, and very rare Pseudoemiliania lacunosa occur in Sample 149-898-A-2H-CC; these indicate the top of Subzone NN19F The highest occurrence (HO) of Reticulofenestra sp. A (>6.5 µm) was recognized in Sample 149-898A-4H-CC. The lowest occurrence (LO) of Gephyrocapsa omega (>4.0 µm) can be seen in Sample 149-898A-6H-4, 77 cm; it indicates the top of Subzone NN19E. The LO of Reticulofenestra sp. A (>6.5 µm) was recorded in Sample 149-898A-8H-1, 78 cm. Sample 149-898A-10H-CC contains the HO of Gephyrocapsa caribbeanica (>5.5 µm) and very rare Helicosphaera sellii; it corresponds to the top of Subzone NN19D (1.10-1.12 Ma). The consistent occurrence of Helicosphaera sellii from Sample 149-898A-12H-3, 104 cm, was used to determine the top of Subzone NN19C (1.20 Ma). The LO of Gephyrocapsa caribbeanica (>5.5 µm), was seen in Sample 149-898A-13H-4, 134 cm. The HO of Calcidiscus macintyrei (circular, >10.0 µm; in Sample 149-898 A-15X-CC) marked the top of Subzone NN19B. The LO of Gephyrocapsa oceanica (>4.0-5.0 µm = Gephyrocapsa oceanica s.l.) can be seen in Sample 149-898A-16X-1, 59 cm. The LO of Gephyrocapsa caribbeanica (>4.0 µm), in Sample 149-898A-16X-CC, indicated the top of Subzone NN19A (1.64 Ma) and also the location of the Pliocene/Pleistocene boundary (1.65 Ma). Intervals 149-898A-17X-1, 12 cm, and 149-898A-18X-3, 56 cm, were assigned to the uppermost Pliocene Subzone NN19A on the basis of the absence of Discoaster spp. and Gephyrocapsa caribbeanica (>4.0 µm). Sample 149-898A-18X-4, 58 cm, contains abundant Coccolithus doronocoides, rare Gephyrocapsa spp. (small), rare Discoaster brouweri brouweri, very rare Discoaster brouweri triradiatus, and Discoaster variabilis. This assemblage places this sample within the upper Pliocene Zone NN18.
Intervals 149-898A-18X-4, 134 cm, to -18X-5, 5 cm, contain Discoaster kugleri and Coccolithus miopelagicus and lacks Catinaster coalitus and Cyclicargolithus floridanus. This assemblage indicates a middle Miocene age (Zone NN7). The HO of Cyclicargolithus floridanus and Coronocyclus nitescens and the absence of Discoaster kugleri in Sample 149-898A-18X-CC indicate Zone NN6. Sample 149-898A-19X-3, 140 cm, contains Sphenolithus heteromorphus and defines the top of Zone NN5. The LO of Calcidiscus macintyrei is recorded in Sample 149-898A-21X-1, 8 cm. Helicosphaera ampliaperta, in Sample 149-898A-22X-4, 107 cm, indicates the top of Zone NN4, whereas the LO of Calcidiscus leptoporus, in Sample 149-898A-25X-3, 35 cm, indicates the base of Zone NN4. In Sample 149-898A-26X-2, 63 cm, few Triquetrorhabdulus carinatus, common Helicosphaera scissura, very rare Helicosphaera recta, and the absence of Helicosphaera ampliaperta and Cyclicargolithus abisectus indicate Zone NN2. In Sample 149-898A-27X-4, 107 cm, few Zygrhablithus bijugatus and Triquetrorhabdulus carinatus, very rare Cyclicargolithus abisectus, Reticulofenestra bisecta, Sphenolithus ciperoensis and Discoaster woodringii, and common Clausicoccus fenestratus indicate the lowermost Miocene Zone NN1. Sample 149-898A-28X-CC has very rare Cyclicargolithus abisectus, Reticulofenestra bisecta, rare Zygrhablithus bijugatus, and common Clausicoccus fenestratus. Intervals 149-898A-29X-1, 45 cm, to -35X-CC were assigned to the uppermost Oligocene Zone NP25. The common occurrence of Cyclicargolithus abisectus and Reticulofenestra bisecta and the presence of Pontosphaera enormis in Sample 149-898A-29X-1, 45 cm, are used to determine the top of Zone NP25 and, thus, the Oligocene/Miocene boundary. The LO of Triquetrorhabdulus carinatus and the HO of Sphenolithus distentus can be seen in Sample 149-898A-32X-CC. Sample 149-898A-35X-CC contains abundant Cyclicargolithus abisectus, common Helicosphaera recta and Reticulofenestra bisecta, few Sphenolithus ciperoensis, rare Sphenolithus distentus, and very rare Chiasmolithus altus. Sample 149-898A-34X-CC contains few Sphenolithus ciperoensis, rare Sphenolithus distentus, and very rare Sphenolithus predistentus. Intervals 149-898A-36X-4, 90 cm, to -36X-CC contain few Helicosphaera compacta and Sphenolithus distentus, rare Chiasmolithus altus, and lacks Sphenolithus ciperoensis. This assemblage indicates Zone NP24.
One core was recovered from Hole 898B. Sample 149-898B-1H-3, 149 cm, has very abundant, well-preserved calcareous nannofossils. The assemblage has very abundant small Gephyrocapsa spp. (>2.5 µm) having an open central area (morphogroup 1 of Gard and Backmann, 1990), but lacks Emiliania huxleyi and Pseudoemiliania lacunosa and was assigned to Zone NN20 from the uppermost Pleistocene. Sample 149-898B-1H-CC contains few, poorly preserved calcareous nannofossils. The low diversity of this assemblage does not allow for a precise age assignment.
Planktonic foraminifers in core-catcher samples from Holes 898A and 898B were examined to establish preliminary ages for the sediments. Samples from the upper sections of Hole 898A contained a moderately diverse planktonic foraminiferal assemblage having good to moderate preservation. Samples from the lower intervals of Hole 898A contained a less diverse planktonic foraminiferal assemblage (Table 5). Similar patterns were observed in the benthic foraminiferal assemblages.
A late Pliocene to Pleistocene age sequence (Zones N22-N23) was identified in Intervals 149-898A-1H-CC to -9H-CC. These samples are characterized by the co-occurrence of Globorotalia tosaensis and Globorotalia truncatulinoides. Globigerinoides extremus, which is usually used as a marker for the base of Zone N22, is present at the top, and is scattered within, this interval. These occurrences were interpreted as having been reworked into the upper part of Zones N22 and N23, taking into consideration the younger nannofossil zones and ages in the upper part of this interval. Abyssal and reworked shallow-water benthic foraminifers were identified throughout this interval. Sample 149-898B-1H-CC also belongs in Zones N22 to N23.
Intervals 149-898A-10H-CC to - 16X-CC are characterized by the absence of Globorotalia truncatulinoides and the rare occurrence of Globorotalia tosaensis. Although the absence of Globorotalia truncatulinoides suggests that this interval may be assigned to Zone N21, which is of late Pliocene age, we have not assigned this age to the assemblage, because of the absence of other zonal markers as well. Calcareous nannofossil evidence does not support a late Pliocene age; nannofossil zonal markers restricted to the early Pleistocene have been found in this interval. The discrepancy in age dating may be the result of mass transport of upper Pliocene sediments into lower Pleistocene sediments, which were barren of planktonic foraminifers, or at least the zonal markers.
Changes in the characteristics of the water mass may also account for the absence of Globorotalia truncatulinoides and the rare occurrence of Globorotalia tosaensis in this interval. Both of these species are warm subtropical to tropical species and are rare in temperate regions (Kennett and Srinivasan, 1983). The relatively common presence of cooler water species, such as Neogloboquadrina pachyderma, indicates sedimentation under cool-water conditions and also may explain the absence of Globorotalia truncatulinoides. Additional analysis of hemipelagic Samples 149-898A-14H-3, 98-99 cm; -15X-2, 45-47 cm; -16X-1, 59-61 cm; and -17X-4, 108-110 cm, did not yield any specimens of Globorotalia truncatulinoides.
Sample 149-898A-17X-CC contains Globorotalia tosaensis and Globorotalia inflata and can be assigned to the upper part of Zone N19, which is of late early to early late Pliocene age. Additional analysis of Sample 149-898A-17X-4, 108-110 cm, yielded Sphaeroidinellopsis paenedehiscens, Globorotalia tosaensis, Globorotalia inflata, and Neogloboquadrina atlantic, allowing us to assign it to the top of Zone N19. Samples 149-898A-18X-CC, -19X-CC, and -20X-CC contain no zonal markers.
If, on the basis of calcareous nannofossils, the Pliocene/Pleistocene boundary is just below Sample 149-898A-16X-CC, then planktonic foraminiferal evidence indicates that a hiatus occurs between this and Sample 149-898A-17X-4, 108-110 cm, where Zone N21 is missing. The planktonic foraminifer assemblage suggests an age of early late Pliocene for Sample 149-898A-17X-CC, which conflicts with the calcareous nannofossil age of latest Pliocene for the same sample.
Sample 149-898A-21X-CC contains Cassigerinella chipolensis, Neogloboquadrina mayeri, and Sphaeroidinellopsis disjuncta, while Globigerina nepenthes is absent. This sample can be assigned to Zones N9 to N13, which indicates a middle Miocene age. This sample also contains Globorotalia sp. l of Iaccarino and Salvatorini (1979), which was recorded in lower Miocene sediments at DSDP Site 398. Samples 149-898A-22X-CC and -23X-CC are barren.
Intervals 149-898A-24X-CC to -28X-CC are characterized by the presence of Catapsydrax dissimilis at the top and Globoquadrina dehiscens at the base. The interval can be assigned to the top of Zones "N4" to N6, which are of early Miocene age. No zonal marker species were present in Samples 149-898A-29X-CC, -30X-CC, -31X-CC, -32X-CC, -33X-CC, and -34X-CC.
Samples 149-898A-35X-CC and -36X-CC contain Globigerina angulisuturalis and Globigerina ciperoensis. This interval can be assigned to the lower part of Zone P22, which is of late Oligocene age.