Calcareous nannofossils and diatoms were examined in core catcher samples from Holes 1233A, 1233B, and 1233C and samples from all sections of Hole 1233B. Planktonic and benthic foraminifers were examined in core catcher samples from Hole 1233B. All groups exhibit moderate to high abundances and good preservation (Fig. F16).
Precise age assignments are not possible, although the continued presence of Emiliania huxleyi to the bottom of Hole 1233B indicates that the entire sequence is younger than 0.26 Ma.
This fossil group is generally abundant and well preserved at the site. The continued presence of E. huxleyi to the bottom of Hole 1233B indicates that the entire sequence is younger than 0.26 Ma.
E. huxleyi is generally rare to few and significantly less abundant than Gephyrocapsa spp. in the samples examined. This suggests that the E. huxleyi acme zone of Thierstein et al. (1977) (0-0.08 Ma) may be missing at the site, or more likely, the zone cannot be applied to this part of the southeast Pacific.
Nannofossil taxa commonly seen at the site include various species of the genus Gephyrocapsa, E. huxleyi, Calcidiscus leptoporus, Helicosphaera carteri, Braarudosphaera bigelowii, Rhabdosphaera clavigera, Umbilicosphaera sibogae, and Umbellosphaera tenuis (Table T13). The generally low fluctuating abundance of tropical (warm water) taxa, such as R. clavigera, U. sibogae, and U. tenuis, at the site is believed to indicate variations in the influence of the northward-flowing colder water. This colder water may also be responsible for the generally low abundance of the index species E. huxleyi.
Planktonic foraminifers are abundant and relatively well preserved in all samples. The planktonic foraminiferal assemblage includes Globorotalia truncatulinoides, Globorotalia inflata, Globorotalia scitula, Globorotalia ungulata, Globigerina bulloides, Globigerinita glutinata, Neogloboquadrina dutertrei, Neogloboquadrina pachyderma, and Orbulina universa. This assemblage is present down to 127.05 mcd, indicating the late Pleistocene Subzone Pt1b of Berggren et al. (1995) (Table T5 in the "Explanatory Notes" chapter). The composition of the assemblage generally reflects temperate transitional climatic conditions.
Benthic foraminifers are generally abundant (representing between 15% and 50% of the total foraminiferal assemblage) and well preserved, although a small proportion of tests are pyritized. For a preliminary assessment of assemblage composition and variability downhole, ~200 specimens from the >150-µm fraction were picked from each core catcher sample and mounted onto slides prior to identification and counting. A total of 44 taxa were identified (Table T14). Common species are Bulimina marginata, Cassidulina teretis, Chilostomella ovoidea, Ehrenbergina serrata, Eubuliminella exilis, Nonionella auris, Globobulimina pyrula, Globobulimina affinis, Hoeglundina elegans, Melonis affinis, Planulina wuellerstorfi, Praeglobobulimina spinescens, Pullenia bulloides, Pullenia quinqueloba, Pyrgo serrata, Rutherfordoides mexicanus, Trifarina tricarinata, and Uvigerina peregrina.
This upper bathyal assemblage is characteristic of environments receiving an enhanced carbon flux at the seafloor. Many of the species found at Site 1233 are present in assemblages from upwelling regions (e.g., off Pakistan [Maas, 2000; W. Kuhnt and A. Thies, unpubl. data] and off Peru [Oberhänsli et al., 1990; Andresen, 1995]). Oxygen is often depleted in these environments, and distinctive downslope foraminiferal successions are observed that are related to the ambient oxygen and carbon-flux gradients (Jannink et al., 1998; W. Kuhnt and A. Thies, unpubl. data). Species with a high tolerance to oxygen depletion (B. marginata, C. ovoidea, E. exilis, G. pyrula, G. affinis, and P. spinescens) provide useful indicators for seafloor oxygenation. At Site 1233, their highest abundance in Samples 202-1233-1H-CC, 2H-CC, 4H-CC, 5H-CC, 6H-CC, 7H-CC, and 10H-CC indicates relatively intense dysoxia at the seafloor during sediment deposition. The species C. teretis and U. peregrina thrive when food is abundant but require higher oxygenation levels. These species show higher abundance in Samples 202-1233-3H-CC, 8H-CC, 9H-CC, and 11H-CC, reflecting more oxic conditions.
Diatom abundance is generally high, and assemblages are moderately to well preserved in both the core catcher samples of Holes 1233B and 1233C and the split-core samples from Hole 1233B. Silicoflagellates, radiolarians, ebridians, sponge spicules, and phytoliths are also observed in most samples.
The major constituents of the flora are taxa typical from intermittent upwelling conditions, such as Chaetoceros spores, bristles and vegetative cells, and Thalassiosira species (Schuette, 1980; Abrantes 1988; Schrader and Sorknes, 1990; Abrantes and Moita, 1999). Delphineis sp., Pseudo-nitzschia sp., and Thalassionema nitzschioides are secondary components of the flora (Table T15).
Pelagic warm water-related species, such as Azpetia nodulifer, Coscinodiscus radiatus, and Nitzschia marina, are found in a few levels and more consistently from Samples 202-1233B-8H-2, 40 cm, through 9H-CC; at 10H-2, 40 cm; and from 8H-CC through 10H-CC. Pelagic cold water-related forms, such as Actinocyclus curvatulus and the Rhizosolenia hebetata group, were present at the following levels: Samples 202-1233B-2H-3, 40 cm, through 2H-7, 40 cm; 5H-5, 40 cm, through 5H-7, 40 cm; 6H-1, 40 cm, through 6H-5, 40 cm; 8H-2, 40 cm, through 8H-4, 40 cm; 10H-2, 40 cm; 10H-4, 40 cm; 2H-CC; and 7H-CC.
Occasionally, floras are enriched in heavily silicified neritic forms, such as Actinoptychus senarius, Stephanopyxis turris. Freshwater diatoms, mainly Aulacoseira granulata, are also present in low abundance (trace/rare) throughout the core but more consistently between Sections 202-1233B-6H-CC and 11H-CC (57-113 mcd). Marine benthic forms are found sporadically in the uppermost five cores and more consistently between Sections 202-1233B-6H-CC and 11H-CC (57-113 mcd), indicating some redeposition.
Aside from the presence of Fragilariopsis doliolus in Sample 202-1233B-9H-CC, which assigns the core to the Quaternary, no other biostratigraphically useful marker species are observed.