Calcareous nannofossils, diatoms, and planktonic and benthic foraminifers were examined in mudline and core catcher samples from Hole 1234A. Calcareous nannofossils and diatoms were also examined in additional samples from all sections of same hole. Drilling at Site 1234 revealed one major biostratigraphic unit of late Pleistocene age. The abundance of calcareous microfossils varies from rare to common, and a few samples are barren (Fig. F15). Preservation of the four groups (calcareous nannofossils, diatoms, and benthic and planktonic foraminifers) is generally moderate to good. Reworking of microfossils, mainly from Neogene sediments, is apparent to some extent in all the fossil groups examined. In particular, benthic diatoms are present in most of the samples, indicating relatively persistent redeposition from the upper continental margin. Changes in benthic foraminiferal assemblages likely reflect variations in the oxygenation of bottom water masses. Calcareous nannofossils suggest that the base of the cored sequence is younger than 0.26 Ma (Zone NN21).
Calcareous nannofossils are common to abundant and well preserved in most of the samples examined, except for the core catcher samples from the lowermost six cores (Cores 202-1234A-17X through 22X [180.5-238.9 mcd]), where nannofossils are generally rare and poorly preserved. Reworking of nannofossils is minimal in the sequence, as only a few specimens of Neogene-Cretaceous nannofossil taxa were observed in a small proportion of the samples.
Calcareous nannofossil taxa commonly encountered at the site include various species of the genus Gephyrocapsa, Emiliania huxleyi, Calcidiscus leptoporus, Coccolithus pelagicus, and Helicosphaera carteri (Table T9). However, large fluctuations in the relative abundance of these taxa occur, presumably in response to changes in surface-water properties. The abundance of E. huxleyi also fluctuates significantly, which makes the placement for the base of the E. huxleyi acme zone (0.08 Ma) difficult at this site. The abundance of E. huxleyi first exceeds that of medium-sized gephyrocapsids (mainly Gephyrocapsa muellerae) at ~70 mcd. This abundance reversal generally occurs near the boundary between oxygen isotope Stages 4 and 5 in the Southern Ocean (Flores et al., 2000). More reliable and precise delineation of the E. huxleyi acme zone at this site awaits further detailed studies. E. huxleyi was found down to Sample 202-1234A-22X-5, 40 cm (237.1 mcd), in the last core, indicating an age younger than 0.26 Ma for the entire sedimentation sequence (Table T9). All of the sequence corresponds to Zone NN21 (Martini, 1971).
Planktonic foraminifers were examined in all core catcher samples from Hole 1234A. They are present in all samples, but abundance and preservation vary markedly (Table T10). At the base of Hole 1234A (Sections 202-1234A-17X through 22X; 180.9-238.9 mcd), the benthic to planktonic foraminiferal ratio is generally high and the abundance of planktonic foraminifers shows a marked decrease relative to the total foraminiferal abundance. 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 238.9 mbsf, indicating the upper Pleistocene Subzone Pt1a of Berggren et al. (1995) (Fig. F12 in the "Explanatory Notes" chapter). Changes in assemblage composition offer the potential to monitor variations in the intensity and position of the upwelling system with time.
Benthic foraminifers were studied in all core catcher samples from Hole 1234A. They are generally frequent to common (representing between 20% and 98% of the total foraminiferal assemblage) and are moderately or well preserved, although a significant proportion of tests are pyritized at some levels. To assess 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 34 taxa were identified (Table T10). Common species are Bolivina seminuda, Bolivina costata, Bulimina mexicana, Cassidulina teretis, Chilostomella ovoidea, Eubuliminella exilis, Globobulimina pyrula, Globobulimina affinis, Nonionella auris, Nonionella stella, Planulina wuellerstorfi, Praeglobobulimina spinescens, Rotaliatinopsis semiinvoluta, Rutherfordoides mexicanus, and Uvigerina peregrina.
The assemblage is dominated by high carbon-flux indicators that are typical in assemblages from oxygen minimum zone (OMZ) environments. Oxygen often becomes the limiting factor in the central part of the modern OMZ, and the downslope succession of species reflects local variations in carbon flux and oxygen gradients. At Site 1234, marked changes in assemblage composition are recorded downhole, particularly for Bolivina spp., Globobulimina spp., Uvigerina spp., and Nonionella spp. As these taxa have different tolerance of oxygen depletion, they can be used to detect shifts in bottom water oxygenation related to upwelling and circulation patterns. For instance, the distinct peak in the abundance of Bolivina spp. at ~175-215 mcd (>40% of total benthic foraminifers) points to an intense episode of seafloor dysoxia (Fig. F16) that may be linked to the intensification of nutrient-rich PCW and the decreased influence of AAIW.
Diatoms are abundant and generally well preserved both in the core catcher and the split-core samples. Silicoflagellates, radiolarians, sponge spicules, and phytoliths are also observed in most samples.
This site is within the highly productive coastal upwelling area near Concepción, and diatom floras are dominated by the upwelling-related genus Chaetoceros spores, bristles, and vegetative cells (Schuette, 1980; Abrantes, 1988; Schrader and Sorknes, 1990; Abrantes and Moita, 1999). Thalassiosira sp., Delphineis sp., Pseudonitzschia sp., and Thalassionema nitzschioides appear as secondary components of the flora.
Despite the dominance of the coastal upwelling forms, the presence of more oceanic species gives indications of cold- and warm-water masses at various times. Warm water-related species such as Azpeitia nodulifer, Fragilariopsis doliolus, and Nitzschia marina are present in higher abundances (frequent/common) below ~68 mcd, from Samples 202-1234A-7H-CC through 8H-CC; 11H-CC through 12X-CC; 14X-3, 40 cm, to 18X-2, 40 cm; 18X-4, 40 cm, to 19X-3, 40 cm; and 21X-1, 40 cm, to 21X-3, 40 cm. Cold water-related forms, such as Actinocyclus curvatulus and the Rhizosolenia hebetata group, are found in higher abundances (frequent/common), mainly above ~78 mcd at the following levels: mudline to Samples 202-1234A-1H-CC; 4H-1, 40 cm, to 4H-5, 40 cm; 4H-CC to 5H-3, 40 cm; 6H-CC to 8H-3, 40 cm; and 13X-1, 40 cm, to 13X-5, 40 cm (Table T11).
Floras are occasionally enriched in heavily silicified neritic forms, such as Actinoptychus senarius and Stephanopyxis turris. Freshwater diatoms are present (trace/rare) in the following intervals: mudline to Samples 202-1234A-1H-3, 42 cm; 2H-6, 40 cm, to 2H-CC; 5H-CC to 10H-4, 40 cm; and 16X-3, 40 cm, to 16X-4, 40 cm. Displaced shallow-water marine benthic diatoms are found throughout the core, with higher abundances between the mudline and Samples 202-1234A-2H-2, 40 cm, and from 14X-CC to 16X-4, 40 cm. Large diatom cells of the genus Coscinodiscus were found in the >63-µm fraction in Samples 202-1234A-2H-CC and 6H-CC. The presence of F. doliolus down to Sample 202-1234A-21X-CC places this sequence in the F. doliolus Zone; thus, the whole section cored at this site is of Quaternary age.