Calcareous nannofossils, diatoms, and planktonic and benthic foraminifers were examined in mudline and core catcher samples from Hole 1235A. Calcareous nannofossils and diatoms were also examined in additional samples from all core sections of Hole 1235A. Drilling at Site 1235 revealed one major biostratigraphic unit of late Pleistocene age. The microfossils of this site are similar to those of Site 1234. The abundance of calcareous microfossils varies from rare to common, although a few samples are also barren (Fig. F18). Preservation of the four groups (calcareous nannofossils, diatoms, and planktonic and benthic foraminifers) is generally moderate. 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 reflect variations in oxygenation in the bottom water masses. Calcareous nannofossils suggest that the base of the cored sequence is younger than 0.26 Ma (Zone NN21).
Calcareous nannofossil distribution and biostratigraphy at Site 1235 are similar to those at Site 1234. The main difference appears to be the generally lower abundance of nannofossils at this shallower water site. Calcareous nannofossils are mostly common and moderately well preserved in the upper 16 cores (Hole 1235A; 0-171.8 mcd) and are generally rare to few and poorly preserved in the lowermost four cores (Cores 202-1235A-17H through 20H; 171.8-214.7 mcd) (Table T8).
Common Emiliania huxleyi is found down to Sample 202-1235A-3H-CC (below which the species is generally rare to few) and is observed down to Sample 20H-7, 40 cm (213.8 mcd) at the base of the hole; this suggests an age younger than 0.26 Ma for the entire sediment sequence cored at this site (Table T8), corresponding with Zone NN21 (Martini, 1971). As at Site 1234, the E. huxleyi acme zone is difficult to delineate because of the abnormal abundance of the species. The reversal in abundance between medium-sized gephyrocapsids (mainly Gephyrocapsa muellerae) and E. huxleyi is not clear, based on current data. As at Site 1234, the relative abundances of the nannofossil taxa, especially those of Helicosphaera carteri, Calcidiscus leptoporus, and Coccolithus pelagicus, show significant changes at the site, presumably in responding to changes in surface-water properties.
Planktonic foraminifers are present in all samples, but abundance varies markedly (Table T9). For instance, Sample 202-1235A-10H-CC (103.1 mcd) is virtually barren, containing only a few poorly preserved tests. Except for a few samples, tests are moderately well preserved overall and a significant number of tests are pyritized at distinct levels. 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 Sample 202-1235A-20H-CC (214.7 mcd), 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 are generally frequent to common (representing between 15% and 92% of the total foraminiferal assemblage) and are moderately well preserved, although a significant proportion of tests are pyritized at discrete levels. To assess assemblage composition and variability downhole, ~200 specimens from the >150-µm fraction were picked from each sample. Specimens were mounted onto slides prior to identification and counting. A total of 34 taxa were identified (Table T9). Common species are Bolivina seminuda, Bolivina costata, Cassidulinoides bradyi, Eubuliminella exilis, Globobulimina pyrula, Globobulimina affinis, Nonionella auris, Nonionella stella, Planulina wuellerstorfi, Protoglobobulimina pupoides, Rotaliatinopsis semiinvoluta, Rutherfordoides mexicanus, and Uvigerina peregrina.
The assemblages from Sites 1234 and 1235 are similar. Both are strongly dominated by high carbon-flux indicators typical of oxygen minimum zone environments and show marked changes in composition downhole, particularly in the relative proportion of Bolivina spp., Globobulimina spp., Uvigerina spp., and Nonionella spp. (Fig. F19). However, there are significant differences in species distribution between the two sites. For instance, the species Bulimina mexicana is virtually absent at Site 1235, whereas it shows marked fluctuations in numbers at Site 1234. The species C. bradyi, E. exilis, R. semiinvoluta, and R. mexicanus overall are more abundant at Site 1235 than at Site 1234. A distinct peak in the abundance of Bolivina spp. is observed at ~75-115 mcd (>40% of total benthic foraminifers), which points to an intense episode of seafloor dysoxia (Fig. F19) that may be linked to intensification of the GUC or perhaps a deepening of this undercurrent relative to the site, which is associated with Pleistocene sea level change.
In spite of the somewhat poorer preservation at Site 1235 than at Site 1234, benthic foraminifers have good potential for investigation of temporal and spatial fluctuations in the upwelling system and water-mass distribution and, in particular, to decipher the relative influence of the GUC, PCW, and AAIW at the two sites.
Given the location of this site, closer to the coastal upwelling area of Concepción than Site 1234, abundant and well-preserved diatoms were expected. However, the core catcher and the split-core samples observed from Hole 1235A revealed lower abundances (frequent to common) and less well preserved diatom assemblages than those found in Hole 1234A (Table T10).
The typical upwelling-related genus, Chaetoceros spores, bristles, and vegetative cells dominated the assemblages (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.
Floras are enriched in neritic and heavily silicified forms such as Actinoptychus senarius and Stephanopyxis turris between the mudline and Samples 202-1235A-3H-CC; 10H-CC; 12H-3, 40 cm; 14H-CC; and 15H-4, 40 cm. Freshwater diatoms are rare to frequent only in three levels: mudline to Sample 202-1235A-3H-CC; Section 8H-3, 40 cm, to 9H-CC; and Section 11H-3, 40 cm, to 13H-CC.
Displaced shallow-water marine benthic diatoms are found throughout the sequence above Core 202-1235A-17H (180 mcd), with higher abundances between the mudline and Section 3H-CC; from Section 10H-CC to 12H-3, 40 cm; and from Section 14H-CC to 15H-4, 40 cm.
Large diatom cells of the genus Coscinodiscus were found in the >63-µm fraction in Sample 202-1235A-18H-CC.
Given that biostratigraphically useful diatom species are not observed in Hole 1235A, no age assignments are possible.
Silicoflagellates, radiolarians, sponge spicules, and phytoliths are also observed in most samples.