BIOSTRATIGRAPHY

Calcareous nannofossils, planktonic and benthic foraminifers, and diatoms were examined at Site 1232. The abundance of calcareous microfossils is generally low, and their preservation is generally poor to moderate. Reworking of microfossils is apparent in all the fossil groups examined. In particular, freshwater and benthic diatoms are present in most of the samples, indicating relatively persistent redeposition from the upper continental margin. Calcareous nannofossils and planktonic foraminifers suggest that the entire sequence cored at the site is Pleistocene in age.

Calcareous Nannofossils

All the core catcher samples, plus additional samples from the split cores, were examined for calcareous nannofossils. Calcareous nannofossils are generally rare to few in most samples and barren in a number of samples. The preservation of nannofossils is generally poor to moderate. Reworking of nannofossils (mostly Neogene in age) is apparent in many samples.

Emiliania huxleyi was observed down to 143.7 mcd (Sample 202-1232A-13X-CC) (Table T8). Thus, an age of 0.26 Ma can be assigned to the interval of 137.9-138.8 mcd (Samples 202-1232A-13X-CC and 14X-1, 29 cm). One specimen of Pseudoemiliania lacunosa was found in each of the following samples: Samples 202-1232A-26X-CC (263.5 mcd), 30X-CC (303.0 mcd), and 32X-CC (322.7 mcd). This suggests that the interval from Section 202-1232A-26X-CC (273.4 mcd) downhole is older than 0.46 Ma. Medium-sized Gephyrocapsa spp. are present down to the bottom of the hole, which suggests a basal age younger than 1.69 Ma.

Planktonic Foraminifers

Planktonic foraminifers were examined in all core catcher samples from Hole 1232A and in selected samples from Cores 202-1232A-1H and 8H. The soft clay in the samples was easily removed by being soaked in warm water and washed through a 63-µm sieve. The abundance of planktonic foraminifers at Site 1232 fluctuates markedly (between >50% and <1% of the >63-µm residue), and a few samples are barren. Preservation varies significantly, and evidence of dissolution, frequent test breakage, and test infills are observed, particularly in samples with low test abundance.

The planktonic foraminiferal assemblage includes Globorotalia truncatulinoides, Globorotalia inflata, Globigerina bulloides, Globigerinita glutinata, Globigerina falconensis, Neogloboquadrina pachyderma, and Orbulina universa (Table T9). This assemblage is present intermittently down to the base of Hole 1232A (Sample 202-1232A-39X-CC), indicating the Pleistocene Zone Pt1 of Berggren et al. (1995). Globorotalia tosaensis and Globorotalia crassaformis are both present in the lower part of the section in the interval from 273.4 to 317.5 mcd (interval 202-1232A-26X-CC through 36X-CC). G. tosaensis is the species used by Berggren et al. (1995) to subdivide the zone into two the Subzones Pt1a (present-0.65 Ma) and Pt1b (0.65-1.77 Ma). However, it is difficult to apply here for a similar purpose, as poor preservation makes reliable recognition of the taxon difficult. Therefore, the true last appearance of the taxon cannot be precisely determined in this section. The composition of the assemblage generally reflects temperate transitional climatic conditions.

Benthic Foraminifers

Benthic foraminifers were studied in all core catcher samples from Holes 1232A and in selected samples from Cores 202-1232A-1H and 8H. Benthic foraminifers are present in many of the samples studied. However, abundance varies markedly, and the ratio of benthic to planktonic foraminifers fluctuates significantly. Preservation is generally moderate or poor. The Pleistocene assemblage includes Cibicidoides spp., Coryphostoma sp., Eggerella bradyi, Globobulimina pacifica, Globocassidulina subglobosa, Globulina spp., Gyroidinoides orbicularis, Hoeglundina elegans, Melonis affinis, Melonis pompilioides, Oridorsalis umbonatus, Pullenia bulloides, Pyrgo murrhina, Pyrgo serrata, Uvigerina peregrina, and Uvigerina hispida.

Tests appear frequently blackened with mud infill and abraded, showing clear evidence of reworking. Samples 202-1232A-8H-5, 136-138 cm; 8H-5, 145-147 cm; and 8H-5, 148-150 cm, respectively (below, within, and above a distinct turbidite layer), were analyzed to detect any changes in provenance and assemblage composition related to lithology. Composition did not vary significantly in the three samples studied. Even samples from the presumed "hemipelagic" intervals above and below the distinct turbiditic interval were found to contain some blackened abraded tests with mud infills (in particular Uvigerina spp. and Cibicidoides spp.), suggesting significant reworking prior to sediment deposition.

Diatoms

All core catcher samples from Hole 1232A as well as some additional samples from the split cores were analyzed. Most samples contain diatoms; abundance varies between common and trace, and preservation is poor to moderate. Preservation deteriorates and abundance decreases below Core 202-1232A-20X. Diatom assemblages vary in character between typical upwelling and freshwater dominated.

Upwelling-related assemblages were defined based on the dominance of resting spores of the genus Chaetoceros and the common occurrence of Thalassiosira, mainly forms of the eccentrica group. Freshwater-dominated assemblages are mainly composed of Aulacoseira granulata with minor contributions of Eunotia, Fragilaria, Melosira, Cymbella, and Cyclotella spp. Freshwater assemblages are mainly associated with the coarser sediments but are present in most samples. Benthic forms such as Cocconeis and Navicula are also present in most samples, except for the intervals between Cores 202-1232A-11H and 13X, 31X and 33X, and 34X and 39X. Given that these forms can only live in shallow waters (water depth of <100 m), they were interpreted as displaced.

Typical oceanic warm-water forms such as Azpeitia nodulifer, Fragilariopsis doliolus, and Hemidiscus cuneiformis are found as minor contributors in "hemipelagic" layers sampled from the split cores (Samples 202-1232A-4H-2, 10 cm; 7H-3, 121 cm; 7H-5, 44, 60, and 65 cm; 8H-5, 55 and 80 cm; 19X-1, 93 cm; 34X-3, 120 cm; and 34X-5, 54 cm) (Table T10).

Because no biostratigraphically useful marker species were found, age assignments were not possible.

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