Calcareous nannofossil biostratigraphy for Site 1148 was done mainly on core-catcher samples from Holes 1148A and 1148B and from selected samples within the cores (Table T7; Fig. F15). Sediments at Site 1148 yielded abundant nannofossils whose preservation varied downhole. Above 367.4 mcd (middle Miocene through Pleistocene), the nannofossils exhibit moderate to good preservation with moderate to strong etching on placoliths in different intervals (e.g., good preservation at 0.7 mcd and from 213.5 to 261.7 mcd; strong etching at 54.2 mcd, from 104.6 to 115.3 mcd and from 329.0 to 338.5 mcd). These variations in nannofossil preservation are possibly caused by fluctuations in the lysocline since the middle Miocene. Nannofossils are moderately preserved below 367.4 mcd with Discoaster spp. showing various degrees of postburial overgrowth within different intervals.
Reworked nannofossils are commonly seen at this site. Early Miocene Sample 184-1148A-44X-CC (425.3 mcd) contains common late Oligocene nannofossil species (e.g., Reticulofenestra bisectus, Sphenolithus delphix, and Zygrhablithus biijugatus) that indicate pronounced reworking.
Thirty-nine nannofossil biostratigraphic datums were recognized in the early Oligocene to Pleistocene sediments at Site 1148 (Table T8). The first occurrence (FO) of Discoaster druggii, a marker for the bottom of Zone NN2, is difficult to identify between 405.7 and 472.7 mcd because of heavy overgrowth. A few moderately preserved D. druggii were observed at 425.3 and 454.4 mcd, which place these two samples within Zone NN2 (19.2-23.2 Ma). Several discoaster specimens that resembled D. druggii were observed at 454.4 mcd, suggesting that this sample is probably still in Zone NN2. No nannofossil index species can be identified confidently in Sample 184-1148A-49X-CC (472.7 mcd). At 477.3 mcd, common Sphenolithus ciperoensis, the marker for Zone CP19 (Zone NP24 and the lower part of Zone NP25), was observed. Its common occurrence places the sample in the lowermost part of Zone CP19b (lower part of Zone NP25). Abundant Sphenolithus distentus was also seen at 477.3 mcd, which places this sample in the uppermost part of Zone NP24 (>27.5 Ma). These data suggest that sediments between the lowermost part of Zones NP25 and NN2 are missing at Site 1148. This interpretation agrees with the lithologic and physical property changes observed in the same interval (see "Lithostratigraphy" and "Physical Properties").
S. distentus does not occur continuously at Hole 1148A; therefore, its first occurrence was not used as a biozone marker. Nannofossil assemblages and biomarkers above 716.4 mcd in Hole 1148B are similar to those in Hole 1148A. Reticulofenestra umbilicus does not occur between 716.4 and 851.4 mcd, which suggests that the bottom of Hole 1148B is still within Zone NP23 (<32.3 Ma).
Planktonic foraminifers were examined in all core-catcher samples from Hole 1148A and in core-catcher samples from Hole 1148B below 710 mcd. Removing the clay in these samples often required soaking in a heated solution of equal parts Calgon, hydrogen peroxide, and tap water for several hours before washing through a 150-µm sieve. A 63-µm sieve was used for washing samples below ~490 mcd because of the smaller size of the planktonic foraminiferal biomarkers. Site 1148 yields poor to moderately preserved planktonic foraminifers as documented by frequent test breakage (fragmentation >10%), clear evidence of genera-specific dissolution (e.g., Globigerinoides), and recrystallization in the lower section (below ~510 mcd). In general, the upper part of Hole 1148A has moderate preservation that rapidly degrades below ~110 mcd.
At several intervals in Hole 1148A, the abundance of the planktonic foraminifers varied greatly from one core catcher to the next. Qualitatively, although the amount of core washed may have varied by as much as a factor of two between samples, the yield obtained after washing may have changed by an order of magnitude or more. This difference was particularly noticeable in the intervals 300.0-309.4 mcd, 329.0-338.5 mcd, and 357.7-367.4 mcd.
The planktonic foraminiferal biostratigraphy for Site 1148 is based on the combined study of Hole 1148A and the lower part of Hole 1148B (see Tables T7, T9). The biostratigraphy of Site 1148 had several notable conventions and exceptions.
Within Zone N22, we used the last occurrence (LO) (0.12 Ma; Thompson et al., 1979) and FO (0.40 Ma; Li, 1997) of pink Globigerinoides ruber as two biostratigraphic control points. We noted that both the FO of Globigerinoides fistulosus and the FO of Globorotalia truncatulinoides (bottom of Zone N22; Blow, 1969) appeared at 125.8 mcd.
For Zone N21, the LO of Globorotalia multicamerata was found at 135.5 mcd, which corresponds to an age of 2.4 instead of 3.09 Ma (see Table T3 in the "Explanatory Notes" chapter). This observation is supported by studies of the South China Sea northern shelf (Wang et al., 1991). The coiling change of Pulleniatina from sinistral to dextral was quite distinct and served as a useful marker for the bottom of Zone N20 (169.5 mcd). At Site 1148, we observed that the FO of Sphaeroidinella dehiscens s.l. appeared at 184.5 mcd and used it as a marker for the bottom of Zone N19.
Although Pulleniatina primalis is observed higher in the section, we did not find its FO to be a useful marker for the bottom of Zone N17b. Instead, we relied on the FO of Globigerinoides conglobatus as an indicator of Zone N17b (203.8 mcd). Because we observed both dextral and sinistral forms of Neogloboquadrina acostaensis throughout Zone N17a, its coiling change was not used as a datum to mark 6.6 Ma. The FO of Globorotalia pleisotumida was seen at 261.66 mcd and marked the bottom of Zone N17a. Gradational specimens of N. acostaensis and Paragloborotalia mayeri were found down to a depth of 279.5 mcd; we thus assigned cores above this level an age of N16-N15.
The FO of Orbulina spp. was quite distinct and clearly marked the bottom of Zone N9 (309.5 mcd), whereas the FO of Praeorbulina sicana was used to mark the lower boundary of Zone N8 (348.3 mcd). At this site, we used the multiple occurrence of Paragloborotalia kugleri, Globoquadrina dehiscens, and Globigerinoides trilobus to indicate the lowermost Miocene (Zone N4) at 454.4 mcd. Below this level, we found Paragloborotalia opima opima of Zone P21b at 477.3 mcd. This suggests a hiatus between the lowermost Miocene and the upper Oligocene and that sediments between the lowermost Zone N4 and the lowermost Zone P22 are missing at Site 1148.
Although gradational specimens Globoturborotalia anguilisuturalis and Globigerina ciperoensis were observed, we used the FO of typical G. anguilisuturalis and the LO of Turborotalia ampliapertura to define the top (593.6 mcd) and bottom (639.3 mcd), respectively, of Zone P20. In the last core-catcher sample, we observed Paragloborotalia opima nana but not P. opima opima and Pseudohastigerina. This confines the bottom of Hole 1148A (716.4 mcd) to within Zone P19 (30.6 to 32.0 Ma).
Selected core-catcher samples from Hole 1148B were examined from 733.9 to 869.5 mcd (bottom of hole). We observed T. ampliapertura and Cassigerinella chipolensis—but not Pseudohastigerina—in all samples in this interval. These observations led us to assign an age of P19 to the interval and the bottom of Hole 1148B.
Site 1148 yields few to abundant deep-sea benthic foraminifers. The ratio of benthic to planktonic foraminifers is high in Hole 1148A because of strong carbonate dissolution. The benthic foraminifers (e.g., Heterolepa, Gavelinopsis, Globocassidulina, Martinottiela, Sigmoilopsis, Textularia, and Uvigerina) in the lower part of Hole 1148A (>~ 510 mcd) are comparable with those observed at 1000-2000 m in the modern South China Sea. An increase in the abundance of Globobulimina and Chilostomella (indicative of high productivity) was observed in the upper (above ~50 mcd) and lower (below ~500 mcd) sections of Hole 1148A. This corresponds to the higher organic carbon content and abundant siliceous fossil content (radiolarians and diatoms) found in the two intervals. The LO of Stilostomella was observed at a depth of 43.7 mcd, which (for the latitude of this site) assigns it an age of 0.75 Ma (Schönfeld, 1996).
At Site 1148, calcareous nannofossils are common to abundant, but preservation varies in different intervals. Planktonic foraminifers are abundant but with poor to moderate preservation. Benthic foraminifers are generally few to common but become more abundant in lower sections.
An age-depth plot shows that the biohorizons from the three fossil groups generally agree except for the interval of ~454 to ~477 mcd (Fig. F15). The Pleistocene/Pliocene boundary is constrained by the LO of Calcidiscus macintyrei and Discoaster brouweri and is located between 125.8 and 135.5 mcd. The Pliocene/Miocene boundary is constrained by the LO of Triquetrorhadulus rugosus and the LO of Discoaster quinqueramus between 184.5 and 193.8 mcd. Both the calcareous nannofossil and planktonic foraminifer data suggest a hiatus between the lowermost Miocene and upper Oligocene (~454 and ~477 mcd, respectively). The sedimentation rate at Site 1148 has been calculated based on biostratigraphic data (Table T7) and is depicted in Figure F15.