Calcareous nannofossil biostratigraphy for Site 1146 used core-catcher samples of Hole 1146A, selected samples within the cores of Hole 1146A, and core-catcher samples from Hole 1146B (Tables T7, T8; Fig. F11). Sediments recovered at Site 1146 yield abundant nannofossils that are generally well preserved above 531.2 mcd but are moderately overgrown below that level. Many samples exhibited some degree of reworking; however, this was more pronounced in the interval between 419.9 and 460.4 mcd.
Thirty-two nannofossil biostratigraphic datums were recognized in the lower Miocene to Pleistocene sediment sequence at Site 1146 (Table T7). The last occurrence (LO) of Sphenolithus spp. (Sphenolithus abies and Sphenolithus neoabies) was noted in Sample 184-1146B-25X-CC (250.6 mcd); no sediments were recovered from Core 184-1146A-25X.
The Pliocene/Miocene boundary is constrained by the LO of Triquetrorhabdulus rugosus and the LO of Discoaster quinqueramus between 308.4 mcd and 317.9 mcd, respectively. Discoaster loeblichii and Discoaster neorectus are marker species used by Okada and Bukry (1980) to subdivide late Miocene Zone CN8. At Site 1146, specimens that resemble these two species were found to occur down to 438.7 mcd (Zone CN7) and 460.4 mcd (Zone CN6) in Hole 1146A. Thus, these two species are not useful markers for the South China Sea. Sphenolithus belemnos was observed between 635.3 and 643.1 mcd, which places the bottom of Hole 1146A in Zone NN3 (18.3 to 19.2 Ma).
Planktonic foraminifers were examined in all core-catcher samples from Hole 1146A and selected core-catcher samples from Hole 1146C. Site 1146 yields abundant, well-preserved planktonic foraminifers as documented by infrequent test breakage (fragmentation <10%), little to no evidence of dissolution or diagenetic alteration, and the observation of numerous clear tests. In addition, the soft clay in the samples was easily removed by soaking in a warm Calgon and hydrogen peroxide solution and washing through a 150-µm sieve.
A complete planktonic foraminiferal biostratigraphy for Site 1146 is based on all core-catcher samples from Hole 1146A and three core-catcher samples from Hole 1146C (see Tables T7, T9 for details). The biostratigraphy of Site 1146 had several notable conventions and exceptions.
Within Zone N22, we used the LO (0.12 Ma; Thompson et al., 1979) and first occurrence (FO) (0.40 Ma; Li, 1997) of pink Globigerinoides ruber as two biostratigraphic control points. The FO of Globorotalia truncatulinoides was used to mark the bottom of Zone N22 (185.4) (Blow, 1969).
For Zone N21, the LO of Globorotalia multicamerata was found to occur at 206.1 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). Because there was no recovery for Core 184-1146A-25X, we used three core-catcher samples from Hole 1146C to constrain the LOs of Globoquadrina altispira and Sphaeroidinellopsis seminulina and the FO of Globorotalia tosaensis (Table T7). The coiling change of Pulleniatina from sinistral to dextral was quite distinct and served as a marker for the bottom of Zone N20 (256.5 mcd). At Site 1146, we observed that the FO of Sphaeroidinella dehiscens appeared at 337.7 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 of the bottom of Subzone N17b. Instead, we relied on the FO of Globigerinoides conglobatus as an indicator of N17b (360.3 mcd). Because we observed both dextral and sinistral forms of Neogloboquadrina acostaensis throughout Subzone N17a, its coiling change was not used as a datum to mark 6.6 Ma. At this site, we used the FO of N. acostaensis and the LO of Globorotalia mayeri to define the top (419.8) and bottom (449.1 mcd), respectively, of biozone N15. No transitional specimens of these two species were found within this biozone. The FO of Globigerina nepenthes was quite distinct and clearly marked the bottom of Zone N14 (460.4 mcd).
The FOs of Orbulina spp. and Praeorbulina sicana were used to mark the lower boundary of Zones N9 (531.2 mcd) and N8 (594.7 mcd), respectively. In the last core-catcher sample, we observed Globoquadrina binaiensis but not Paragloborotalia kugleri. This confines the bottom of Hole 1146A (643.1 mcd) to within Zone N5, with an age between 19.1 and 21.5 Ma.
Site 1146 yields rare to common deep-sea benthic foraminifers, but the ratio of benthic to planktonic foraminifers increased greatly in the lower part of Hole 1146A, indicating a shallower water depth for sediments below ~560 mcd (Heterolepa, Gavelinopsis, Uvigerina, and Globocassidulina were observed). However, we found no clear evidence for reworked benthic foraminifers from the shelf and upper slope. The LO of Stilostomella was observed at a depth of 104.8 mcd, which is assigned an age of 0.75 Ma for the latitude of this site (Schönfeld, 1996).
At Site 1146 calcareous nannofossils are abundant and well preserved, although preservation deteriorates below ~530 mcd. Planktonic foraminifers are abundant and have good preservation for the site's entire interval. Benthic foraminifers are generally few but become more abundant in the lower part.
An age-depth plot shows that the biohorizons from the three fossil groups generally agree with each other (Fig. F11). The Pleistocene/Pliocene boundary is constrained by the FO of medium-sized Gephyrocapsa spp. and the LOs of Globigerinoides fistulosus and Discoaster brouweri and is located between 185.4 and 195.1 mcd. The Pliocene/Miocene boundary is constrained by the LO of T. rugosus and the LO of D. quinqueramus, between 308.4 and 317.9 mcd, respectively. The sedimentation rate at Site 1146 has been calculated based on biostratigraphic data (Table T7) and is depicted in Figure F11.