Calcareous nannofossils from Site 1143 were investigated in smear slides from core-catcher samples and from selected samples to better constrain the biohorizons. Site 1143 yielded abundant and well-preserved nannofossils in Holocene to Pliocene sections. In the upper Miocene, the nannofossils remained abundant, but their preservation gradually deteriorated downhole. The deterioration is indicated by increasing calcite overgrowth on discoasters and dissolution of placoliths, until finally the discoasters are difficult or impossible to identify to the species level. A complete calcareous nannofossil biostratigraphy for Site 1143 is based on the compilation of detailed studies on Hole 1143A and the lower part of Hole 1143C (see Tables T7, T8).
Emiliania huxleyi is present continuously from the top of Hole 1143A down to 13.2 mcd, assigning the interval to Zone NN21/CN15. A distinct decrease in the relative abundance of E. huxleyi from 2.9 mcd to 13.2 mcd constrains the uppermost 2.9-m interval to younger than 90 ka. The interval between 15.7 mcd and 26.8 mcd contains abundant Gephyrocapsa spp. but lacks E. huxleyi and Pseudoemiliania lacunosa; it was assigned to Zone NN20/CN14b with an age range of 260 to 460 ka. At 27.7 mcd, the last occurrence (LO) of P. lacunosa marks the NN19b/CN14a to NN20/CN14b zonal boundary. Samples 184-1143A-6H-CC (53.7 mcd) and 7H-CC (63.4 mcd) contain rare to very rare, medium and large Gephyrocapsa spp. and an overwhelming dominance of various small Gephyrocapsa spp. (if excluding Florisphaera profunda): hence, these samples were assigned to the mid-Pleistocene small Gephyrocapsa acme zone (1.01-1.22 Ma). The next lower biohorizon, the LO of Calcidiscus macintyrei, was encountered at 74.2 mcd and indicates an age of 1.59 Ma (NN19a) for this stratigraphic level.
In terms of calcareous nannofossils, the Pliocene/Pleistocene boundary (1.77 Ma, Berggren et al., 1995) is located below the first occurrence (FO) of medium-sized Gephyrocapsa spp. (>4 mm; i.e., Gephyrocapsa lumina, Gephyrocapsa carribeanica, or Gephyrocapsa oceanica). This biohorizon has been dated at 1.69 Ma and is above the LO of Discoaster brouweri, which has an age of 1.95 Ma. The identification of the latter is unreliable at Site 1143 because of the consistent occurrence of reworked discoasters in the uppermost Pliocene and the lowermost Pleistocene. The process of redeposition will likely obscure the LO datum but will not affect the FO datum. The FO of medium-sized Gephyrocapsa spp. was identified at 93.5 mcd, but rare D. brouweri and Discoaster pentaradiatus also occurred in the same sample. D. brouweri is always very rare approaching its extinction level, and its LO is estimated at 94.3 mcd. Its abundance increases distinctly from 98.2 mcd downward. The LO of D. pentaradiatus is tentatively placed at 111.2 mcd because of its consistent occurrence and higher abundance below this level. The LO of D. surculus was found at 119.7 mcd, which assigns an age of 2.53 Ma to this level.
Sphenolithus abies and Sphenolithus neoabies are the last two survivors of sphenoliths, and they disappeared from the oceans in the latest Zanclean. Their highest occurrence in Hole 1143A was at 153.0 mcd, which assigns an age of 3.66 Ma to this level. Reticulofenestra pseudoumbilicus is identified here as specimens whose maximum diameter is larger than 7 µm in its uppermost range (the lower Pliocene), which is in accord with the size of the holotype (Gartner, 1967). Based on this definition, the LO of R. pseudoumbilicus is identified slightly below that of S. abies at 153.0 mcd. Discoaster asymmetricus occurred sporadically in the upper Miocene and the lowest Pliocene but became slightly more abundant and consistently present in samples above 195.2 mcd. This first "common" occurrence datum of D. asymmetricus is used to separate Zones NN13/CN11a and NN14/CN11b. The extinction of Triquetrorhabdulus rugosus (5.23 Ma) occurred shortly after the end of the Messinian and predates the evolution of Ceratolithus rugosus (5.1 Ma). These two datums are useful bioevents to constrain the Pliocene/Miocene boundary, but, unfortunately, they were not found in the uppermost Miocene and lowermost Pliocene at Site 1143. Thus, the Pliocene/Miocene boundary at Site 1143 is located between the FO (5.37 Ma) and LO (4.99 Ma) of Ceratolithus acutus at 213.0 and 200.6 mcd, respectively.
The LO of Discoaster quinqueramus was found at 220.7 mcd, which defines the upper boundaries of Zone NN11/CN9d and gives an age of 5.54 Ma to this level. Amaurolithus amplificus occurs at 244.0 through 259.7 mcd, which constrains this interval from 5.99 to 6.76 Ma. The oldest nannofossil datum documented in Hole 1143A is the FO of Amaurolithus primus, which is identified at 347.6 mcd. The interval from 357.8 mcd to 413.6 mcd yielded few D. quinqueramus and common Discoaster berggrenii, placing the bottom of Hole 1143A in Subzone CN9a (the lower part of Zone NN11).
Hole 1143C was drilled down to 500 mbsf, 100 m deeper than Hole 1143A, in expectation of penetrating the lower upper Miocene. The FO of D. berggrenii that defines the boundary between Zones NN10 (CN8) and NN11(CN9) was documented at 412 mcd, and it assigns an age of 8.2 Ma to this level. Sample 184-1143C-49X-CC (470.61 mcd) consists mostly of volcanic ash with few calcareous nannofossils. The nannofossil assemblage in this sample contains Discoaster hamatus, which occurred exclusively in Zone NN9, and Eocene to early Miocene species Reticulofenestra umbilicus, Sphenolithus ciperoensis, Sphenolithus belemnos, and Sphenolithus heteromorphus. This suggests that the deposition of volcanic ash was accompanied by extensive redeposition from older strata. Lithologies in the interval below Core 184-1143C-49X are very similar to those above. Nannofossil assemblages in the interval between Samples 184-1143C-50X-CC (480.0 mcd) and 54X-CC (512.9 mcd) indicate increased dissolution, whereas discoasters are very difficult to identify to species level because of heavy overgrowth. However, Discoaster neohamatus can still be recognized in these samples. This places the bottom of Hole 1143C in Zone NN9/CN7 with an age greater than 9.6 Ma (Berggren et al., 1995).
Planktonic foraminifers were examined from all core-catcher samples at Site 1143. 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.
Site 1143 yielded a moderate abundance of planktonic foraminifers, except for Sample 184-1143C-49X-CC (470.6 mcd), which consisted almost entirely of volcanic ash (glass) and very few planktonic foraminifers. Sample 184-1143A-31X-CC (289.1 mcd) also had a very low yield of foraminifers and contained mostly clay. Samples 184-1143A-37X-CC (347.6 mcd) and 184-1143B-35X-CC (327.4 mcd) yielded high amounts of planktonic foraminifers, possibly because of downslope transport.
Planktonic foraminiferal preservation at Site 1143 was good to very good, as documented by infrequent test breakage (fragmentation <10%), little to no evidence of dissolution or diagenetic alteration, the presence of delicate species such as Beella digitata, and the observation of numerous clear tests.
A complete planktonic foraminiferal biostratigraphy for Site 1143 is based on the compilation of detailed studies on Hole 1143A and the lower part of Hole 1143C (see Tables T7, T9 for complete details). The biostratigraphy of Site 1143 had several notable conventions and exceptions.
Within Zone N22 we used the LO (0.12 Ma; Thompson et al., 1979) and FO (0.40 Ma; Li, 1997) of pink Globigerinoides ruber as biostratigraphic references. We noted that the LO of Globigerinoides fistulosus and the FO of Globorotalia truncatulinoides (bottom of Zone N22) occurred within Sample 184-1143-10H-CC (93.5 mcd) (Blow, 1969).
Within Zone N21, the LO of Globorotalia multicamerata was found to occur at a level corresponding to an age of 2.4 Ma instead of at its datum of 3.09 Ma. This observation is supported by studies of the South China Sea northern shelf (Wang et al., 1991). The bottom of Zone N21 was defined by the FO of Globorotalia tosaensis at 142.4 mcd. The coiling change of Pulleniatina spp. from sinistral to dextral (161.7 mcd) was quite distinct and served as a marker for the bottom of Zone N20. At Site 1143, Sphaeroidinella dehiscens disappeared within Zone N20, and so we were not able to use this species as a marker for the bottom of Zone N19. Thus, having only the FO of Globorotalia tumida to mark the bottom of Zone N18, we assigned the ages of several core catchers to Zones N19-N18 (213.0-241.0 mcd). The FO of common Globorotalia margaritae was used to help indicate the bottom of Zone N19 and the latest late Miocene.
Although Pulleniatina primalis was 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 and G. margaritae as markers for Subzone N17b. Also, the coiling change of Neogloboquadrina acostaensis at 6.6 Ma (within Subzone 17a) was found to be inconsistent, and it was not used as a datum. We marked the bottom of Subzone 17a by both the LO of Globigerinoides extremus and Globorotalia plesiotumida, resulting in an age of 8.58 Ma. Because gradational forms of the species N. acostaensis and Globorotalia mayeri were observed from 452 mcd to 470.6 mcd, we assigned this interval to a combined zone of N15-N16 (8.58 to 10.49 Ma). Below this level and to the bottom of the hole, we observed typical G. mayeri and Globoturborotalita nepenthes. Thus, we assigned an age of <11.19 Ma (Zone N14) to the bottom of the hole (512.9 mcd).
Although the paleontological focus of Leg 184 was to investigate calcareous nannofossils and planktonic foraminifers, we also tried to make note of the benthic foraminifers in the samples collected at Site 1143. Generally, Site 1143 yielded rare deep-sea benthic foraminifers that comprised ~3% of the total foraminiferal fauna. The proportion of benthic foraminifers increased markedly at ~44 and 224 mcd, possibly indicating high organic carbon flux and/or increased carbonate dissolution. The LO of one useful genus of benthic foraminifer Stilostomella was observed at 53.71 mcd, which indicates an age of ~0.75 Ma (Schönfeld, 1996) for this latitude.
At Site 1143, calcareous nannofossils are abundant and well preserved above the Pliocene, although preservation deteriorates through the Miocene. Planktonic foraminifers are abundant and have good preservation for the entire interval at this site. Benthic foraminifers are generally very rare but become more abundant in two short intervals.
An age-depth plot shows that biohorizons from the three fossil groups generally agree with each other (Fig. F11). The Pleistocene/Pliocene boundary is constrained by the FO of medium Gephyrocapsa spp., the LO of G. fistulosus, and the LO of D. brouweri and is located between 93.5 and 94.3 mcd. The Pliocene/Miocene boundary is constrained by the FO and LO of C. acutus, between 213.0 and 200.6 mcd. The sedimentation rate at Site 1143 has been calculated based on biostratigraphic data (Table T7; Fig. F11) and is 54 m/m.y. in the Pleistocene, 32 m/m.y. in the Pliocene, 63 m/m.y. in the uppermost Miocene, and 72 m/m.y. between 7.39 and 8.58 Ma. Below 432.8 mcd, the sedimentation rate was not calculated because the last two datums (LO G. mayeri and FO D. neohamatus) showed a wide disparity.