Calcareous nannofossil biostratigraphy for Site 1144 was done mainly on core-catcher samples from Hole 1144A and supplemented by selected core-catcher samples from Holes 1144B and 1144C (Tables T6, T7; Fig. F9). Sediments recovered at Site 1144 yield abundant nannofossils that are generally well preserved above 387.8 mcd but are slightly to moderately dissolved below that level.
A middle to upper Pleistocene calcareous nannofossil assemblage was recognized, which is characterized by abundant Emiliania huxleyi, Gephyrocapsa caribbeanica, Gephyrocapsa oceanica, and Pseudoemiliania lacunosa (Table T7). Florisphaera profunda was less abundant (<20%) in comparison to the Pleistocene nannofossil assemblage at Site 1143 (>50%). This probably reflects the greater overall productivity at Site 1144 (i.e., less light penetration to the thermocline where F. profunda resides).
Five nannofossil datums were determined, as given in Table T6 and Fig. F9. Because of the frequent occurrences of reworked Miocene to lower Pleistocene nannofossils, the two late Pleistocene biostratigraphic markers, P. lacunosa and Reticulofenestra asanoi, are almost continuously present in Pleistocene sediments (Table T7; Fig. F10). This leads to difficulties in determining the levels of their last occurrences. To obtain better biostratigraphic control, the relative abundances of P. lacunosa and R. asanoi were determined by counting nannofossils. The abundance of P. lacunosa shows two distinct intervals: above 288.7 mcd, its abundance is lower than 20 specimens/10 fields of view; below this level, its abundance increases significantly. Also, observations of other reworked fossils show no distinct increase in reworking below this level (Fig. F10A). This suggests that the increase in P. lacunosa below 288.7 mcd is not a result of reworking and places the last occurrence (LO) of P. lacunosa at this level. A significant increase in the abundance of R. asanoi was noted at 417.0 mcd (Fig. F10B). Furthermore, various gradational morphological changes between P. lacunosa and R. asanoi were observed below 422.7 mcd. These morphological changes occur only within the interval of R. asanoi (1.16 to 0.90 Ma) in the Northeast Atlantic (Su, 1996). This evidence allows us to assign the LO of R. asanoi at 422.7 mcd. The interval from 505.3 to 518.5 mcd is assigned to the Gephyrocapsa (small) acme zone based on the disappearance of large and medium Gephyrocapsa spp., which indicates an age range of 1.01-1.20 Ma.
Planktonic foraminifers were also examined in all core-catcher samples from Hole 1144A and selected core-catcher samples from Holes 1144B and 1144C. The very soft sediments at this site allowed the clay to be easily removed from these samples by soaking them in a dilute Calgon and hydrogen peroxide solution and washing through a 150-µm sieve. Site 1144 yielded a moderate abundance of well-preserved planktonic foraminifers. This preservation was documented by infrequent test breakage (fragmentation <8%; see Fig. F11), little to no evidence of dissolution or diagenetic alteration, and the observation of numerous clear tests.
The planktonic foraminiferal biostratigraphy for Site 1144 is based on Hole 1144A (see Table T8 for details). Because Globorotalia truncatulinoides is present continuously down the full length of this hole (518.5 mcd), this interval is assigned to Zone N22 (Blow, 1969). The complete absence of Globigerinoides fistulosus constrains the age of the sediments in Hole 1144A to younger than 1.77 Ma. 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 (Table T6).
As shown by previous studies in the South China Sea and Pacific Ocean (Wang et al., 1995; Le and Shackleton, 1992), the ratio between tropical species Globorotalia menardii and temperate species Globorotalia inflata, the relative abundance of Pulleniatina obliquiloculata, and the planktonic foraminiferal fragmentation increased during the interglacial stages, whereas the ratio between pteropods and planktonic foraminifers increased during the deglacial and glacial stages (Wang et al., 1997) (Fig. F11C, F11D, F11E, F11F). On the basis of planktonic foraminiferal and calcareous nannofossil events (Table T6), we can tentatively recognize glacial-interglacial MISs 1-9 from changes in the above-mentioned four indicators (Fig. F11). However, we cannot identify any glacial-interglacial cycles before MIS 9 (below ~250 mcd); sample resolution is very low because of the decreased sedimentation rate (Fig. F9; Table T6).
The benthic foraminifers observed at Site 1144 comprised ~4% of the total foraminiferal fauna. The relative abundance of benthic foraminifers, indicative of high organic carbon flux (e.g., Globobulimina spp., Chilostomella ovoidea, and Uvigerina peregrina), is very high above ~240 mcd and then decreases downhole (Fig. F11). This trend corresponds well to the changes in the group abundance of siliceous microfossils such as radiolarians and diatoms (Fig. F11) and organic carbon content in Hole 1144A, indicating that productivity decreases downhole. In particular, Hole 1144A can be divided into three sections based on the changes in productivity and sedimentation rate. For the upper section, 0 to ~240 mcd, the high productivity corresponds to a high sedimentation rate. The middle section, between ~240 and ~420 mcd, has a low sedimentation rate with a moderate productivity. However, the low productivity in the lower section, below 420 mcd, corresponds to a high sedimentation rate rather than a low one (Fig. F11).
One useful genus of benthic foraminifer, Stilostomella, was observed at 365.5 mcd (Sample 184-1144A-35X-CC). This genus has several species that disappeared from the global ocean at different latitudes during the time interval of 1.0-0.6 Ma. For the latitude of this site, we used 0.75 Ma as the LO of Stilostomella (for the northern South China Sea) (Schönfeld, 1996).
At Site 1144, both planktonic foraminifers and calcareous nannofossils are abundant and well preserved, whereas the nannofossils are occasionally to commonly reworked. Benthic foraminifers are generally few in number.
An age-depth plot shows that biohorizons from the three fossil groups generally agree with each other (Fig. F9). The maximum ages of Holes 1144A, 1144B, and 1144C are >1.01 Ma, >1.01 Ma, and <0.26 Ma, respectively. The sedimentation rates at Site 1144, calculated based on biostratigraphic data (Table T6; Fig. F9), are 930 m/m.y. after 0.26 Ma, 750 m/m.y. before 0.83 Ma, and 310 m/m.y. in between.