During Leg 184, calcareous nannofossils and planktonic foraminifers were studied at six sites, one located on the southern margin and the remainder on the northern continental slope of the South China Sea. With the exception of Site 1148, all sites were triple cored using the APC and XCB systems.
Preliminary ages were assigned to core-catcher samples. Samples from within the cores were examined when a more refined age determination was necessary and when time permitted.
The ages for biostratigraphic datums were compiled mainly from Berggren et al. (1995a, 1995b), Raffi and Flores (1995), Backman and Raffi (1997), Kameo and Bralower (2000), and I. Raffi (unpubl. data). Two Quaternary nannofossil datums, the last occurrence (LO) and first occurrence (FO) of Gephyrocapsa (small) acme (Gartner, 1988), were also used. Astrochronologically tuned biostratigraphic datums were used whenever possible (Table T2). Estimates of biostratigraphic ages were calibrated against the magnetic polarity time scale of Cande and Kent (1995) and Berggren et al. (1995a, 1995b).
For simplicity, the astrochronologically tuned planktonic foraminifer ages of Berggren et al. (1995a, 1995b) were used for the Holocene to 4.70 Ma. The tuned ages of Chaisson and Pearson (1997), Pearson and Chaisson (1997), and Curry, Shackleton, Richter, et al. (1995) were used for 4.70 to 13.42 Ma. Ages older than 13.42 Ma were untuned and taken from Berggren et al. (1995a, 1995b) (Table T3). In addition to the above ages, we also adopted the use of the LO (0.12 Ma; Thompson et al., 1979) and FO (0.40 Ma; Li, 1997) of Globigerinoides ruber (pink) as biostratigraphic references.
The biostratigraphic zonation of calcareous nannofossils is based upon the studies of Okada and Bukry (1980) and Martini and Müller (1986). Planktonic foraminifer biostratigraphic zonation is derived from the studies of Blow (1969) (Fig. F5).
Ages of Cenozoic chronostratigraphic boundaries were established by Berggren et al. (1995a, 1995b). According to this study, the age of the Pliocene/Pleistocene boundary is 1.77 Ma, the age of the Miocene/Pliocene boundary is 5.32 Ma, and that of the Oligocene/Miocene is 23.80 Ma, as given in Figure F5.
Whenever possible, we also tried to note the LO of the benthic foraminifers Stilostomella. Several species of this genus disappeared from the global ocean at different latitudes during the interval 1.0-0.6 Ma. For the latitudes of Leg 184, we used 0.75 Ma as the LO of Stilostomella (Schönfeld, 1996).
Several Gephyrocapsa species are commonly used as biostratigraphic markers. However, only two morphological groups, medium and small Gephyrocapsa spp., were used during shipboard study. Gephyrocapsa (medium) spp. includes specimens from G. oceanica, G. lumina, and G. caribbeanica with maximum lengths >3.5 µm. Gephyrocapsa (small) spp. is used for various Gephyrocapsa species with a maximum length <3 µm.
Several Reticulofenestra species have been used as Neogene and Quaternary biostratigraphic markers. They are mainly distinguished by coccolith size and the relative magnitude of the central opening. However, some species show a great range of variation in these parameters, causing problems in identification (Backman, 1980; Gallagher, 1987; Young, 1990; Su, 1996). Therefore, only two Reticulofenestra species were used as stratigraphic markers during Leg 184. Reticulofenestra umbilicus can be easily distinguished from other species by its enormous size. Similarly, R. pseudoumbilicus is identified by specimens having a maximum coccolith length >7 µm in its uppermost range (the lower Pliocene), which is in accord with the size of the holotype (Gartner, 1967). Identification of other calcareous nannofossils mainly follows the compilation of Perch-Nielsen (1985).
Calcareous nannofossils were examined using standard light microscope techniques under crossed polarizers and transmitted light at 1000×-2000× magnification. The degrees of preservation and abundance of calcareous nannofossil species were noted as follows:
VG = very good preservation (no evidence of dissolution and/or overgrowth);
G = good preservation (slight dissolution and/or overgrowth; specimens are identifiable to the species level);
M = moderate preservation (exhibit some etching and/or overgrowth; most specimens are identifiable to the species level); and
P = poor preservation (severely etched or with overgrowth; most specimens cannot be identified at the species and/or generic level).
The relative abundance of individual calcareous nannofossil species is estimated (at 1000× magnification) based on a five-category scheme:
D = dominant (>50%, or 100 specimens per field of view);
A = abundant (10%-50%, or 10-100 specimens per field of view);
C = common (1%-10%, or 1-10 specimens per field of view);
F = few/frequent (0.1%-1%, or 1 specimen per 1-10 fields of view); and
R = rare (<0.1%, or <1 specimen per 10 fields of view).
Taxonomic concepts for Neogene and Paleogene taxa follow those of Kennett and Srinivasan (1983), Bolli and Saunders (1985), and South China Sea Branch of Petroleum Corporation of the People's Republic of China et al. (1981).
Core-catcher samples (plus one sample per section where necessary) were soaked in a weak Calgon/hydrogen peroxide solution, warmed on a hot plate, and washed over a 150-µm mesh sieve (with the exception of Site 1148, where a 63-µm sieve was used for deeper cores). Before washing, each sieve was dipped in a solution of methyl blue dye to identify contaminants from previous samples. Lithified material was crushed to pea size, heated in a Calgon/hydrogen peroxide solution, and then sieved as before. All samples were dried on a hot plate. The dried samples were examined under a binocular microscope, and planktonic foraminifer faunal composition was recorded in qualitative terms based on an assessment of forms observed in a random sample of 200-400 specimens from the >150-µm size fraction. Relative abundances were reported using the following categories:
D = dominant (>30%);
Preservation of planktonic foraminifer assemblages was recorded as
VG = very good (no evidence of breakage or dissolution);
G = good (>90% of specimens unbroken with only minor evidence of diagenetic alteration);
M = moderate (30%-90% of the specimens unbroken); and
P = poor (strongly recrystallized or dominated by fragments and broken or corroded specimens).