The biostratigraphy of high-latitude regions in the Southern Hemisphere was vastly improved by ODP Legs 113, 114, 119, 120, and 177 (e.g., Gersonde et al., 1990; Thomas et al., 1990; Barron et al., 1991; Harwood et al., 1992; Gersonde and Bárcena, 1998; Shipboard Scientific Party, 1999a). An important outcome of these studies was the direct correlation of species-stratigraphic ranges to the geomagnetic polarity time scale (GPTS). The biochronological framework used during Leg 178 basically follows that established during Legs 119 (Barron et al., 1991) and 120 (Harwood et al., 1992; Harwood and Maruyama, 1992). We tie the biochronology to the GPTS of Berggren et al. (1995; BKSA95), which uses the magnetic polarity chrons of Cande and Kent (1995; CK95) for correlation between the magnetostratigraphy and the chronologic time scale (Table T1; Fig. F8). BKSA95 also follows the astronomical time scale values of polarity boundaries from Chron 1n to 3n.4n (Thvera) (0-5.23 Ma) (Shackleton et al., 1990; Hilgen, 1991a, 1991b), which results in coherent and congruent magnetostratigraphic and astronomic chronologies back to 5.23 Ma. Extension of the astronomical time scale into the Miocene Epoch (Shackleton et al., 1995; Shipboard Scientific Party, 1995) and fine tuning of the chronology of the astronomical time scale (e.g., Langereis et al., 1994) are an ongoing process, and differences between astronomical and geomagnetic time scales still exist (Table T2).

Figure F9 illustrates the magnetic calibration and estimated ages of biostratigraphic zones used during Leg 178. Absolute radiolarian and diatom datum ages were converted from published age estimates based on Berggren et al. (1985a, 1985b) to the time scale of Berggren et al. (1995) through linear interpolation between the nearest geomagnetic reversal boundaries (Fig. F8).

Age assignments of the standard epoch/stage boundaries are shown in Table T3. Throughout this volume, "m.y." denotes duration in millions of years, whereas "Ma" denotes an absolute age in millions of years.

The preliminary biostratigraphic age assessments for Leg 178 are determined on the basis of analysis of calcareous nannofossils, foraminifers, radiolarians, and diatoms from core-catcher samples. Stratigraphic constraint of calcareous nannofossil and diatom datums is generally determined on board ship by examining one sample per stratigraphic section (1.5 m). Foraminifers and radiolarian datums are placed on the basis of core-catcher material. Improved dating of Neogene biostratigraphic ranges can be accomplished by correlation with orbitally tuned isotopic signals or other data sets with high temporal resolution, such as color reflectance, magnetic susceptibility, and gamma-ray attenuation porosity evaluator (GRAPE) density.

Numerous diatom biostratigraphic studies have been completed for the Southern Ocean (Gersonde and Burckle, 1990; Baldauf and Barron, 1991; Harwood and Maruyama, 1992; Gersonde and Bárcena, 1998; Shipboard Scientific Party, 1999a). The Neogene and Quaternary diatom zonal scheme used during Leg 178 was primarily that proposed by Harwood and Maruyama (1992, Leg 120) (Figs. F9, F10). Some zone names have been revised following the taxonomic conversion of genus Nitzschia to Fragilariopsis (Round et al., 1990; Hasle, 1993; Gersonde and Bárcena, 1998). The Fragilariopsis kerguelensis Zone was removed, and the Fragilariopsis barronii datum was retained as a subzonal marker because early forms of F. kerguelensis are not distinctive and can be mistaken for F. barronii or Fragilariopsis ritscherii. This change follows the scheme used by Leg 177; thus, some continuity between the legs has been attempted.

All absolute ages for the marker species datums were recalculated to Berggren et al. (1995) (Table T4). The change in ages repositioned some of the boundaries but did not necessitate any major revisions. A new marker species, Thalassiosira oliverana, was adopted for the base of the Nitzschia reinholdii Zone. This species is more structurally distinctive than Thalassiosira miocenica, the species used originally for this datum. The lower boundary age of this zone was not changed by this substitution. Several new zones for the Pleistocene were established in a recent paper (Gersonde and Bárcena, 1998), and the Proboscia barboi Zone of Leg 177 (Shipboard Scientific Party, 1999a) is included. The detailed diatom biostratigraphy of Gersonde and Bárcena (1998) was considered during this cruise, but as its data were drawn from sediment recovered farther north, we chose not to incorporate these zones into our initial biostratigraphic scheme. Note that Gersonde, Hodell, Blum, et al. (Shipboard Scientific Party, 1999a) indicated that the first occurrence (FO) of Thalassiosira vulnifica, which marks the base of the Thalassiosira insigna-T. vulnifica Zone of Harwood and Maruyama (1992), is a diachronous event. They tentatively replaced this zone with the T. insigna Zone and divided it into Subzones a-c. This new zone was defined wholly by the FO and last occurrence (LO) of T. insigna, which was observed in lower abundances at sites closer to the Antarctic continent. Thus, the older zone definition was retained for this study.

Figure F10 illustrates the zonal scheme, paleomagnetic calibration, and marker species datums used during this leg. However, this zonation scheme was not fully applicable to the diatom flora in all the sediments recovered during Leg 178 because of the absence or low abundance of several marker species. Additional investigation will be necessary when examining material with reworked specimens, especially for LO datums.

Smear slides from core-catcher samples were examined routinely for stratigraphic marker species. When required (in material with few specimens), selected samples were processed using hydrogen peroxide and 10% hydrochloric acid. Strewn slides and sieved slides (>20 µm) were also prepared from the acid-cleaned samples, when necessary, to remove excess quantities of clays. Slides were routinely examined using a Zeiss compound microscope at of 630× and 1000×, with the higher power being reserved mainly for taxonomic identification.

Abundance of diatoms was determined by the number of specimens observed per field of view at a magnification of 630× . These abundance estimates were recorded as follows:

A = abundant (>10 valves per field of view);

C = common (>1 valve per field of view);

F = few (>1 valve per 10 fields of view and <1 valve per field of view);

R = rare (>3 valves per traverse of coverslip and <1 valve per 10 fields of view);

+ = present (<3 valves per traverse of coverslip, including an appearance as fragments); and

B = barren (no valves observed in slide).

Preservation of diatoms was determined qualitatively and recorded as follows:

G = good (slight to no fragmentation and dissolution);

M = moderate (moderate fragmentation and dissolution); and

P = poor (severe effects of fragmentation and dissolution).

The radiolarian biostratigraphic scheme proposed by Lazarus (1990, 1992) was applied to Leg 178 material. This zonation is based on the earlier schemes of Hays (1965), Chen (1975), Weaver (1976), Keany (1979), and Caulet (1991) and refined by Lazarus (1992) using sediments recovered during Legs 119 and 120. Species concepts follow those used by Keany (1979), Lazarus (1990, 1992), and references therein. The species events and their respective zones are illustrated with the magnetostratigraphy of Berggren et al. (1995) in Figure F11. Stratigraphic constraint of radiolarian events is determined on the basis of an examination of one sample per core (core catcher).

Sample preparation for radiolarian analysis followed the procedure of Sanfilippo et al. (1985). Samples were sieved at 63 µm. Only samples with significant carbonate fraction were treated with hydrochloric acid. Strewn slides of the residue were made and mounted in Picolyte.

Abundance of total radiolarians is defined as follows:

A = abundant (>100 per traverse);

C = common (50-100 per traverse);

F = few (10-50 per traverse);

R = rare (<10 per traverse);

+ = present (<10 per slide); and

B = barren (no skeletons observed).

Species abundances were recorded as follows:

A = abundant (> 20 per slide);

C = common (10-20 per slide);

F = few (5-10 per slide);

R = rare (<5 per slide); and

X = present (1 per slide).

Preservation is defined as follows:

E = excellent (nearly pristine);

G = good (most specimens complete, minor breakage and dissolution);

M = moderate (dissolution and small amount of breakage evident); and

P = poor (extensive breakage, dissolution, or recrystallization).

During Leg 178, the zonal schemes referred to were those of Martini (1971) and Bukry (1973, 1975, 1978), code numbered by Okada and Bukry (1980) (Figs. F9, F12). These zonations provide the framework for the biostratigraphic subdivision of low-latitude Cenozoic marine sediments on the basis of calcareous nannofossils. In addition to the traditional use of first/last occurrences of index species, ranges of taxa were used to improve the stratigraphic resolution of the Pleistocene interval. These include the commonly used Emiliania huxleyi acme Zone, which roughly covers the last 90 k.y. (Thierstein et al., 1977), and the "small Gephyrocapsa Zone" of Gartner (1977), an interval that defines the last 300 k.y. of Okada and Bukry's CN13b biozone. According to the Leg 175 Shipboard Scientific Party (1998c), this interval ranges from marine isotope Stage 30 to 44. The top of the Gephyrocapsa caribbeanica acme Zone, dated at 260 ka, is synchronous with the FO of E. huxleyi (Pujos, 1988) and thus provides a useful alternative to identify the base of Martini's NN21 zone. Ages of most calcareous nannofossil data employed to construct the Leg 178 age model for the Pliocene-Pleistocene interval come from the work of Raffi et al. (1993) and Wei (1993).

Smear slides were prepared using Norland Optical Adhesive as a mounting medium. Calcareous nannofossils were examined using a polarizing microscope at a magnification of 1000× . Unless otherwise noted, the taxonomic concepts summarized by Perch-Nielsen (1985) were followed. For morphometric concepts concerning the Gephyrocapsa group, the Raffi et al. (1993) proposal was utilized.

Etching and overgrowth are the most important features of nannofossil preservation. To establish a ranking of preservation, the code system adopted by the Leg 172 Shipboard Scientific Party (1998b) was used:

G = good (little or no evidence of dissolution and/or secondary overgrowth of calcite; diagnostic characters fully preserved);

M = moderate (dissolution and/or secondary overgrowth; partially altered primary morphological characteristics [however, nearly all specimens can be identified at the species level]); and

P = poor (severe dissolution, fragmentation, and/or secondary overgrowth with primary features largely destroyed; many specimens cannot be identified at the species level and/or generic level).

Total abundance of calcareous nannofossil for each sample was estimated as follows:

VA = very abundant (>100 nannoliths per field of view);

A = abundant (10-100 nannoliths per field of view);

C = common (1-10 nannoliths per field of view);

R = rare (<1 nannolith per 10 fields of view); and

B = barren.

Several zonal schemes have been developed for the mid- and high latitudes of the Southern Hemisphere (e.g., Jenkins and Srinivasan, 1986; Berggren et al., 1995). However, these schemes are not fully applicable to the planktonic foraminiferal fauna in all the sediments recovered during Leg 178 because of the absence or low abundance of foraminiferal species. In general, high-latitude foraminiferal assemblages contain low-diversity and long-ranging species that are of limited biostratigraphic use. Planktonic foraminiferal zonation and classification follow Berggren (1992).

Benthic foraminifers provide limited biostratigraphic age control as currently applied to Leg 178 samples, and all zones recognized are local assemblage zones. Individual benthic foraminiferal datums are recognized and discussed for each site.

Core-catcher sediment samples were soaked in tap water, disaggregated, wet sieved over a 63-µm sieve, and dried in an oven at temperatures <60ºC. Several different methods were used for disaggregation, including ultrasonic treatment, heating on a hot plate, Calgon solution, and hydrogen peroxide for consolidated sediments. Well-indurated samples were subjected to drying and wetting to break up the sample. The sieves were soaked in water containing methylene blue between successive samples to stain specimens left in the sieves from previous samples. Foraminifers were separated and identified under the microscope. A 20-cm³ wet sample was used for shipboard studies and in calculating abundances. The abundance of planktonic foraminifers as a group relative to the total sieved residue was categorized as follows:

A = abundant (>50%);

C = common (25%-50%);

F = few (10%-25%);

R = rare (<5% of the residue); and

B = barren (no specimens in sample).

Benthic foraminifer species abundances were recorded as follows:

D = dominant (>50% of total assemblage);

A = abundant (10%-50% of total assemblage);

C = common (1%-10% of total assemblage);

F = few (0.1%-1% of total assemblage);

R = rare (<0.1% of total assemblage); and

B = barren (no specimens observed).

Foraminifer preservation was categorized as follows:

G = good (dissolution effects are rare);

M = moderate (dissolution damage, including etched and partially broken tests or fragments, is common); and

P = poor (a high degree of fragmentation is common and specimens are small, encrusted, and possibly reworked).

Bolboforma are an extinct group of calcareous plankton that lived in temperate to cold conditions that have characterized subantarctic water masses, and may provide useful biostratigraphy in southern high-latitude sites. Thirteen zones have been established for the Eocene-upper Pliocene (Spiegler and von Daniels, 1991). The work of Spiegler (1991) on Leg 114 material from the South Atlantic Ocean and that of Mackensen and Spiegler (1992) on Leg 120 material from the Kerguelen Plateau, Southern Indian Ocean, suggest that the biostratigraphic utility of this group in the Southern Ocean may be improved further. In addition, Bolboforma were identified at Site 1092 of Leg 177 (Shipboard Scientific Party, 1999b). The preparation methods used to obtain Bolboforma were the standard techniques used to obtain foraminifers described above. The occurrences of Bolboforma are designated as follows:

A = abundant (>25 specimens per 20 cm³);

C = common (11-25 specimens per 20 cm³); and

R = rare (1-10 specimens per 20 cm³).