ODP Legs 113, 114, 119, and 120 resulted in an enormous improvement of southern high-latitude biostratigraphy. Cenozoic sequences recovered during these legs allowed the establishment of biostratigraphic zonations using calcareous and siliceous microfossils, and the resolution of species stratigraphic ranges that could be tied directly to the geomagnetic polarity time scale (GPTS) (e.g., Gersonde at al., 1990; Thomas et al., 1990; Barron et al., 1991; Harwood et al., 1992). Drilling during Leg 177 of a north-south transect will allow further improvement and refinement of these biostratigraphic schemes and the intercalibration of high- and mid-latitude zonations and species ranges. 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 paleointensity records. In addition, the transect of sites across the Southern Ocean provides a unique opportunity for documenting and understanding evolutionary processes (patterns, modes, and timing of speciation and diversification), the development of southern hemisphere bioprovinces (e.g., endemisms), and the response of the biota to long- and short-term environmental changes related to paleogeographic and cryospheric evolution in southern high latitudes.
Calcareous nannofossils, planktic and benthic foraminifers, diatoms, and radiolarians were examined for biostratigraphic zonation. The presence of other siliceous groups was routinely investigated (silicoflagellates, chrysophycean cysts, opal phytoliths, sponge spicules, ebridians, and the dinoflagellate Actiniscus). Depths (mbsf and mcd) given in the text for CC samples refer to the top of the sample interval.
Preliminary ages were assigned primarily by analysis of CC samples. Samples from within the cores were examined when a refined age determination was necessary. Correlations to standard chronostratigraphic frameworks will be determined postcruise by magnetobiostratigraphic studies and oxygen isotopic stratigraphy.
Ages for calcareous nannofossil, foraminifer, diatom, and radiolarian datum events, and epoch boundaries are based on the GPTS of Berggren et al. (1995a, 1995b) (Fig. F6).
Micropaleontological data, including total and species abundance and preservation, are summarized in separate tables in the "Biostratigraphy" section of each site chapter.
During Leg 177, we employed the zonal schemes of Martini (1971) and Bukry (1973, 1975) with code numbering by Okada and Bukry (1980). These zonations are regarded as the standard framework for the biostratigraphic subdivision of low-latitude Cenozoic marine sediments based on calcareous nannofossils, and some of these events are also identifiable in middle to high latitudes. In addition to the classical concept of first/last occurrences of index species, we used ranges of taxa to improve the stratigraphic resolution of the Pleistocene interval. This includes the commonly used Emiliania huxleyi acme zone that roughly spans 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 (1998b), this interval ranges from marine isotopic Stage (MIS) 30 to 44. The top of the Gephyrocapsa caribbeanica acme Zone, dated at 260 ka, is approximately synchronous with the first occurrence (FO) of E. huxleyi (Pujos, 1988), and thus provides a useful alternative to identify the base of Martini's NN21 Zone. Likewise, ages of most calcareous nannofossil data employed to construct the Leg 177 age model for the Pliocene-Pleistocene interval come from the work of Raffi et al. (1993) and Wei (1993). For the Miocene-Eocene interval we follow the biochronology proposed by Berggren et al. (1995b) as well as other authors referenced in Table T1. Where datums used by Martini (1971) and Okada and Bukry (1980) were not identifiable, additional data were used from the Paleogene-lower Neogene zonal schemes of Wise (1983), Wei and Wise (1990), and Crux (1991) (Table T1; Figs. F6, F7).
Standard smear slides were made for all samples using Norland Optical Adhesive as a mounting medium. Calcareous nannofossils were examined by means of a light-polarized microscope at 1000× magnification. Unless otherwise noted, we followed taxonomic concepts summarized in Perch-Nielsen (1985). For morphometric concepts concerning the Gephyrocapsa group, we mainly followed the scheme proposed by Raffi et al. (1993). We also utilized (1) G. caribbeanica (3-4 µm), whose acme is coincident with the FO of E. huxleyi, and (2) Gephyrocapsa sp. 3 (Rio, 1982), whose FO is coincident with the reentrance of medium Gephyrocapsa (4-5.5 µm) in low-latitude regions.
Etching and overgrowth are the most important preservation features of nannofossils. To establish a ranking of preservation, we have followed previous code systems, such as the one adopted by the Leg 172 Shipboard Scientific Party (1998a):
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 of calcite; partially altered primary morphological characteristics; how-ever, nearly all specimens can be identified at the species level); and
P = poor (severe dissolution, fragmentation, and/or secondary overgrowth of calcite with primary features largely destroyed; many specimens cannot be identified at the species level and/or generic level).
Relative abundance of nannofossils was described by five categories:
D = dominant, >50% of the total assemblage;
A = abundant, 10%-50% of the total assemblage;
C = common, 1%-10% of the total assemblage;
F = few, 0.1%-1% of the total assemblage; and
R = rare, <0.1% of the total assemblage.
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 for 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, 1992a; Berggren et al., 1995b). On the basis of previous work (Brunner, 1991; Pujol and Bourroiulh, 1991), the late Neogene subantarctic zonation scheme of Jenkins and Srinivasan (1986) (Table T2) was selected to biostratigraphically subdivide the sequences recovered during Leg 177 (Fig. F6). However, this zonation scheme was not fully applicable to the foraminifer fauna in all sediments because of the absence or low abundance of several marker species.
Sediment samples were soaked in tap water and then washed over a 63-µm sieve. The sieves were soaked in water containing Methylene Blue between successive samples to stain specimens left in the sieve from previous samples. All samples were dried under heat lamps.
Planktic foraminifer species abundances (as percentages of the total assemblage) were defined as follows:
D = dominant, >30%;
A = abundant, 10%-30%;
F = few/frequent, 5%-10%;
R = rare, 1%-5%;
P = present, less than 1%; and
B = barren.
Preservation was categorized as follows:
G = good (dissolution effects are rare);
M = moderate (dissolution damage, such as etched and partially broken tests, occurs frequently and fragments are abundant); and
P = poor (the degree of fragmentation is often high and the specimens are often small, compact, and encrusted).
The abundance of planktic foraminifers as a group relative to the total residue was categorized as follows:
A = abundant, >50%;
C = common, 25%-50%;
F = few, 10%-25%;
R = rare, less than 5% of the residue;
T = trace, trace was used in the case where only a few broken tests were recorded in a sample; and
B = barren, no specimens in sample.
The generic classification used mainly follows that of Kennett and Srinivasan (1983). However, the taxonomy of Globorotalia follows Scott et al. (1990). Species identification of Paleogene and early Neogene planktic foraminifers follows Stott and Kennett (1990) and Berggren (1992b).
Benthic foraminifers provide limited biostratigraphic age control for Leg 177 samples, and all zones recognized are local assemblage zones. Individual benthic foraminifer datums are recognized and discussed for each site. Particularly useful, but requiring further study, is the last occurrence (LO) of Stilostomella lepidula at about 0.9 Ma. Additional age-diagnostic taxa include the LO of Alabamina dissonata at Site 1090 which marks the latest Eocene, the LO of Nuttallides truempyi which is considered by Berggren and Aubert (1983) to provide a useful marker of the Eocene/Oligocene boundary, or the presence of Aragonia aragonensis which provides a potentially valuable indication of latest middle Eocene age.
Taxonomic assignments follow those of van Morkhoven et al. (1986), Thomas (1990), and Mackensen (1992).
Bolboforma were recovered from Site 1092 and taxonomic concepts are based on Qvale and Spiegler (1989) and Spiegler (1991). Stratigraphic subdivision is largely based upon Norwegian Sea material and is therefore of limited value for a detailed stratigraphic subdivision of the middle to late Miocene at Site 1092. However, the work of Spiegler (1991) on Leg 114 material from the South Atlantic Ocean and Mackensen and Spiegler (1992) on Leg 120 material from the Kerguelen Plateau in the southern Indian Ocean, suggests that there is some potential to improve the biostratigraphic utility of this group in the Southern Ocean.
To obtain planktic and benthic foraminifers and Bolboforma from CC samples, a 20-cm3 sample was disaggregated and washed over a 63-µm sieve. At the southernmost sites, highly abundant, needle-shaped remains of the diatom genus Thalassiothrix in the >63-µm fraction made it necessary to wet-sieve sediment samples at 150 µm. Cursory examination of the 63- to 150-µm fraction reveals that significant components of the benthic assemblage are retained within this size range, notably phytodetrital taxa such as Alabaminella weddellensis and Epistominella exigua. Between samples, sieves were soaked in a solution of Methylene Blue to stain foraminifers and identify potential contamination. The samples were dried under heat lamps and benthic foraminifers were examined from the entire >63- or >150-µm fraction under the binocular microscope and identified, where possible, to species level. Species abundances were determined from numeric population counts that typically ranged from 100 to 300 specimens per sample. The relationship between number of specimens and number of taxa observed in all Leg 177 samples is summarized in Figure F8. The figure clearly illustrates that counts of >250-300 specimens are required to obtain reliable estimates of species richness and diversity; it should be noted, however, that many of the 20-cm3 samples yielded only low foraminifer numbers. Quantitative estimates of foraminifer abundance were made at each site by multiplying foraminifer sums by the fraction of the coarse residue studied. Where time did not allow for large counts, 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.
Preservation was categorized as G (good), M (moderate), or P (poor).
Diatom biostratigraphic studies made during Leg 177 follow the zonal schemes developed for the southern high latitudes by Gersonde and Burckle (1990), Baldauf and Barron (1991), Harwood and Mayurama (1992), and Gersonde and Bárcena (1998), as combined by Gersonde et al. (1998). Two biostratigraphic zones have been preliminarily revised. The FO of Thalassiosira vulnifica, which marks the base of the Thalassiosira insigna-T. vulnifica Zone of Harwood and Maruyama (1992), is probably diachronous, and we replaced this zone by one tentatively named the T. insigna Zone. The bottom and top of this zone are defined by the FO and LO of the nominate taxon, respectively. On the basis of preliminary biostratigraphic age assignments, the basal age of the Fragilariopsis reinholdii Zone, marked by the FO of the nominate taxon, was placed in Chron C4 at ~8.1 Ma, an age that is close to the age of its FO in the equatorial Pacific Ocean, as reported by Barron (1992).
Because of the presence of warm and temperate species in the northernmost sites of Leg 177, additional stratigraphic ranges have been added following the compilation of Barron (1992). This zonation and the individual species ranges cover a time interval spanning the early Oligocene to Pleistocene and are tied to the GPTS of Berggren et al. (1995a, 1995b) as presented in Figures F6 and F9.
Age assignments of the zones and species ranges employed are compiled in Tables T3 and T4. For stratigraphic age assignment of Paleogene sections we combined diatom zones and species ranges published by Gombos (1983), Gombos and Ciesielski (1983), and Fenner (1984). These are not yet tied to the GPTS.
The diatom assemblages observed in sediments recovered during Leg 177 also provide paleoenvironmental information, such as the thermal isolation of the Southern Ocean, changes in the late Quaternary surface-water temperatures, and Antarctic sea-ice extent. The presence of freshwater and benthic marine diatoms at Leg 177 sites indicates advection by eolian and/or lateral transport.
Two type of slides were prepared for diatom analysis, depending on overall diatom abundance. For intervals rich in biogenic silica (e.g., sites in the circum-Antarctic opal belt; see "Leg 177 Summary" chapter), smear slides were prepared from a small amount of raw material in a CC or from additional core material when required. In intervals dominated by carbonate and poor in silica, a small amount of sample was immersed in 10% HCl to remove calcium carbonate. The carbonate-free residue was repeatedly washed with distilled water to remove the acid. In each case, aliquots of raw and cleaned sample were mounted as a thin film on microslides, protected by 18-mm-diameter cover glass slips using Mountex mounting medium. All slides were examined in their entirety with a Zeiss compound microscope at a magnification of 400× for stratigraphic markers and paleoenvironmentally sensitive taxa. Species identification was confirmed when necessary at 1000× . The counting convention of Schrader and Gersonde (1978) was adopted. For documentation of poorly known or undescribed taxa, photomicrographs were made using a video-print system at 1500× final magnification.
Overall diatom abundance was determined based on smear-slide evaluation at 400×, using the following convention:
A = abundant, >300 valves per traverse of microslide (>10 per field of view);
C = common, 100-300 valves per traverse of microslide (3-10 per field of view);
F = few, 30-100 valves per traverse of microslide (1-3 per field of view);
R = rare, 5-30 valves per traverse of microslide;
T = trace, <5 valves per traverse of microslide; and
B = barren, no diatoms in sample.
The species relative abundance was defined as follows:
D = dominant, >60% of assemblage;
A = abundant, 30%-60% of assemblage;
C = common, 15%-30% of assemblage;
F = few, 3%-15% of assemblage;
R = rare, <3% of assemblage;
T = trace, sporadic occurrence; and
X = present, presence was indicated in cases where diatom valves could not be counted individually (e.g., Ethmodicus rex fragments) or for all taxa encountered in samples with trace/rare overall abundance.
Preservation of diatoms was determined qualitatively as follows:
G = good (lightly silicified forms present and no alteration of frustules observed);
M = moderate (lightly silicified forms present, but with some alteration); and
P = poor (finely silicified forms absent or rare and fragmented, and the assemblage is dominated by robust forms).
At several sites, preservation and abundance estimates were plotted vs. depth to visualize the distribution of diatom abundance and preservation. For computing purposes, a number was assigned to each overall diatom abundance category: abundant = 18, common = 9, few = 3, rare = 1, trace = 0.5, and barren = 0. To show individual species or species group abundance we assigned the following numbers: dominant = 100, abundant = 60, common = 30, few = 15, rare = 3, trace = 0.5, and barren = 0. Assemblage preservation categories were designated as good = 3, moderate = 2, or poor = 1.
Neogene to Paleogene radiolarian zones used in this study mainly follow those of Abelmann (1992), Lazarus (1992), and Takemura (1992). However, radiolarian assemblages from the northern sites of the Leg 177 transect include low- to mid-latitude zonal markers, which prevent application of the Antarctic/Subantarctic zonal scheme. In such cases, a tentative age assignment was made using the mid-latitude zonation of Foreman (1975) and Morley (1985), established in the North Pacific region. Applicability of the Indian and Pacific low-latitude zonations (Johnson et al., 1989; Moore, 1995) is not yet confirmed. Figure F6 and Table T5 show the radiolarian zonations and datums, respectively, used during Leg 177. Precise correlation between the Antarctic and mid-latitude zonations remains preliminary. It is expected that additional radiolarian datums will be identified by shore-based biostratigraphic studies.
To obtain radiolarians from CC samples, ~10 cm3 of sediment was disaggregated and boiled with 10% H2O2, 10% HCl, and ~1% Calgon solutions. Brief treatment of samples in an ultrasonic bath was followed by washing on a 63-µm sieve. The residue was transferred to a beaker, and a strewn slide was made using a pipette. Canada Balsam was used as a mounting medium. Additional random strewn slides will be prepared on shore to locate biostratigraphic events more accurately within cores.
Overall radiolarian abundance was determined by strewn slide evaluation at 100×, using the following conventions:
A = abundant, >100 specimens per slide traverse;
C = common, 50-100 specimens per slide traverse;
F = few, 10-50 specimens per slide traverse;
R = rare, <10 specimens per slide traverse; and
B = barren, no radiolarians in sample.
The abundance of individual species was recorded relative to the fraction of the total assemblage as follows:
A = abundant, >10% of the total assemblage;
C = common, 5%-10% of the total assemblage;
F = few, <5% of the total assemblage;
R = rare, a single to few specimens per slide; and
B = barren, absent.
Preservation was recorded as follows:
E = excellent (nearly pristine, complete skeleton, lacking any indication of dissolution, recrystallization or breakage);
G = good (majority of specimens complete; minor dissolution, recrystallization and/or breakage);
M = moderate (minor but common dissolution, small amount of recrystallization or breakage of specimens); and
P = poor (strong dissolution, recrystallization or breakage, many specimens unidentifiable).