BIOSTRATIGRAPHY

Calcareous nannofossils, planktonic foraminifers, diatoms, palynomorphs (notably dinocysts), and radiolarians were the main groups examined and used for biostratigraphic zonation. Benthic foraminifers and bolboformids were used to estimate paleobathymetry. Paleoenvironmental analysis was based on the integrated results of all microfossil groups. The abundance, preservation, and biostratigraphic assignment were entered into the Janus database (see the "Related Leg Data" contents list).

Preliminary ages were assigned based mainly on core-catcher samples. Additional samples (toothpick) were examined for nannofossils routinely. Where necessary to refine an age determination, additional samples from within the cores were also utilized.

The geochronologic scale for the various bioevents and epoch boundaries used is based on the geomagnetic polarity scheme as integrated by Berggren et al. (1995a, 1995b).

Calcareous Nannofossils

For Cenozoic sediments from Leg 189, the zonal schemes of Martini (1971) and Okada and Bukry (1980) were employed with some modifications. Previous southwest Pacific studies have shown these standard zonations are not always applicable in higher latitudes because of the absence or rarity of index species (Edwards and Perch-Nielsen, 1975). The main bioevents used are given in Table T1.

Nannofossil bioevents used in this study have been tentatively tied to the geochronologic scale of Berggren et al. (1995a, 1995b). Biomagnetostratigraphic correlations at several Southern Hemisphere high-latitude sites have shown considerably different ages (Wei and Wise, 1992) relative to those compiled from the mid-latitudes by Berggren et al. (1995a, 1995b). Correlation with shipboard magnetostratigraphy was essential for constraint of the nannofossil bioevents.

Methods

Smear slides were prepared for calcareous nannofossil study using standard techniques. Slides were examined using the light microscope under cross-polarized light, transmitted light, and phase-contrast light at 1000-1200× magnification. Relative abundance of each nannofossil species, overall preservation of the nannofossil assemblage, and the relative abundance of nannofossils were recorded for each sample. Taxonomic concepts follow those of Perch-Nielsen (1985) unless otherwise noted.

Preservation and abundance of calcareous nannofossil species varied significantly because of etching, dissolution, or calcite overgrowth. Preservation of assemblages was ranked according to the following criteria:

VG = very good (no evidence of dissolution and/or overgrowth was found; no alteration of primary morphological characteristics were present; specimens were identifiable to the species level).
G = good (little or no evidence of dissolution and/or overgrowth was found; primary morphological characteristics were only slightly altered; specimens were identifiable to the species level).
M = moderate (specimens exhibit some etching and/or overgrowth; primary morphological characteristics were sometimes altered; however, most specimens were identifiable to the species level).
P = poor (specimens are severely etched or exhibit overgrowth; primary morphological characteristics were largely destroyed; fragmentation had occurred; specimens could not be identified at the species and/or generic level).

Six calcareous nannofossil abundance levels for species and assemblages were recorded as follows in number of specimens per field of view:

V = very abundant (11-100).
A = abundant (1-10).
C = common (1 per 2-10).
F = Few (1 per 11-100).
R = rare (1 per 101-1000).
B = barren.

Planktonic and Benthic Foraminifers and Bolboformids

Zonation

The planktonic foraminiferal cool-subtropical (temperate) biostratigraphic scheme of Jenkins (1985, 1993a, 1993b), the Antarctic schemes of Stott and Kennett (1990) and Berggren et al. (1995b) formed the basis for the zonation used during Leg 189. The main bioevents (and others used during this leg) on which this scheme is based are given in Table T2. The geochronologic scale to which these bioevents are tied is that of Berggren et al. (1995a, 1995b).

Planktonic foraminiferal bioevents used in this study have been tentatively tied to the geochronologic scale (Fig. F5) of Berggren et al. (1995a, 1995b).

Benthic foraminifers provide only limited biostratigraphic age control, but they are useful in paleobathymetric and paleoenvironmental determinations. Interpretations in bathyal and abyssal depths, however, are difficult to standardize because the faunas are not strictly limited to specific water depths, but rather are a function of a large number of interrelated factors of which the supply of organic carbon and bottom-water oxygen content are considered most important. During Leg 189, use was made of guidelines for benthic foraminiferal depth distribution according to van Morkhoven et al. (1986) and Tjalsma and Lohmann (1983).

Bolboforma is an extinct group of spherical (70-250 µm in diameter) calcareous plankton that lived in temperate to cold waters of northern and southern middle to high latitudes. The group is tentatively placed in the Class Chrysophyaceae (Tappan, 1980) with ~100 described species ranging from early Eocene to late Pliocene in age. The studies of Spiegler (1991) on Leg 114 material from the South Atlantic Ocean, Mackensen and Spiegler (1992) on Leg 120 material from the Kerguelen Plateau and Southern Indian Ocean, Kennett and Kennett (1990) on the Weddell Sea, Antarctica, and Spiegler (1999) on Leg 162 material from the North Atlantic Ocean suggest that the biostratigraphic utility of this group is high.

Sample Preparation

Most samples were prepared by soaking in a 10% hydrogen peroxide solution with Calgon for a short period and then washed over a 63-µm sieve. Harder samples were boiled for some time. All samples were washed with alcohol and then dried in an oven. Benthic foraminifers were analyzed from the >125-µm-size fraction and, in only a few exceptions, from the >63-µm-size fraction.

Abundance and Preservation

For estimates of species abundance the following scale was used:

P = present.
T = trace (1-2 tests on tray).
R = rare (3-5 tests on tray).
F = frequent (6-14 tests on tray).
C = common (>14 tests on tray).

The preservational state is described as follows:

G = good (little or no fragmentation).
M = moderate (some signs of fragmentation or alteration).
P = poor (severe fragmentation or alteration).

Paleodepth

The depth-zonation scheme used is based on van Morkhoven et al. (1986), with the exception of the uppermost zone, which was redefined as follows:

Neritic to upper bathyal = 0-200 m.
Upper bathyal = >200-600 m.
Middle bathyal = >600-1000 m.
Lower bathyal = >1000-2000 m.
Upper abyssal = >2000-3000 m.
Lower abyssal = >3000 m.

Diatoms

Diatom zonation for Leg 189 is based upon several zonal schemes developed for the southern high latitudes by Gersonde and Burckle (1990), Baldauf and Barron (1991), Harwood and Maruyama (1992), Gersonde and Bárcena (1998), and combined by Gersonde et al. (1998). Effectively, the scheme used during Leg 177 (Shipboard Scientific Party, 1999b), including their slight changes, was adopted for continuity in this work (Fig. F6). For instance the Thalassiosira insigna-Thalassiosira vulnifica Zone of Harwood and Maruyama (1992) is replaced by the T. insigna Zone (Shipboard Scientific Party, 1999b). This change was made because of the probable diachroneity of the first occurrence of T. vulnifica. It was also adopted during Leg 181 (Shipboard Scientific Party, 1999a) and was retained for Leg 189. In addition, the basal age of the Fragilariopsis reinholdii Zone, defined by the first occurrence (FO) of the nominate taxon, is placed at ~8.1 Ma within Chron C4 in accordance with Legs 177 and 181. This datum is close to that of the equatorial Pacific zonation (Barron, 1992). In addition to southern high-latitude diatoms, warm and temperate species were also encountered during Leg 189. Therefore, additional stratigraphic ranges have been added following the compilation of Barron (1992).

Age assignments for the zones used are listed in Table T3. The zonation is tied to the geomagnetic polarity time scale of Berggren et al. (1995a, 1995b) and is presented with species ranges in Figure F7.

Methods

Two methods of diatom analysis were followed. For assessment of the overall abundance of the group, smear slides were prepared from a small amount of core-catcher material or from additional core material. For intervals rich in carbonate, as much as 10 cm3 of raw sample was treated with 35% HCl. For organic-rich intervals, samples were treated with up to 30% H2O2 before carbonate removal. Residues were washed with distilled water several times to remove the acid. For both methods, permanent slides were made by mounting the sediment on glass slides protected by a glass cover slip using Hyrax or Norland Optical Adhesive mounting media. All slides were examined in their entirety for stratigraphic markers and paleoenvironmentally sensitive taxa using a Zeiss compound microscope at 400× magnification. Identifications were confirmed when necessary at 1000× magnification. The counting method of Schrader and Gersonde (1978) was adopted. Photomicrographs of poorly known taxa were made using a video-print system at 1500× final magnification.

Overall diatom abundance was determined from smear-slide analysis at 400× magnification according to the following convention in values per traverse of cover slip:

A = abundant (>150).
C = common (>80-150).
F = few (>30-80).
R = rare (5-30).
T = trace (<5).
B = barren (no diatoms observed).

Species relative abundance was determined with the following convention in percent of assemblage:

D = dominant (>50%).
A = abundant (>30%-50%).
C = common (>15%-30%).
F = few (3%-15%).
R = rare (<3%).
T = trace (occasional occurrence).
P = present (applicable to diatoms where diatom valves could not be counted individually [e.g., Ethmodiscus rex fragments], or for all taxa with trace/rare overall abundance).

Preservation of diatoms was determined qualitatively according to the following convention:

G = good (finely silicified forms present and no alteration [dissolution or fragmentation] of valves).
M = moderate (finely silicified forms present, but some alteration of valves).
P = poor (finely silicified forms rare or absent, valves fragmented, and assemblage dominated by heavily silicified forms).

Radiolarians

Zonation

No useful radiolarian zones for temperate regions of the Southern Hemisphere have been published. This study temporarily utilizes Antarctic and subtropical zonal schemes. Antarctic zones have been compiled from Abelmann (1990, 1992), Caulet (1991), Chen (1975), Hollis (1993, 1997), Lazarus (1992), Nishimura (1987), Takemura (1992), and Takemura and Ling (1997), and tropical zones are modified from Sanfilippo and Nigrini (1998).

The main bioevents (and others used during this leg) on which this scheme is based are given in Table T4. Radiolarian bioevents used in this study have been tentatively tied to the geochronologic scale (Fig. F7) of Berggren et al. (1995a, 1995b).

Sample Preparation

Radiolarians were extracted using standard leaching techniques. Most samples were disaggregated by boiling in a solution of 10% H2O2 and 5% HCl. After rinsing the disaggregated sediment through a 63-µm sieve, the residues were placed in an oven at 60°C for a minimum of 1 hr. As a final process, the residues were uniformly distributed on glass slides using Norland Optical Adhesive mounting media.

Abundance and Preservation

Several hundred species were encountered under the microscopic observations of 100× to 400× magnifications. For age determination, only age-diagnostic radiolarian taxa were identified. Overall, radiolarian abundance was determined by strewn-slide evaluation of 100 specimens, using the following conventions:

C = common (>50 specimens per slide traverse).
F = few (10-50 specimens per slide traverse).
R = rare (<10 specimens per slide traverse).
T = trace (1 or a few specimens are encountered in one overall slide).
B = barren (no radiolarians in sample).

The relative abundance of individual species was recorded as follows:

A = abundant (>10% of the total assemblage).
C = common (>5%-10% of the total assemblage).
F = few (1%-5% of the total assemblage).
R = rare (<1% of the total assemblage).
T = trace (1 to 3 specimens per slide).

The state of preservation was determined using the following criteria:

VG = very good (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).
P = poor (strong dissolution, recrystallization, or breakage; many specimens unidentifiable).

Palynology, Dinoflagellate Cysts

Zonation

No integrated, calibrated dinoflagellate cyst (dinocyst) Cenozoic zonal schemes have been established for the Southern Ocean. In effect, little is known of Neogene organic walled dinocysts from the Southern Ocean. For the Paleogene, use will be made of the combined regional schemes and/or bioevents from Australia (e.g., Partridge, op. cit., Truswell, 1997) and New Zealand (e.g., Wilson, 1984, 1988; Raine et al., 1997). In addition, available data of the Tertiary from the Southern Ocean region will be taken into account (e.g., Goodman and Ford, 1983; Wrenn and Hart, 1988; Wilson, 1989; Mohr, 1990; Mao and Mohr, 1995, among others). Encountered dinocyst events will be compared with these previous investigations, and with the succession of events and zones established by Bujak and Mudge (1994), Bujak and Brinkhuis (1998), and Williams et al. (1998a) for the Northern Hemisphere Cenozoic to allow cross-hemisphere correlations and assessment of paleoprovincialism.

Initial stratigraphic assessment of occurring Paleogene terrestrial palynomorphs are made using the schemes of Truswell (1997) and MacPhail (1999).

Processing

Up to 10 cm3 of sample was processed according to standard palynological treatment. Briefly, the procedure includes digestion by HCl and HF, followed by HCl leaching, and centrifuging after each step. Residues were sieved using a stainless steel 20-µm sieve and strew-mounted on slides using glycerine jelly. In some cases, heavy-liquid separation using a ZnCl2 solution is applied (in quartz-rich sediments). A minimum of two slides were prepared and counted for a minimum of 150 specimens.

Abundance, Preservation, and Taxonomy

For indications of group abundance, the following scale is used:

B = barren.
T = trace.
F = few.
C = common.
A = abundant.

When possible, counts of >100 palynomorphs (broad categories) and, subsequently, >200 dinocysts were made.

Preservation is classified as one of the following:

P = poor.
M = moderate.
G = good.

Taxonomy is in accordance with that cited in Williams et al. (1998b).

Sediment Accumulation Rates

We used an estimation of sedimentation rates determined by age vs. depth data for each site. Ages follow the geochronological scale outlined in "Biostratigraphy". Paleomagnetic data for each site were calibrated to the calcareous microfossil biostratigraphic events. A table of the bioevents used for the age-depth plots is included with each site chapter.

Biostratigraphic data were used as a first estimate of the age of the sediments at each site, from the time scale used in this volume. This permitted the correlation of the paleomagnetic data to the published paleomagnetic time scale, which yielded age determinations that were used to adjust the age estimates for the stratigraphic section.

Periods of continuous sedimentation on the age-depth curves are indicated by solid lines, and hiatuses are denoted by dashed lines. Average rates of sedimentation during each epoch are given within each figure.

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