BIOSTRATIGRAPHY AND SEDIMENTATION RATES

Results from ODP Legs 113, 114, 119, and 120 greatly improved southern high-latitude Cenozoic and Late Cretaceous biostratigraphy. The study of sediments recovered during these legs provided biostratigraphic zonations using both calcareous and siliceous microfossils with the additional resolution of stratigraphic ranges that could be tied directly to the geomagnetic time scale (e.g., Gersonde et al., 1990; Barron et al., 1991; Harwood et al., 1992). Leg 188 drilling in Prydz Bay allowed further testing and possible refinement of these biostratigraphic schemes and the intercalibration of high- and mid-latitude zonations and species ranges. Improved dating of Neogene biostratigraphic ranges may be accomplished by correlation with isotopic stratigraphies or other data sets with high temporal resolution, such as color reflectance, magnetic susceptibility, and magnetic paleointensity records. In addition, Prydz Bay drilling provided a unique opportunity for documenting and understanding evolutionary processes (patterns, modes, and timing of speciation and diversification), the development of Southern Hemisphere bioprovinces, 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, planktonic and benthic foraminifers, diatoms, and radiolarians were examined for Leg 188 biostratigraphic assessment. Preliminary ages were assigned primarily based upon core-catcher samples; however, selected samples from within the cores were examined for additional resolution. Ages for calcareous nannofossil, foraminiferal, diatom, and radiolarian events and epoch boundaries are based on the geomagnetic polarity time scale (GPTS) of Berggren et al. (1995b) and Cande and Kent (1995). Age models were based primarily on biostratigraphic datums as well as magnetostratigraphic chron and subchron boundaries correlated with the GPTS. For sedimentation rate calculations, it was necessary to employ additional high-latitude biostratigraphic datums (e.g., Wei, 1992; Harwood et al., 1992) not presented in Berggren et al. (1995b).

Biostratigraphy

The cosmopolitan nannofossil biostratigraphic schemes of Martini (1971) and Okada and Bukry (1980) were used with major modifications. The absence of low- to mid-latitude marker species in the Southern Ocean necessitated the combination of many of the zones, particularly in the Neogene (Pospichal et al., 1992) (Fig. F7). Wei and Wise (1992a, 1992b) calibrated several useful Neogene high-latitude nannofossil datums to the paleomagnetic time scale (subsequently recalibrated by Berggren et al., 1995b). About five useful zones were used for the austral high-latitude Neogene.

Higher resolution is possible for the Oligocene to mid-middle Eocene (Wise, 1983; Wei and Wise, 1990; Wei and Thierstein, 1991) (Fig. F7). Ages for key datum levels have been calibrated in the region of the Kerguelen Plateau against magnetostratigraphy by Wei and Wise (1992b); these are indicated in bold type on Figure F7, where they are shown against the Berggren et al. (1995b) time scale.

As noted by Wei and Wise (1992b), biomagnetostratigraphic correlations at several Southern Ocean sites may show considerably different ages relative to those compiled from the mid-latitudes by Berggren et al. (1985, 1995b). Where such differences exist, we have chosen to use ages derived from the high-latitude calibrations. Where such ages differ from those in the lower latitudes, the high-latitude ages are shown in bold type in Figure F7 following the corresponding datum level (similarly, high-latitude biostratigraphic datums are also indicated in bold type). For major differences in age assignment, arrows indicate where on the chart a datum has been repositioned for the purposes of this leg.

Methods

Smear slides were prepared for calcareous nannofossil study using standard techniques. Slides were examined using a light microscope under crossed polarizers, transmitted light, and phase-contrast light at 1000× or 1200× magnification. Preservation and abundance of calcareous nannofossil species varied significantly due to etching, dissolution, or calcite overgrowth. Preservation was indicated as follows:

VG = very good preservation (no evidence of dissolution and/or overgrowth; no alteration of primary morphological characteristics and specimens appear diaphanous; specimens were identifiable to the species level).

G = good preservation (little or no evidence of dissolution and/or overgrowth; primary morphological characteristics are only slightly altered; specimens were identifiable to the species level).

M = moderate preservation (specimens exhibit some etching and/or overgrowth; primary morphological characteristics are sometimes altered; however, most specimens were identifiable to the species level).

P = poor preservation (specimens are severely etched or exhibit overgrowth; primary morphological characteristics are largely destroyed; fragmentation has occurred; most specimens could be identified to the species and/or generic level).

Six calcareous nannofossil abundance levels were recorded as follows:

V = very abundant (>10 specimens per field of view).

A = abundant (1-10 specimens per field of view).

C = common (1 specimen per 2-10 fields of view).

F = few (1 specimen per 11-100 fields of view).

R = rare (1 specimen per 101-1000 fields of view).

B = barren.

Several planktonic foraminiferal zonal schemes have been developed for the mid- and high latitudes of the Southern Hemisphere (e.g., Jenkins and Srinivasan, 1986; Berggren, 1992a, 1992b); however, these schemes are not fully applicable to the planktonic foraminiferal fauna in all the sections recovered during Leg 188 because of the absence or low abundance of foraminiferal species, particularly in the Neogene. In general, high-latitude foraminiferal assemblages contain low-diversity and long-ranging species that are of limited biostratigraphic use. Planktonic foraminiferal zonation and classification followed Berggren (1992a, 1992b). Recent experience on Mac. Robertson Shelf (Quilty et al., 2000) and on the southern Kerguelen Plateau (Berggren, 1992a, 1992b) indicates that the diversity of planktonic foraminifers may have been greater during the Paleogene and that lower latitude zonal schemes may be applicable. Thus, the scheme of Berggren (1992a, 1992b) is illustrated in Figure F7. Future examination of the fine fractions (63-125 µm) may also yield additional species (Li and Radford, 1992).

Although benthic foraminifers generally provided limited biostratigraphic age control, all zones recognized were local assemblage zones (e.g., Mackensen and Berggren, 1992), and they were useful in paleoenvironmental reconstruction.

At Site 1097 (Leg 178), three foraminiferal biofacies (Biofacies A, B, and C) aided the interpretation of late Neogene lithofacies (Shipboard Scientific Party, 1999c). This approach was applied to Leg 188 sediments to further characterize glacial marine sedimentary environments in Prydz Bay. Biofacies A consists of poorly preserved reworked assemblages of benthic foraminifers. Characteristic samples contain <12 robust foraminifer specimens (typically the benthic foraminifers Globocassidulina subglobosa and Cassidulinoides parkerianus), which are commonly yellow colored, broken, or filled with sediment, indicating postmortem transport.

Biofacies B yields moderately preserved, more abundant and diverse assemblages in which G. subglobosa and C. parkerianus are again dominant, but preservation is better than in Biofacies A.

Biofacies C consists of large numbers (as many as 1000 specimens) of well-preserved foraminifers in association with other well-preserved biogenic material.

Biofacies A is characteristic of massive diamictite lithofacies, Biofacies B is typical of stratified and graded diamictites interbedded with bioturbated mudstones, and Biofacies C occurs in massive diamictites that are transitional to bioturbated muddy sands with ice-rafted clasts.

Methods

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

A = abundant (<50% of the total sieved residue).

C = common (>25%-50% of the total sieved residue).

F = few (5%-25% of the total sieved residue).

R = rare (<5% of the residue).

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%-1.0% of total assemblage).

R = rare (<0.1% of total assemblage).

B = barren (no specimens observed).

Foraminifer preservation was categorized as follows:

G = good (dissolution effects were rare).

M = moderate (dissolution damage such as etched and partially broken tests or fragments occurred frequently).

P = poor (the degree of fragmentation was often high, and the specimens present were small, encrusted, and possibly reworked).

Bolboforms are an extinct group of calcareous plankton that lived in temperate to cold conditions; they have characterized sub-Antarctic water masses and may provide useful biostratigraphy in southern high-latitude sites. They are placed in the family Bolboformaceae of the class Chrysophyceae (Tappan, 1980). Thirteen zones have been established for the Eocene to upper Pliocene (Spiegler and von Daniels, 1991). The studies of Spiegler (1991) on Leg 114 material from the South Atlantic Ocean and those 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 and may rival planktonic foraminifers. In addition, Bolboforma were identified at Site 1092 of Leg 177. The preparation methods used to obtain Bolboforma were the standard techniques used to obtain foraminifers (Mackensen and Spiegler, 1992). The occurrences of Bolboforma were designated as follows:

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

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

R = rare (<11 specimens per 20 cm³).

Numerous diatom biostratigraphic studies have been completed in Southern Ocean sediments (Gersonde and Burckle, 1990; Baldauf and Barron, 1991; Harwood and Maruyama, 1992; Gersonde and Bárcena, 1998). The Paleogene and Neogene diatom zonal scheme used during Leg 188 was primarily that proposed by Harwood and Maruyama (1992; Leg 120) (Fig. F8), with some modification. Some zone names were revised using the taxonomic transfer of genus Nitzschia to Fragilariopsis (Round et al., 1990; Hasle, 1993; Gersonde and Bárcena, 1998). Diatom workers on recent legs (Legs 177 and 178) removed the Fragilariopsis kerguelensis Zone because of taxonomic problems in distinguishing the last occurrence (LO) of Fragilariopsis barronii. This is because biostratigraphically younger F. barronii specimens can be mistaken for F. kerguelensis or Fragilariopsis ritscherii. The use of the F. kerguelensis Zone was retained for Leg 188, despite taxonomic problems associated with its upper boundary. Additionally, a new marker species, Thalassiosira oliverana, was adopted for the base of the Nitzschia reinholdii Zone for use during Legs 177 and 178. During Leg 188, we have retained the usage of the first occurrence (FO) of Thalassiosira miocenica for the base of this zone, as the first occurrence of T. oliverana most likely lies well below the FO of T. miocenica.

All of the absolute ages for the marker species datums were recalculated to the Berggren et al. (1995b) time scale (Table T1), and the boundaries in the zonal scheme were repositioned accordingly. Several new diatom zones for the Pleistocene were established in a recent paper (Gersonde and Bárcena, 1998). The detailed diatom biostratigraphy of Gersonde and Bárcena (1998), however, was not used during this cruise because their scheme is derived from sections recovered in more northerly latitudes. Shipboard Scientific Party (1999b; Leg 177) pointed out that the FO of Thalassiosira vulnifica, which marks the base of the Thalassiosira insigna-T. vulnifica Zone of Harwood and Maruyama (1992), is a diachronous event in the Southern Ocean. They replaced this zone with the T. insigna Zone and divided it into Subzones a-c. These new subzones were defined wholly by the FO and LO of T. insigna. The range of T. insigna, however, is not well documented on the Antarctic margin; consequently, the T. insigna-T. vulnifica Zone definition of Harwood and Maruyama (1992) was retained for this study.

Figure F8 illustrates the diatom 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 188 because of the absence or low abundance of several marker species. Recent drilling in western McMurdo Sound, Ross Sea, at Cape Roberts resulted in the development of a new zonal scheme for the Antarctic continental shelf and the identification of numerous new taxa (Harwood et al., 1998; Scherer et al., in press). These new data were applied to lower Oligocene-upper Eocene sediments of Hole 1166A.

Reworking was noted in several Leg 188 samples. Biostratigraphic assignment of these samples was preferentially based upon FO datums over LO or last common occurrence (LCO) datums. Postcruise analyses will provide additional data that will help in delineating true ranges from reworked specimens.

Strewn 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. Sieving (>10 µm) was also performed when necessary to remove excess clays and to break down biosiliceous clasts. Slides were routinely examined on a Zeiss compound microscope at 630× and 1000× magnification, with the higher power being reserved mainly for confirmation of identification.

Abundance of individual diatom taxa was based on the number of specimens observed per field of view at 630×. Diatom abundance estimates were recorded as follows:

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

C = common (1-10 valves 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).

X = present (<3 valves per traverse of coverslip, including an appearance as fragments).

B = barren (no valves observed in slide).