Five sites (Sites 1168–1172) were drilled in water depths of 2463–3568 m during Ocean Drilling Program (ODP) Leg 189 in the Tasmanian Gateway in March–May 2000 (Fig. F1). In all, 4539 m of Late Cretaceous to Quaternary marine sediments were recovered, with an overall recovery of 89%. The sediments of Leg 189 essentially record the Antarctic Cenozoic evolution from "Greenhouse" to "Icehouse" (see Exon, Kennett, Malone, et al., 2001). Postcruise work is presented in this volume and notably in Exon et al. (in press). The biomagnetostratigraphic age models for the four deep sites (Sites 1168, 1170, 1171, and 1172) are presented in this paper. Difficulties in drilling at Site 1169 resulted in a high amount of core disturbance, causing unreliable age assignment. Drilling was aborted beyond one advanced piston corer/extended core barrel (APC/XCB) hole, and no further postcruise attempt has been made to refine the shipboard age model (Shipboard Scientific Party, 2001c) for this site.
The relatively shallow region off Tasmania is one of the few places where well-preserved and almost complete marine Cenozoic carbonate-rich sequences can be drilled in present-day latitudes of 40°–50°S and paleolatitudes of up to 70°S. The broad geological history of all the sites is comparable, although important differences exist east to west and north to south (e.g., Exon, Kennett, Malone, et al., 2001; Brinkhuis, Munstermann, et al., this volume; Brinkhuis, Sengers, et al., this volume; Huber et al., submitted [N1]; McGonigal, in press; Pfuhl et al., in press; Pfuhl and McCave, this volume; Stant et al., this volume; Stickley et al., submitted [N2]). The wide range of lithologies recovered during Leg 189 (Exon, Kennett, Malone, et al., 2001) are almost completely fossiliferous throughout the Cretaceous to Quaternary; they contain a wealth and diversity of microfossils (e.g., diatoms, nannofossils, ostracodes, planktonic and benthic foraminifers, radiolarians, and silicoflagellates) and palynomorphs (dinocysts, acritarchs, spores, and pollen) in varying abundances and associations, providing ample opportunity to develop correlations between groups. The main groups are used here to derive age models for the four deep-penetrating sites. These are the most complete integrated biomagnetostratigraphic data for Leg 189 to March 2003. This is not intended as a final synthesis, however, and ongoing work on individual fossil groups is still refining some intervals.
Five microfossil groups of age significance are present in Leg 189 sediments in varying abundances. These comprise a calcareous grouping (nannofossils and planktonic foraminifers), a siliceous grouping (diatoms and radiolarians), and palynomorphs (notably organic walled dinoflagellate cysts or dinocysts, spores, and pollen). Other groups (e.g., benthic foraminifers, ostracodes, silicoflagellates, and ebridians) are present in varying abundance through selected stratigraphic intervals (see Exon, Kennett, Malone, et al., 2001) but are not used here for age assignment.
The stratigraphic distribution of the main microfossil groups through the drilled sequences changes markedly with depositional environment/lithology. The calcareous groups are most abundant in the pelagic calcareous oozes of the Oligocene–Quaternary (Stant et al., this volume; McGonigal and Wei, this volume; McGonigal, in press; Wei et al., this volume), whereas palynomorphs are the dominant group in the shallow-water siliciclastic sediments of the Maastrichtian and Paleogene (Brinkhuis, Munstermann, et al., this volume; Brinkhuis, Sengers, et al., this volume). However, palynomorphs and calcareous groups are largely abundant throughout the drilled sequences at Site 1168. In addition, well-preserved dinocysts are present in the Quaternary intervals of Sites 1168–1172, making these occurrences the southernmost Quaternary dinocysts records to date. Diatoms are commonly abundant in sediments of late Eocene age and younger at all sites (except Site 1168), with pyritized specimens frequently occurring in the Maastrichtian–middle Eocene intervals. Radiolarians are abundant at Site 1171 from the upper Eocene and at Site 1170 from the Oligocene. At Site 1172 they are common only in the Eocene and Miocene intervals. At Site 1168 diatoms and radiolarians are preserved only in two short intervals in the mid-Oligocene and upper Miocene, and therefore are not useful for age assignment at this site. Benthic foraminifers are present throughout the drilled intervals of all sites except for the upper Eocene glauconitic sands. They give an indication of paleobathymetric history through the sequences (see Exon, Kennett, Malone, et al., 2001).
Extensive postcruise high-resolution calcareous nannofossil stratigraphic work has been undertaken through the Neogene and relevant Paleogene sections, instigating refinement of the initial age models presented in Exon, Kennett, Malone, et al. (2001). The present paper summarizes the latest nannofossil stratigraphies in our integrated age models, and the reader is referred to Stant et al. (this volume), McGonigal and Wei (this volume), McGonigal (in press), and Wei et al. (this volume) for further details. The zonal schemes of Martini (1971), Gartner (1977), 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). 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 magnetostratigraphy was essential for constraint of the nannofossil bioevents. The resulting nannofossil biostratigraphy has produced some very useful subantarctic temperate biostratigraphic records. In particular, the Oligocene to Pliocene interval is among the most detailed of Southern Ocean sites at similar latitudes. This sequence will serve as an important reference section for the Southern Hemisphere.
The planktonic foraminiferal cool subtropical (temperate) biostratigraphic scheme of Jenkins (1985, 1993a, 1993b) and the Antarctic schemes of Stott and Kennett (1990) and Berggren et al. (1995a, 1995b) formed the basis for the zonal scheme used during Leg 189 (see Exon, Kennett, Malone, et al., 2001). Because of the southern location of Australia during the Paleogene, the subantarctic zonal scheme (Stott and Kennett, 1990; Berggren et al., 1995a, 1995b) was used in place of the traditional temperate scheme (Jenkins, 1985, 1993a, 1993b). This temperate scheme was appropriate for the upper Paleogene and Neogene. The low-diversity planktonic foraminiferal assemblages of the Eocene are generally very well preserved, but their abundances are low. The shipboard foraminiferal biostratigraphy is integrated into the age model here. Further work on refining this is ongoing and so far has concentrated on Site 1168 only.
Oligocene to Quaternary holoplanktonic diatoms are abundant at all sites drilled during Leg 189 (except Site 1168) and form an important constituent of the age models for this interval, particularly at Sites 1170 and 1171. Application of existing circum-Antarctic biostratigraphic schemes (Gersonde and Burckle, 1990; Baldauf and Barron, 1991; Harwood and Maruyama, 1992; Gersonde and Bárcena, 1998; Gersonde et al., 1998; Zielinski and Gersonde, 2002; Florindo et al., 2003; Roberts et al., in press) has proved useful for these southern sites. The modifications adopted during ODP Leg 177 (Shipboard Scientific Party, 1999a) and Leg 181 (Shipboard Scientific Party, 1999b) are retained here. 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 (FO) of T. vulnifica. In addition, the basal age of the Fragilariopsis reinholdii Zone, defined by the FO of the nominate taxon, is placed at ~8.1 Ma within Chron C4. 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). The resulting diatom stratigraphy is in generally good agreement with that for the nannofossils for the majority of intervals. Further integration of siliceous and calcareous groups for the subantarctic Oligocene–Quaternary looks encouraging from these initial findings. In addition, higher-resolution diatom biostratigraphy is being undertaken on Leg 189 sediments. Abundant neritic and offshore diatoms of the upper Eocene and lower Oligocene are, in conjunction with dinocysts, proving useful for reconstructing the timing and paleoenvironment of the Eocene–Oligocene (E–O) transition at Sites 1170–1172 (e.g., Stickley et al., submitted [N2]).
Radiolarians are well represented at all Leg 189 sites (except Site 1168) with varying diversity through the sequences. The subantarctic radiolarian biostratigraphic sequence from the middle Eocene through Pleistocene is unique because no useful radiolarian zones for temperate regions of the Southern Hemisphere have been published. These data will provide an important new radiolarian zonation and the potential for correlation between tropical and Antarctic biostratigraphies despite a near absence of tropical and cold-water age-diagnostic species in the Tasmanian region. The radiolarian age assignments used in this age model are (tentatively) based on existing Antarctic and subtropical zonal schemes (e.g., Abelmann, 1990, 1992; Caulet, 1991; Chen, 1975; Hollis, 1993; Lazarus, 1992; Nishimura, 1987; Takemura, 1992; Takemura and Ling, 1997) and modifications of tropical zones (Sanfilippo and Nigrini, 1998). The shipboard radiolarian biostratigraphy, with some deletions, is integrated into the age model here. Further work on refining the radiolarian biostratigraphy is ongoing, and so far has concentrated on the Eocene of Site 1172.
Palynomorphs are extremely abundant in the Paleogene and Cretaceous intervals of Leg 189 sediments and are providing the first well-calibrated Paleogene dinocyst record of the Southern Hemisphere (see Brinkhuis, Munstermann, et al., this volume; Brinkhuis, Sengers, et al., this volume; Sluijs et al., this volume; Williams et al., this volume). A significant number of studies concentrating on Upper Cretaceous to middle Eocene dinocysts from the broad Antarctic realm or Southern Ocean are available, notably from southeast Australia, New Zealand, the Ross Shelf, and Seymour Island as well as several Deep Sea Drilling Project/ODP sites (see overviews in, e.g., Askin, 1988a, 1988b; Wilson, 1988; Wrenn and Hart, 1988; Mao and Mohr, 1995; Truswell, 1997; Hannah et al., 1997; Levy and Harwood, 2000). These studies have documented Southern Ocean Paleogene dinocyst distribution and taxonomy in great detail. However, previous studies concentrating on the E–O transition in the region are but few (e.g., Edbrooke et al., 1998). Moreover, meaningful chronostratigraphic calibration of Paleogene dinocyst events is, typically, largely absent.
Combined dinocyst and diatom stratigraphies in some of the critical boundary intervals has allowed an integrated age model and environmental analysis of the Eocene/Oligocene (E/O) boundary (Sluijs et al., this volume; Stickley et al., submitted [N2]) and the Cretaceous/Tertiary (K/T) boundary (Schellenberg et al., in press) transitions, for example, as well as reconstruction of environmental periodicities and circulation patterns in the Eocene (Huber et al., submitted [N1]; Röhl et al., in press b). In addition, drilling has yielded excellent material to study the variability of dinocyst morphology, notably within the Vozzhennikovia, Deflandrea, and Enneadocysta groups. Postcruise work has focused on dinocyst successions from Sites 1168 (Eocene–Quaternary) and 1172 (Maastrichtian–Oligocene and Quaternary) (Brinkhuis, Munstermann, et al., this volume; Brinkhuis, Sengers, et al., this volume). Age assignment of events for Sites 1168, 1170, and 1171 is derived by correlation to the dinocyst stratigraphy of Site 1172, which is closely calibrated to the magnetostratigraphy of that site. The dinocyst stratigraphy of the Eocene and lowermost Oligocene intervals of Sites 1170 and 1171 are virtually unchanged from the shipboard data, but are summarized here. For further details on the dinocyst scheme for all four deep-penetrating sites as well as site-to-site correlations incorporating early Oligocene diatom events see Sluijs et al. (this volume).
Shipboard paleomagnetic and rock magnetic investigations included routine measurements of natural remanent magnetization (NRM). Both were measured before and after alternating-field demagnetization to 20 mT. Low-field magnetic susceptibility measurements were made with the multisensor track. NRMs and a limited set of rock magnetic observations were made on discrete samples. A nonmagnetic APC core barrel assembly was used for alternate cores in selected holes and the magnetic overprints in core recovered with this assembly were compared with those obtained with standard assemblies. Where magnetic cleaning successfully isolated the characteristic remnant magnetization, paleomagnetic inclinations were used to define magnetic polarity zones. On some occasions, it was possible to recover a satisfactory magnetic stratigraphy even when the inclination was of a single polarity because of a persistent overprint. On such occasions, there were indications of the magnetic stratigraphy in the intensity and associated minor differences in the inclination. To recover the magnetostratigraphy, the z-component alone was used. The z-component was biased in one direction but showed a clear alternating signal superposed upon this. By removing the bias, the magnetization with alternating sign, which carries the magnetostratigraphic signal, is made clearer. Postcruise interpretations of the magnetic polarity stratigraphy, with constraints from the biostratigraphic data, are presented here. The revised timescale of Cande and Kent (1995), as presented in Berggren et al. (1995a, 1995b), was used as a reference for the ages of Cenozoic polarity chrons.
Despite the high carbonate content of sediments recovered through much of the drilled sequences, especially at Sites 1170 and 1171, the generation of a sufficient magnetostratigraphy for useful age-depth reconstruction was possible. Magnetostratigraphic interpretation of the paleomagnetic record at all sites has been possible through most of the sections. Although the quality of the magnetic record deteriorated in older intervals, a relatively complete magnetostratigraphy was achieved, particularly across the E/O boundary. At Site 1168, however, the magnetostratigraphic record has been difficult to establish because of a weak magnetic signal. We present data from only the more strongly magnetized sections of this site. In addition, at Site 1171, the interpretation of the Eocene inclination record was problematic, which led to the development of an approach based upon the sign of the z-component. This approach generated distinctive magnetostratigraphic boundaries, whereas they were almost totally obscured in the inclination record. Further details on the magnetostratigraphy of Site 1168 and Site 1172 are presented in Fuller and Touchard (in press).
The chronology from the benthic oxygen isotope records of Holes 1168A, 1170A, and 1172A, as reported in Nürnberg et al. (in press), are integrated into the biomagnetostratigraphic age models for comparison and further age control for the Quaternary intervals (Tables T1, T2, T4; Figs. F2–F13). In Holes 1168A and 1170A stable oxygen isotope measurements were made on 1–7 tests of foraminifers: Cibicidoides wuellerstorfi, Cibicidoides mundulus, Uvigerina pygmea (Hole 1170A only), and Uvigerina peregrina (Hole 1168A only). The >250-µm size fraction was used to eliminate biases caused by any downslope-displaced smaller tests. For Hole 1172A, oxygen isotope analyses were conducted on 1–3 tests of single benthic foraminiferal species of the genus Cibicidoides. All tests were ultrasonically cleaned in distilled water prior to analysis. The chronostratigraphy is determined by graphic correlation of the benthic oxygen isotope curves with the stacked standard records. The marine oxygen isotope stages (MIS) were recognized using the nomenclature proposed by Prell et al. (1986) and Tiedemann et al. (1994). The record of Martinson et al. (1987) was used as reference curve for the youngest isotope excursions back to Event 8.5. The SPECMAP stack (Imbrie et al., 1984) was used from Event 8.5 to 13.2, and the orbitally tuned benthic isotope record of ODP Site 677 was used as a reference curve for older isotope events (Shackleton et al., 1990). The base of the Holocene plateau (9.7 ka) was recognized in Holes 1168A, 1170A, and 1172A. Initial correlations in Hole 1170A were made with the magnetostratigraphic data. In Hole 1168A the FO of Emiliania huxleyi and the last occurrence (LO) of Calcidiscus macintyrei datums were used for initial correlation. Following these initial correlations datums were tied to the standard records. Correlations were performed with the AnalySeries software (version 1.1) (Paillard et al., 1996). For Hole 1172A, prominent maxima and minima in the oxygen isotope record were correlated to the reference oxygen isotope record of Shackleton et al. (1990) (ODP Site 677 timescale calibration). The age model was verified by comparing it to the oxygen isotope records of Holes 1168A and 1170A. See Nürnberg et al. (in press) for further details of Leg 189 Quaternary benthic oxygen isotope analyses and results.
The reflectance (lightness [L*]) record, measuring color variations, for Hole 1171A (see Shipboard Scientific Party, 2001e) is used for the high-resolution Quaternary chronology presented in Table T3 and Figures F8 and F10, because an oxygen isotope record is not yet available. A chronology is determined from an inter-Site correlation of the Hole 1171A L* record with those from Holes 1170A and 1172A. This detailed correlation allows the transfer of age control points and, thus, the establishment of a stratigraphic framework for Hole 1171A. See Nürnberg et al. (in press) for further information on the use and reasoning behind reflectance data for stratigraphic purposes and, specifically, for the Hole 1171A Quaternary reflectance stratigraphy record and establishment of a chronology.