One aim of ODP Leg 181 was to develop a detailed southern mid- to high-latitude biochronology and magnetochronology for the Oligocene through Pleistocene, for the ocean between the Subantarctic Front (SAF) and just north of the Subtropical Convergence (STC), immediately east of New Zealand. A key objective was to monitor global climate fluctuations as reflected in the waxing and waning of the northeast-flowing Deep Western Boundary Current (DWBC), which is a key driving force in global circulation. During Leg 181, nannofossils, radiolarians, diatoms, and foraminifers were studied at seven sites, which were triple, double, or single cored with the APC and XCB systems. The presence of silicoflagellates, chrysophycean cysts, opal phytoliths, sponge spicules, bolboformids, ebridians, and calcareous dinoflagellate cysts were routinely monitored. With most of the sites cored above the carbonate compensation depth, excellent suites of both calcareous and siliceous microfossil assemblages were recovered.
The only existing Neogene site in this region, DSDP Leg 90, Site 594, just south of the STC on the Chatham Rise, provided a low-resolution late Neogene biozonation (Martini and Jenkins, 1986), with provisional ties to the detailed regional chronostratigraphy developed for New Zealand (e.g., Hornibrook and Jenkins, 1994). The ODP Leg 181 sites, strategically chosen on the Hikurangi Plateau, Chatham Rise, and Campbell Plateau to detail both the Neogene global climate change and the growth of the giant drifts redistributed by the DWBC, will greatly expand such calibration. Because Leg 181 analyses simultaneously develop magnetostratigraphy, isotope stratigraphy, and biostratigraphy, they will provide definitive links to the standard time scale for the subantarctic and subtropical New Zealand zonal schemes of nannofossil and planktonic foraminiferal datums, that is, ties to the Cenozoic New Zealand stages (e.g., Edwards et al., 1988; Hornibrook et al., 1989; Hornibrook, 1990; Scott et al., 1990; Morgans et al., 1996; Naish et al., 1998). Northern sites contain numerous volcanic tephras erupted from New Zealand and distributed eastward in the prevailing westerly winds, which will allow further calibration to standard time scales (e.g., Shane et al., 1995), as will the cyclostratigraphic record from the spliced triple- and double-cored APC holes.
ODP Legs 113, 114, 117, 119, 120, and 177 laid the foundation for a circumpolar southern high-latitude biostratigraphy and chronostratigraphy. Cenozoic sequences recovered during these legs provided biostratigraphic zonations based on calcareous and siliceous microfossils and on key species ranges that could be tied directly to the geomagnetic time scale (e.g., Gersonde et al., 1990; Thomas et al., 1990; Barron et al., 1991; Harwood et al., 1992; Wei, 1992; Wei et al., 1993). Leg 181 drilling of a latitudinal transect across the STC will allow further improvement and refinement of these biostratigraphic schemes, and the intercalibration of high- and mid-latitude zonations and species ranges in key microfossil groups for biochronology. Improved dating of Neogene biostratigraphic ranges can be accomplished by correlation with Leg 181 magnetochronology, by using orbitally tuned signals from stable isotope analysis, or using other data sets with high temporal resolution, such as color reflectance, magnetic susceptibility, and biota abundance records. In addition, data from the transect of sites will lead to a understanding of the evolutionary processes (patterns, modes, and timing of speciation and diversification) of taxa, of the migration of taxa across the STC, and of the origin and development of the Southern Hemisphere bioprovinces.
Ages were assigned primarily based upon core-catcher samples. Samples from within the cores were examined when a more refined age determination was necessary. Correlations to standard chronostratigraphic frameworks will be improved by integration of data from post-cruise magnetobiostratigraphic studies and oxygen isotope stratigraphy.
Ages for calcareous nannofossil, foraminiferal, diatom, and radiolarian events in the Paleogene and Miocene are according to the Geomagnetic Polarity Time Scale (GPTS) of Cande and Kent (1995) in Berggren et al. (1995a, 1995b), with minor modifications for the Pliocene and Pleistocene using the astrochronology of Lourens et al. (1996). Although refined astrochronologies do exist for Miocene and even older sections, none is yet firmly calibrated to either standard biochronology or standard magnetochronology (F. Hilgen, pers. comm., 1998), which led us to prefer the Oligocene-Miocene GPTS for calibration of the microfossil datums. For the Pliocene and Pleistocene, the GPTS and the astrochronologies of Mediterranean sapropels and the oceanic stable isotope records are concordant. A compilation of microfossil events ages updated from the literature for four major microfossil groups was made (Tables T2, T3, T4, T5, all also in ASCII format; Figs. F7, F8). These tables were prepared as an aid to quickly look up the definition of biostratigraphic events and chronostratigraphic boundaries and their links to the time scale.
The Oligocene/Miocene boundary is defined at the 35-m level of the Rigorosa Formation in the Carrosio-Lemme section, northwest Italy, corresponding to the base of polarity chron C6Cn.2n and the first appearance of Globorotalia kugleri. The age estimate in GPTS is 23.8 Ma. This level correlates to middle Waitakian in New Zealand (Morgans et al., 1996; Fig. F7).
The Miocene/Pliocene boundary is defined at the onset of marine conditions in the Mediterranean at the base of the Zanclean, correlated to polarity Chron C3r in the GPTS and 5.32 Ma in the astrochronology of Hilgen (1991a, 1991b). Biostratigraphically, the boundary is bracketed by the highest occurrence of Discoaster quinqueramus and Triquetrorhabdulus rugosus and the lowest occurrence of Ceratolithus acutus (see review in Berggren, 1995b). In New Zealand, the boundary interval occurs within the upper Kapitean stage (Morgans et al., 1996; Fig. F7), immediately below the Opoitian stage.
The Pliocene/Pleistocene boundary, defined in the Vrica Section (Calabria, Italy), is located near the top of magnetic polarity Chron C2n, with an estimated age of 1.81 Ma (Lourens et al., 1996). Biostratigraphically, the boundary may be approximated with the lowest occurrence of medium-sized Gephyrocapsa spp. (bmG event) (Raffi et al., 1993) and the highest occurrence of Discoaster brouweri and D. triradiatus (Berggren et al., 1995a). In New Zealand the boundary occurs within the Nukumaruan stage (Fig. F7).
In the previous section, mention was made of the need to correlate the Leg 181 biostratigraphy and chronostratigraphy both to standard stratigraphy and to regional New Zealand stratigraphy for the Neogene. Hence, in Figure F7, we have reproduced the New Zealand geological time scale for the Cenozoic, produced by the Institute of Geological and Nuclear Sciences (Morgans et al., 1996), with minor modifications to the Pliocene-Pleistocene stage usage after Carter and Naish (1998). Using isotope and cyclostratigraphy it may be feasible to directly tie Leg 181 Pliocene and Pleistocene records to the Southern Hemisphere cyclostratigraphic reference section in the Wanganui Basin, New Zealand, which was calibrated by Naish et al. (1998) to the astrochronology of Lourens et al. (1996). The latter opens the possibility to interpolate Leg 181 paleoceanographic events at a remarkable level of accuracy beyond that of conventional biostratigraphy.
Paleobathymetric interpretations in bathyal and abyssal depths are difficult to standardize, because none of the biotic boundaries have absolute paleo-water-depth limits, but rather are a function of many interrelated factors. Organic carbon supply and bottom-water oxygen content are considered principal determinants of the benthic biota within discrete paleo-water-depth units. During Leg 181, we made use of guidelines for benthic and planktonic foraminiferal distributions according to Hayward (1986), van Morkhoven et al. (1986), and Gradstein and Berggren (1981).
Interpretations of the distribution of overlying sea-surface water masses through time were made using the presence and abundance of distinctive subtropical and subantarctic species and assemblages of planktonic foraminifers (e.g., Fig. F9).
During Leg 181, we attempted to correlate our zonations to those of Martini (1971) and Bukry (1973, 1975, 1978), code numbered by Okada and Bukry (1980). These zonations are regarded as a standard framework for the biostratigraphic subdivision of low-latitude Cenozoic marine sediments based on calcareous nannofossils. As indicated by many previous studies in this area of the Southwest Pacific (e.g., Edwards and Perch-Nielsen, 1975; Rade, 1977; Lohman, 1986), the standard zonation schemes are not always applicable because of the paucity/absence of some of the index species. As a mitigation, we adopted the age assignments of several nannofossil datum levels that are well tied with the time scale of New Zealand Stages (Morgans et al., 1996; Naish et al., 1998) (see Table T2). These ages were used as a tentative basis upon which to establish depth-age models for preliminary investigation. Some of the datum levels might be synchronous across the sites, but others are not. There is a need for further investigation and calibration using magnetostratigraphy, chemostratigraphy, tephrachronology, and cyclostratigraphy.
It has been shown that several important Neogene nannofossil markers show substantial diachroneity across the latitudes between the equator and the warm subtropics (36° S) in the Southwest Pacific west of New Zealand (Dowsett, 1988). Ranges of nannofossils at high-latitudes (50°-65°S) in the South Atlantic and Indian Oceans also show significant diachroneity (Wei et al., 1993). It is conceivable that the distribution, evolution, and extinction of calcareous nannofossils in the studied areas have been modulated by the development of oceanic front systems, and, therefore, many of these datum levels could be time transgressive. Detailed magnetostratigraphies across the studied area will assist with estimates of their numerical ages of bioevents in various holes. The depth of each datum level was recorded at the midpoint between observed samples.
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).
Regarding state of preservation, we consider that etching and overgrowth are the most important features. In order to establish a ranking of preservation we have followed the code systems prescribed in the JANUS database system:
Five categories of relative abundance of individual taxa were applied:
For Site 1121, an additional category was added:
Total abundance of calcareous nannofossils for each sample was estimated as follows:
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; Berggren et al., 1995a). Based on previous work (Brunner, 1991; Pujol and Bourrouilh, 1991), the late Neogene subantarctic zonation scheme by Jenkins and Srinivasan (1986), as modified by Scott et al. (1990) and Hornibrook and Jenkins (1994) (Fig. F7), was deemed best to divide the sequences recovered during Leg 181. However, because of the absence or low abundance of several marker species, this zonation scheme was not fully applicable to the foraminiferal assemblages recovered during Leg 181, and both planktonic and local New Zealand benthic datums (Table T3) were used extensively to assist with age assignments.
The generic and species classification used for Neogene planktonic foraminifers mainly follows Kennett and Srinivasan (1983) and Scott et al. (1990, 1995). The latter deals exclusively with genus Globorotalia. Species identification of Paleogene and early Neogene planktonic foraminifers are based on Hornibrook et al. (1989), Stott and Kennett (1990), and Berggren (1992b).
Benthic foraminifers provide limited biostratigraphic age control as currently applied to Leg 181 samples. Individual benthic foraminiferal datums recognized are mostly regional datums established for on-land and nearshore New Zealand (e.g., Hornibrook et al., 1989) and are discussed for each site. Benthic foraminiferal taxonomy follows van Morkhoven et al. (1986), Hornibrook et al. (1989), Thomas (1990), and Mackensen and Spiegler (1992).
Bolboformid taxonomy is based on Qvale and Spiegler (1989), Spiegler (1991, in press), Mackensen and Spiegler (1992), Grützmacher (1993), and Spezzaferri and Spiegler (1998). It is questionable if the local stratigraphies derived from the bolboformid zonations in the North Atlantic and Indian Oceans are applicable to the Leg 181 region, hence the biostratigraphy derived from this group has been used with caution.
To obtain planktonic and benthic foraminifers and bolboformids from core-catcher samples, a 20-50 cm3 sample was soaked in tap water, disaggregated, and washed over a 63-µm sieve. At deeper levels in Site 1123, the low abundance of foraminifers and high degree of chalk cementation required larger samples to be broken down using a combination of water, hydrogen peroxide, and ultrasonic bath. The sieves were immersed in a methylene blue solution between successive samples in order to stain specimens left in the sieves from previous samples. All samples were dried under heat lamps.
Species abundances (as percentages of the total planktonic foraminiferal, benthic foraminiferal, or bolboformid assemblages) were defined as follows:
Preservation was categorized as
The abundance of planktonic foraminifers, benthic foraminifers, and/or bolboformids as groups relative to the total residue were categorized as
During Leg 181 the diatom zonation for the Neogene by Harwood and Maruyama (1992) was used with slight changes as proposed by Gersonde and Zielinski (Shipboard Scientific Party, 1999) in their study of ODP Leg 177 samples (Fig. F8). Gersonde and Zielinski replace the Thalassiosira insigna-T. vulnifica Zone of Harwood and Maruyama (1992) with the Thalassiosira insigna Zone, because the first occurrence of Thalassiosira vulnifica is probably diachronous. Instead, these authors use the total range of T. insigna. Also, these authors place the base of the Fragilariopsis reinholdii Zone, marked by the first occurrence (FO) of the nominate taxon, in polarity Chron C4 at around 8.1 Ma, an age that is close to the age of its FO in the Equatorial Pacific, as reported by Barron (1992). For the Oligocene, the diatom zonation for the high southern latitudes by Fenner (1984, 1985) and modified by Harwood and Maruyama (1992) is followed.
The ages of diatom datums (Table T4) follow Gersonde and Bárcena (1998) and Gersonde et al. (1998). The ages are based on the correlation of the stratigraphic ranges of the marker species to the GPTS of Berggren et al. (1995a, 1995b). This correlation to magnetostratigraphy, which started with work on piston cores (e.g., McCollum, 1975; Ciesielski, 1983) was refined as a result of Legs 113, 114, 119, and 120 (e.g., Gersonde and Burckle, 1990; Fenner, 1991; Baldauf and Barron, 1991; Harwood and Maruyama, 1992; Gersonde and Bárcena, 1998). Because of the occurrence of warm and temperate species in the northernmost sites of Leg 181, additional stratigraphic ranges have been added following the compilation of Barron (1992). The ages for the diatom datums given for the Oligocene in Table T4 follow Harwood and Maruyama (1992) and the Shipboard Scientific Party (1999) for Leg 177.
The diatom assemblages observed in sediments recovered from Leg 181 also provide environmental information, which helps in reconstructing (1) the thermal isolation of the Southern Ocean, (2) the initiation of Antarctic Bottom Water Currents, (3) changes in the late Quaternary surface-water temperatures, and (4) definition of water masses and positions of oceanic fronts.
The presence of freshwater and benthic marine diatoms at Leg 181 sites indicates input from New Zealand and its coastal regions, but may also be in part aeolian input from Australia through the prevailing strong westerly winds. The common presence of reworked diatoms in the sites from the Campbell Plateau allowed the recognition of phases of intensified currents. Displaced subantarctic diatoms from the Bounty Trough area characterize the Chatham Drift sediments.
Depending on overall diatom abundance, two types of slides were prepared for diatom analysis. For intervals rich in biogenic silica, smear slides were prepared from a small amount of raw material from a core catcher or from additional core material when required. At most sites the sediments were too rich in calcium carbonate and/or terrigenous detritus to allow this method to be used. Hence, for most samples it was necessary to boil the sediment 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 on microslides, covered with 18-mm-diameter cover glasses using Mountex mounting medium. All slides were scanned with a Zeiss microscope at a magnification of 400× for stratigraphic markers and paleoenvironmentally sensitive taxa. Species identification was confirmed when necessary at 1000× . For documentation of poorly known or undescribed taxa, photomicrographs were taken using a video-print system at 1500× final magnification.
Overall diatom abundance in the HCl-insoluble residue, as well as species abundance within the diatom assemblages, were estimated based on smear-slide evaluation at 400× , using the following categories:
Preservation of diatoms was differentiated as follows:
At several sites, preservation and abundance estimates were plotted vs. depth to visualize the distribution of diatom abundance and preservation.
Neogene radiolarian biostratigraphy during Leg 181 in the Southwest Pacific was based largely on the southern mid- to high-latitude zonations established by Caulet (1986, 1991), Abelmann (1992), and Lazarus (1992). Because the Leg 181 transect sites cover the SAF and STC, the Neogene radiolarian assemblages include low- to mid-latitude zonal markers, which prevented application of the Antarctic/subantarctic zonal scheme. Tentative age assignments, therefore, were based on the mid-latitude zonation of Foreman (1975) and Morley (1985) established in the North Pacific region. The Paleogene radiolarian zonation used in this study follows those by Takemura (1992) and Hollis (1997). Low-latitude radiolarian zonation and code numbers, which were tied to the GPTS of Berggren et al. (1995a, 1995b) by Sanfilippo and Nigrini (1998), were compiled for the purpose of correlation between mid- and high-latitude zonations. The ages of radiolarian datums follow Sugiyama (Shipboard Scientific Party, 1999) with some additions. Figure F10 and Table T5 show radiolarian zonations and datums used during Leg 181, respectively.
To obtain radiolarians from core-catcher samples, about 10 cm3 of sediment was disaggregated and boiled with 10% H2O2, 10% HCl, and about 1% Calgon solutions. Brief treatment of samples in an ultrasonic bath was followed by washing on a 63-µm mesh sieve. The residue was moved into 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 onshore to locate biostratigraphic events more accurately within cores.
Overall radiolarian abundance was determined based on strewn-slide evaluation at 100× , using the following convention:
The abundance of individual species was recorded relative to the fraction of the total assemblage as follows: