SCIENTIFIC OBJECTIVES

Leg 181 drill sites are mostly located in sediment drift sites across a depth range of 315-4460 m, and will provide a moderate resolution record (2-5 cm/k.y.) of climatic and paleohydrographic changes since the early Miocene. We aim to recover material that will allow the following scientific problems to be studied.

1. Delineate the Cenozoic development of zonal water masses and the ACC system.
Current understanding of paleoclimate suggests that the earliest major meltwater events from Antarctic glaciation occurred at ~38 Ma (Eocene/Oligocene boundary; Shackleton and Kennett, 1975; Miller et al., 1990). At ~32 Ma (mid-Oligocene), the South Tasman Gateway opened, including the Balleny Fracture Zone (Lonsdale, 1988), thereby connecting the Indian and Pacific Oceans for the first time (Kennett, Burns, et al., 1972). Finally, it was not until ~20 Ma (earliest Miocene) that the opening of Drake Passage (Boltovskoy, 1980) allowed the establishment of the full pattern of circum-Antarctic ocean circulation. The evolution of this system, including periods when the boundary current component may have extended into shallow depths or reversed (Mikolajewicz et al., 1993), is the target of Sites SWPAC-5B and 6B. These sites may also penetrate to the regionally widespread 29 Ma mid-Oligocene unconformity (Marshall Paraconformity; Carter, 1985), the genesis of which may relate to the inception of ACC-DWBC activity as much as to global sea-level change (cf. the postulated large 29 Ma lowstand of Haq et al., 1987). Site SWPAC-1C is targeted on large Miocene-Pliocene platform drifts that grew from a paleowater depth of 1000 m in the head of the Bounty Trough (Fulthorpe and Carter, 1991).

2. Infer the changing paleohydrography of the CDW supply to the Pacific Ocean; in particular, to trace the history of mixing of Weddell Sea Deep Water (WSDW) and NADW components.
Measurement of d18O, d13C, and Cd/Ca from microfossil tests will be used to distinguish the relative contribution of nutrient-enriched NADW and d13C depleted components (Boyle and Keigwin, 1987; Oppo and Fairbanks, 1987; Charles and Fairbanks, 1992; Bertram et al., 1995). Sites SWPAC-2B and 5B were selected to maximize the chance of the high-quality carbonate-rich records required for such measurements. For the deeper water sites, should carbonate percentages be low, the bulk carbonate technique of Shackleton et al. (1993) may yield a satisfactory isotope stratigraphy. However, we have obtained monospecific benthic and planktonic oxygen isotope records for mid-Holocene to Stage 3 from a core at 4802 m near Site SWPAC-16A, and McCave and Carter (1997) estimate the carbonate compensation depth (CCD) to lie at ~4750 m. The late Neogene stratigraphy may be strongly supported by a tephrochronology derived from the numerous widespread Cenozoic ash beds deposited east of New Zealand. Global understanding of the history of these water masses will require the comparison of gradients of water composition between sites in the North Atlantic, South Atlantic, North Pacific, and Southern Oceans.

3. Determine the relative paleoflow speeds of deep and intermediate waters, and thereby estimate the changing flux of CDW into the Pacific through time.
The noncohesive sortable silt (10-63 mm) fraction has been shown at widely separated locations to yield coherent indications of flow speed and hence water mass movement (McCave et al., 1995a, b; Manighetti and McCave, 1995; Robinson and McCave, 1994; Haskell et al., 1991). Evaluation of such grain-size signals in the North Chatham Drift (Site SWPAC-5B) and Campbell Drift (Site SWPAC-7B) will permit estimation of the velocity behavior of the DWBC. Sites SWPAC-1C, 2B, and 6B will yield indications of the behavior of low salinity AAIW. Site SWPAC-7B, located a little to the south of the Bounty Trough, will be important for assessing the extent to which the ACC acts as a driving force for CDW inflow, because the site is at the latitude of the mouth of the trough at which the modern ACC veers east into the Pacific, at that point becoming decoupled from the deep boundary flow (cf. Semtner and Chervin, 1992).

4. Establish the history and depth ranges of AAIW across the New Zealand Plateau.
In the North Atlantic, an intermediate water (possibly paleo-Labrador sea water) has been shown to increase both in depth range and speed during glaciations, concomitant with a decrease in NADW production in the Norwegian-Greenland Sea (Boyle and Keigwin, 1987; Manighetti and McCave, 1993). As this change is associated with suppression of NADW, there is little reason to expect the same glacial/interglacial changes to AAIW in the Southern Ocean. However, Pudsey et al. (1988) have argued that AABW production also diminished during glacials as a result of the grounding of ice sheets, in which case the thickness of AAIW may well have increased concomitantly. If the vigor of global deep circulation was decreased by these North Atlantic and Antarctic events, then during glacial times the Indian/Pacific upper CDW should have become even more nutrient enriched and oxygen depleted than it is today. Material from Site SWPAC-5B (depth 3308 m) will be used for d13C and trace element analysis (e.g., Cd/Ca in calcite and opal) to allow ocean paleochemistry to be used to determine whether during glaciations the site lay under severely depleted AAIW or enriched CDW.

5. Determine the history of productivity and surface water mass fluctuations in the vicinity of the Subtropical Convergence.
Near zones of upwelling, such as the Subtropical Convergence (STC), it is usually difficult to distinguish between climatically controlled temporal and spatial changes in productivity because the convergence moves. Unusually, however, for at least the last full glacial/interglacial cycle the STC has apparently been topographically trapped over the Chatham Rise (Fenner et al., 1992; Nelson et al., 1993; Weaver et al., in press). This raises the prospect of being able to obtain temporal records of productivity change from faunal, isotopic, and chemical data without the aliassing usually produced by shifts in the position of such convergences. DSDP Site 594, in 1200 m of water just south of the present STC, is a valuable control because it shows that cold water reached there in the last glaciation (Nelson et al., 1993), probably representing waters wind-drifted from the Subantarctic Front (SAF), which itself remained bounded by the Campbell Plateau. Site 594 provides a high-quality record extending to the middle Miocene, and we aim to match it by similar records from beneath the STC and farther north (Site SWPAC-5B) and south (Sites SWPAC-6B, 7B).

6. Examine the shifting positions of the Subantarctic Front and Antarctic Convergence.
The zone of cold water between the STC and the Antarctic Convergence (AAC) at 60S is divided by the SAF at about 51S. Nelson et al. (1993) found a sharp cooling of waters at glacial levels in Site 594 (latitude near 45S), suggesting that the SAF lay nearby. Our most southerly drift sites (Sites SWPAC-6B, 7B) and Site SWPAC-8A on the levee of the Bounty Channel will allow us to assess the shift in position of these climatically important fronts, using faunal and coarse fraction analysis, stable isotopes, and magnetic susceptibility measurements to trace ice-rafted detritus.

7. Test the record of circum-Antarctic flow against the Milankovitch orbital model, including estimates of simultaneity with Northern Hemisphere records.
Achieving this objective requires the retrieval of high-quality, long-term faunal and isotopic records from Southern Ocean sites to assess changes in temperature, salinity, and CO2 as components of the climate system. The relative timing of events between the Northern and Southern Hemisphere will be evaluated (cf. Nelson et al., 1985). We anticipate the best long-term records will come from Sites SWPAC-2B and 5B on the north flank of Chatham Rise, where the North Chatham Drift is up to 1000 m thick and probably extends back to the late Oligocene. The earlier period of drift formation will be principally examined farther north (Site SWPAC-9B), where the thickness is reduced, although obviously with less stratigraphic resolution.

8. Study the effect of an oscillating sediment source controlled by Pleistocene sea-level cyclicity on fan overbank turbidite deposition and sediment supply to the DWBC.
The Bounty Channel and Fan are fed with terrigenous sediment through a number of submarine canyons, which cut the eastern South Island shelf edge. Located about 30-60 km offshore, these canyons were alternately supplied directly with sediment during Pliocene-Pleistocene glacial lowstands, and cut off during interglacial highstands when sediment was moved directly north along the inner shelf (Carter and Carter, 1993). Consequently, the Bounty Fan was supplied with terrigenous sediment mostly during glacial periods, and the levees of the Bounty Channel comprise a regular sequence of 5- to 8-m-thick packets of glacial silt-mud turbidites alternating with interglacial biopelagic, calcareous ooze (Carter et al., 1990). The regularity of these cycles is such that they can be matched prima facie with the oxygen isotopic record back to Stage 100, and viewed as the deep-sea record of continental shelf stratigraphic sequences (Carter and Carter, 1992). Site SWPAC-8A will penetrate about 50 of the cycles identified on seismic records, providing (1) a high-quality record of turbidity current activity through the Bounty Channel since the early Pleistocene; (2) a test of the correlation between the observed seismic cyclothems and the oxygen isotopic stages; and (3) a quantitative model of rates of sediment supply to the DWBC and deep sea during a time of glacio-eustatic oscillation of sea level.

9. Estimate the quantities of sediment fed into the DWBC from all sources, including terrigenous sediment delivered through the Solander, Bounty, and Hikurangi Channels; to understand the relative importance of tectonic, climatic, sea-level, and water-mass controls and to derive a sedimentary budget for the ENZOSS.
Since the Pliocene, the three largest sources of sediment for the DWBC drifts have been turbidity currents travelling the length of the Solander, Bounty, and Hikurangi channel systems (Carter et al., 1990; 1994; 1996; Lewis, 1994). Other major sediment sources are direct transport into the region by the DWBC, and pelagic, hemipelagic, and volcanic fallout. In addition to the differing primary targets at each site, all SWPAC sites will contribute data towards the development of a quantitative sedimentary model and budget for the ENZOSS.

10. Examine the history of large volcanic eruptions from the Taupo Volcanic Zone.
Some of the drift sequences targeted for drilling, particularly those nearer North Island, will contain Pliocene-Pleistocene volcanic ash layers of potential value for correlation. Several very large explosive eruptions have occurred in the Taupo Volcanic Zone (TVZ) of New Zealand in the last few million years (Shane et al., 1996). The largest in the last 50 k.y. have exceeded 100 km2 in volume. These ashes are generally distributed to the east of New Zealand (Stewart and Neall, 1984) and occur widely in marine cores (Ninkovitch, 1968; Lewis and Kohn, 1973; Watkins and Huang, 1977; Kyle and Seward, 1984; Nelson, 1988; Carter et al., 1995). Volcanic ash horizons may also provide useful stratigraphic markers at horizons as old as late Miocene (van der Lingen, 1968), and perhaps earlier. We will investigate possible climatic links between ash eruptions and sea surface temperature (SST), through oxygen isotope analysis and transfer function paleoecology.

The application of combined isothermal plateau fission-track (ITPFT) for dating and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) for fingerprinting has revolutionized the ash chronostratigraphy of the Pliocene-Pleistocene Wanganui Basin (e.g., Naish et al., 1996), and holds great promise for application to any ash layers that are recovered from Leg 181 sites.

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