Tectonic Creation of the Southern Ocean
The origin of the modern thermohaline ocean circulation system must postdate the tectonic creation of a continuous Southern Ocean. Particularly important for the origin of the ACC-DWBC was the opening of the Australian-Antarctic (South Tasman) and South American-Antarctic (Drake Passage) deep-water flow gateways (Lawver et al., 1992). The South Tasman Gateway, including the Balleny Fracture Zone (Lonsdale, 1988), opened to deep water in the early Oligocene (~32 Ma), thereby allowing connection between the Indian and Pacific Oceans for the first time (Kennett, Burns, et al., 1972). Later, at ~20 Ma (earliest Miocene), the opening of Drake Passage (Boltovskoy, 1980) allowed the establishment of the full circum-Antarctic ocean circulation. During the critical late Eocene to Miocene period, the New Zealand Plateau was located downcurrent from the evolving South Tasman Gateway (Watkins and Kennett, 1971), and directly in the path of the evolving ACC-DWBC system.
The supply of deep water to the Pacific Ocean is dominated by a single source, the Deep Western Boundary Current (DWBC) that flows north out of the Southern Ocean along the east side of the Campbell Plateau-Chatham Rise-Hikurangi Plateau, east of New Zealand (Figs. 1, 2, 3, 4, and 5, Table 1). The volume transport of the DWBC in this region is about 20 Sv, which comprises ~40% of the total input of deep water to the world's oceans (Warren, 1973; 1981). (A secondary, but minor, flow of ~3 Sv of deep water flows north into the Peru-Chile basin; Lonsdale, 1976). The magnitude of DWBC flow, and the low temperature of the water involved, are major determinants of the oceanography of the Pacific Ocean and of the global heat balance. Monitoring the DWBC flow at its entry into the Pacific is a key area where the "global salt conveyor" hypothesis (Broecker et al., 1990; Schmitz, 1995) can be tested, as the flow, thereafter, is believed to spread out to fill the Pacific. Some water upwells and returns at shallower depths to the Atlantic, whereas other waters return south as North Pacific Deep Water (NPDW).
The supply of cold water to the deep Pacific from the main generating regions in the Weddell and Ross Seas is modulated by the ACC, which mixes these waters with North Atlantic Deep Water (NADW) in the South Atlantic to form Circumpolar Deep Water (CDW). Deep-water output to the Pacific, therefore, carries the combined signatures of Southern Ocean processes in the region of deep-water formation, chemical composition related to Southern Ocean gas exchange, and NADW. Despite its turbulent passage around Antarctica, CDW is not completely mixed, and a distinct NADW salinity maximum can be recognized at depths of 2800 m (at 55°S) deepening northwards to 3400 m (at 28°S). In the Southwest Pacific, the DWBC comprises three main divisions (1) lower CDW, a mixture of bottom waters generated around Antarctica, in particular cold Weddell Sea deep water and NADW; (2) salinity-maximum middle CDW, representing the NADW core; and (3) strongly nutrient-enriched and oxygen-depleted upper CDW, mainly derived from Indian Ocean outflow added to Pacific outflow returning through Drake Passage. The DWBC has its upper boundary at depths around 2000-2500 m. On the eastern side, the DWBC is overlain between 2550 and 1450 m depth by south-flowing NPDW, and is marked by an oxygen minimum and high silica. Regionally, both DWBC and NPDW are overlain by low-salinity, Antarctic Intermediate Water (AAIW) (Figs. 4, 6).
The ACC-DWBC enters the Southwest Pacific through gaps in the Macquarie Ridge complex before passing along the 3500-m-high margin of the Campbell Plateau. Near the mouth of the Bounty Trough, the ACC uncouples and continues its eastward path, whereas the DWBC flows north around the eastern end of Chatham Rise and through Valerie Passage, where a small part of the flow diverges through gaps in the Louisville Ridge. Valerie Passage, the 250-km-wide gap between the Chatham Rise and the Louisville seamount chain, therefore marks the gateway to the Pacific for the DWBC.
Record of the ACC-DWBC
Sediments on the eastern New Zealand margin at shelf to upper bathyal depths (100-1000 m) are known to have been strongly affected by currents since at least the late Oligocene (Ward and Lewis, 1975; Carter, 1985; Fulthorpe and Carter, 1991; R. Carter et al., 1996). This evidence for strong paleoflows, together with the confirmation that substantial Antarctic glaciation commenced at least as early as the early Oligocene (Shackleton and Kennett, 1975; Barron, Larsen, et al., 1989), implies that Pacific hydrography has been fundamentally affected by an evolving circumpolar current and western boundary current system since the mid-Cenozoic.
To reconstruct the paleoflow of the DWBC and overlying current system requires drill sites through thick, undisturbed, fine-grained sediment masses constructed under the influence of the current. Seismic records indicate the presence of suitable sedimentary drifts at many points along the eastern edge of the New Zealand Plateau, in water and paleowater depths between 300 m and 5500 m (Carter and McCave, 1994; L. Carter et al., 1996). Three sediment sources are involved in building these drifts: (1) transport into the area via the DWBC itself (e.g., subantarctic diatoms present in the drifts at 40°S; Carter and Mitchell, 1987); (2) pelagic and hemipelagic rain, and airfall rhyolitic ash, which over the last 20 k.y. has been input at a rate of up to one third that of fluvial terrigenous sediment (Carter et al., 1995); and (3) terrigenous muddy sediment from New Zealand, deposited at the shelf edge by slope progradation (Fulthorpe and Carter, 1991) or delivered into the path of the DWBC from turbidity currents travelling down the Solander, Bounty, and Hikurangi channel systems. Each of these sediment sources can be constrained, and the sedimentary dynamics and transport paths of the modern system are moderately well delineated (e.g., Carter and Carter, 1993; Carter and McCave, 1994; Lewis, 1994). In contrast, little is known regarding the earlier Cenozoic record of the DWBC.
The available seismic records show that the DWBC has been active along the eastern New Zealand margin since at least the Miocene, and probably since the mid-Oligocene (32 Ma) (Carter and McCave, 1994). After ~10 Ma, abundant terrigenous material was shed from rising mountains along the Alpine Fault plate boundary (Kennett, von der Borch, et al., 1986) and fed into the Solander, Bounty, and Hikurangi channel systems, especially at times of late Neogene glacial sea-level lowstand. Much of this sediment was then entrained in the DWBC drift system, which carried it northwards to be eventually subducted into the Kermadec Trench.
Sediment is delivered into the DWBC through two newly described transport conduits, the Bounty (Carter and Carter, 1993) and Hikurangi (Lewis, 1994) channel-fan systems. A third feeder channel, Solander, is poorly known, but extends for >450 km before discharging into the DWBC at Emerald Basin between Macquarie Ridge and the western side of Campbell Plateau (Carter et al., 1996; Carter and McCave, in press). The Hikurangi Fan has been termed a "fan-drift" by Carter and McCave (1993) because it apparently represents the extreme case of a fan whose thickness and facies pattern are directly remolded by a deep current into the form of a sediment drift. In contrast, the Bounty Fan, located in a bathymetric embayment, has retained its fan morphology and has developed directly across the path of the DWBC (Carter and Carter, 1993), the only evidence of drift formation being scour of the northern fan and redeposition of material as a series of small, discrete ridges. Compared to Hikurangi Fan Drift, Bounty Fan has formed in a region where the DWBC is slowed because of a gently sloping western boundary and the shelter provided by Bollons Seamount.
The two described abyssal fans are supplied with sediment by turbidites passing through the Bounty and Hikurangi Channels, each of which is over 1000 km long. The Hikurangi Channel heads in the Kaikoura Canyon, only a few hundred meters from shore, and less than 10 km from the rapidly rising, 2.5-km-high Seaward Kaikoura Mountains. The Hikurangi system is therefore active today, in interglacial times. In contrast, the Solander and Bounty Channels head in a number of canyons that indent the edge of the continental shelf. The Bounty and Solander systems are, therefore, strongly sea-level (i.e., climatically) controlled with most sediment being fed into them during glacial lowstands, whereas during interglacials the same sediment stream is diverted along the inner shelf, some of it even reaching the Hikurangi System via the Kaikoura Canyon (Carter and Herzer, 1979).
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