In summary, our knowledge of Southwest Pacific Ocean history, and, in particular, the development of the ACC-DWBC system, is extremely poor. Leg 181 researchers, therefore, drilled seven holes in the eastern New Zealand region to attempt to reconstruct the stratigraphy, paleohydrography, and dynamics of the DWBC and related water masses (Figs. F1, F2A). The sites composed a transect of water depths from 393 to 4460 m and spanned a latitudinal range from 39° S to 51° S (Fig. F2). Leg 181 drilling has provided the data needed to study a range of problems in Southern Ocean Neogene paleohydrography, sedimentology, paleoclimatology, and micropaleontology.
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 (Molnar et al., 1975; Lawver et al., 1992). The South Tasman gateway, including the Balleny Fracture Zone, opened to deep water in the early Oligocene (~32 Ma), thereby allowing connection between the Indian and Pacific Oceans for the first time (Kennett et al., 1972; Kennett, 1977). Later, at ~20 Ma (earliest Miocene), the opening of Drake Passage (Boltovskoy, 1980) allowed the establishment of the full circum-Antarctic oceanic circulation. During the critical late Eocene to Miocene period, the New Zealand microcontinent was located downcurrent from the evolving South Tasman gateway (Watkins and Kennett, 1971, 1972), directly in the path of the evolving ACC-DWBC system (Fig. F3).
The evolution of the ACC-DWBC system took place during the Oligocene and earliest Miocene (32-20 Ma), when plate movements created the first deep-water oceanic gaps south of Australia and South America. The stratigraphic record of these events, and of the development of the modern ACC-DWBC, occurs in Cenozoic sediments located on and just east of the New Zealand microcontinental plateau (Fig. F4). South Island, New Zealand, is today transected by the Alpine Fault boundary between the colliding Australian and Pacific plates, but the greater part of eastern New Zealand has been unaffected by major tectonic events since the phase of Late Cretaceous rifting that created the South Pacific Ocean and first delineated the New Zealand Pacific continental margin. In short, since the Late Cretaceous, eastern New Zealand--which includes the submerged continental crust of Campbell Plateau and Chatham Rise, separated by the rift re-entrant of the Bounty Trough--has been a trailing-edge passive margin and subject to thermotectonic subsidence and marine transgression (Cotton, 1955; Carter, 1988b).
The geological history of eastern New Zealand therefore consists of Late Cretaceous rift-valley fill (Bishop and Laird, 1976), followed by peneplanation and a marine transgression that reached its climax in the Oligocene, when almost the entire New Zealand plateau was submerged, terrigenous sources were flooded or buried, and regional carbonate sedimentation reigned supreme (Fleming, 1975; Suggate et al., 1978). In the west, tectonic activity, associated with the development of the transform plate boundary through New Zealand, started in the late Eocene (Turnbull, 1985; Turnbull and Uruski, 1995; Sutherland, 1995), and, by the early Miocene, copious volumes of terrigenous sediment from mountains along the Alpine Fault were being shed eastward into the Canterbury Basin, where they built the progradational sedimentary prism that underlies the modern coastal lowland and continental shelf of eastern South Island (Carter and Norris, 1976; Norris et al., 1978). Mountain building accelerated, and, presumably, sediment yields increased, at ~6.5 Ma in the late Miocene, when a shift in the pole of rotation resulted in a stronger element of collision across the Alpine Fault boundary (Walcott, 1998). These various plate boundary events had only minor effects in the eastern (offshore) parts of the New Zealand plateau (Fig. F4B). Apart from localized episodes of volcanism and mild folding-faulting associated with changes in regional stress patterns (e.g., Oliver et al., 1950; Carter, 1988a; Carter et al., 1994; Campbell et al., 1993), stasis or very slow postrift subsidence continued, and major sources of terrigenous sediment were absent. Sediment accumulation on bathymetric highs was either precluded by strong water motion (e.g., Chatham Rise) or consisted of biopelagic ooze and chalk (e.g., Campbell Plateau). In contrast, terrigenous sedimentation was restricted mainly to bathymetric lows (e.g., Bounty Channel-Fan complex) or to sites adjacent to the prograding eastern New Zealand sediment prism (e.g., DSDP Site 594; Kennett, von der Borch, et al., 1986).
Modern New Zealand is transected by the plate boundary between the Pacific and Australian plates (Fig. F5). In South Island, the boundary is marked by the Alpine Fault transform, which passes north into the active arc volcanism of central North Island, east of which the Hikurangi subduction complex marks the subduction of the Pacific Plate under eastern North Island. The geology of eastern North Island (the East Coast Basin) is therefore complex and largely allochthonous (Stoneley, 1968), within which individual tectonic slices represent the emplacement of forearc shelf and slope-basin sediments throughout the Neogene (Ballance, 1976; Lewis, 1980; Pettinga, 1982; Lewis and Pettinga, 1993). The tectonic activity, basin formation, and enhanced terrigenous sedimentation rates that relate to the modern plate boundary commenced in the late Oligocene at ~25 Ma (Field et al., 1997).
All Leg 181 sites are located on the Pacific Plate and penetrated the typical eastern New Zealand rift-drift, or passive margin, succession. However, the results from sites that transected Paleogene sediments (e.g., Sites 1121 and 1124), and also those that are located north of Chatham Rise and contained abundant Miocene and younger tephras (e.g., Sites 1123, 1124, and 1125), are extremely relevant to the interpretation of North Island East Coast Basin geology. The Leg 181 Paleogene successions include deep-marine siliceous mudstone, brown mudstone, and nannochalks, which have close counterparts, respectively, in the Whangai Shale, Waipawa Black Shale, and Amuri Limestone facies onland. The succession of siliceous shale-black shale-nannofossil ooze may be regionally homotaxial and results partly from the postrift subsidence of the margin and partly from change in oceanographic factors through time (cf. Andrews, 1979). Particular sediment facies may therefore be of different ages offshore and onshore, but, in all cases, the occurrence of Leg 181 Paleogene facies within the North Island allochthon represents the accretion onto the Australian Plate of former Pacific Plate deep-marine continental margin sediments. This is an important conclusion, given that there has been a protracted controversy over a shallow vs. a deep-water origin for the onland East Coast Basin Paleogene sediment facies (e.g., Field et al., 1997). Finally, study of the tephra-rich Miocene and younger sediments from Leg 181 sites will help establish a greatly improved timetable of volcanic and tectonic activity for the Hikurangi margin.
The supply of deep water to the Pacific Ocean is dominated by a single source, the DWBC, which flows north out of the Southern Ocean along the east side of the Campbell Plateau-Chatham Rise-Hikurangi Plateau, east of New Zealand (Figs. F2, F6). The volume transport of the DWBC is ~20 × 106 m3 s-1 (Sv), which composes ~40% of the total input of deep water to the world's oceans (Warren, 1973; 1981). A secondary, but minor, deep flow of ~3 Sv passes 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 thermohaline conveyor" hypothesis (Gordon, 1986; 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 across the Indian Ocean and on 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 Atlantic sector of the Southern Ocean 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 northward to 3400 m (at 28° S). In the Southwest Pacific, the DWBC comprises three main divisions: lower CDW, a mixture of bottom waters generated around Antarctica, in particular cold Weddell Sea Deep Water (Pudsey et al., 1988); salinity-maximum middle CDW, representing the NADW core; and strongly nutrient-enriched and oxygen-depleted upper CDW, mainly derived from Indian Ocean outflow added to Pacific outflow returning through Drake Passage (Fig. F6). The DWBC has its upper boundary at depths around 2000-2500 m. On the eastern side, the DWBC is overlain between 2500 and 3500 m depth by south-flowing NPDW, marked by an oxygen minimum and high silica content. Regionally, both DWBC and NPDW are overlain by low-salinity AAIW (Figs. F2B, F6).
The ACC-DWBC enters the Southwest Pacific around and 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 along the eastern margin of the Louisville Seamount chain (McCave and Carter, 1997; cf. Lonsdale, 1988). 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 (Warren, 1973).
In the shallower ocean, the seas east of South Island are crossed by two major frontal systems that exhibit intensified meridional gradients in temperature, salinity, and density (Fig. F2). At around 55° S, the east-flowing ACC is bounded to the north by the SAF (Orsi et al., 1995), but it then curls north around the southeastern corner of the Campbell Plateau to almost 50° S. South of the SAF, the annual mean surface-water temperature is <10° C, and the nutrient-rich polar ocean is rich in both phosphate and silica. The observations of Bryden and Heath (1985), together with global circulation models (Carter and Wilkins, in press), indicate that another unnamed front may extend east into the Pacific from the south side of the Bounty Trough. This front, at about latitude 46° S, probably marks the northern limit of the ACC east of New Zealand, rather than the 56° S SAF that is usually taken as the ACC limit (e.g., Orsi et al., 1995).
About 10° of latitude north of the SAF, the STC separates subantarctic water of salinity 34.5 and an annual surface-temperature range of 8° -15° C from subtropical water with salinity >35 and annual temperatures of >15° C. East-flowing currents occur on both sides of the STC, the warm East Cape Current (ECC; a continuation of the East Australian current) on the north side, and the cold West Wind Drift on the south. Interaction of the ECC with the bathymetry, the strong density gradients of the STC, and tides all interact to produce a strong, variable current regime (e.g., Chiswell, 1994a; Heath, 1985). Eastward flow on the south side of the STC is augmented by the Southland Current, which passes around the south end of South Island and then flows north along the eastern South Island continental margin before splitting, with one branch flowing north through Mernoo Saddle and the other branch turning east along the southern side of the crest of Chatham Rise (Heath, 1985).
Heat transfer from the equator to the pole takes place across the SAF and STC by a combination of wind drift and dynamic eddying, with a cold return flow at depth in the DWBC. In the open ocean, as demonstrated in the Indian sector of the Southern Ocean, these fronts may migrate backward and forward by up to 6° of latitude during a glacial/interglacial cycle (e.g., Howard and Prell, 1992). However, modern seasonal movements of the front of at least 2° of latitude occur also, as for example for the STC east of New Zealand (Chiswell, 1994b). In contrast to the seasonal oceanic mobility of the STC, several authors have shown that, over the long term, the STC has probably remained in the vicinity of the shallow Chatham Rise throughout at least the most recent climatic cycle (Fenner et al., 1992; Nelson et al., 1993; Weaver et al., 1998).
The pronounced bathymetry around New Zealand exercises a controlling influence on the disposition of both deep currents and oceanic frontal zones. Notably, the ACC and DWBC are steered around or through gaps in Macquarie Ridge and are then guided northward along the eastern escarpment of the Campbell Plateau. Concomitantly, in near-surface waters, the ACC bounded by the SAF is forced north around and along the southeastern corner of the Campbell Plateau, whereas the position of the STC is probably regulated by the eastward currents flowing along north and south flanks of Chatham Rise. Some Leg 181 sites were chosen to track whether or not past changes in the position of these fronts have occurred.
Sediments on the eastern New Zealand margin at shelf to upper bathyal depths (50-1000 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; Barrett, 1996; 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 middle 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 candidate sediment drifts at many points along the eastern edge of the New Zealand microcontinent, in water and paleo-water depths between ~300 and 5500 m (Fulthorpe and Carter, 1991; Carter and McCave, 1994; L. Carter et al., 1996). There are, however, five possible origins for any particular body of sediment: (1) deposition as part of the deepening- and fining-upward rift-drift cycle that characterizes New Zealand's Cretaceous to Oligocene history (i.e., Matakaea and Onekakara Group equivalents [Carter, 1988b]; cf. Fig. F4); (2) transport into the area via the DWBC (e.g., subantarctic diatoms present in the drifts at 40° S; Carter and Mitchell, 1987); (3) biopelagic snow (Nodder, 1998); (4) airfall rhyolitic and andesitic tephra, which derives from explosive Miocene-Holocene arc volcanism in New Zealand (Ninkovich, 1968; van der Lingen, 1968; Lewis and Kohn, 1973; Nelson et al., 1986a; Froggatt et al., 1986; Froggatt and Lowe, 1990; Shane, 1990; Shane and Froggatt, 1991; Alloway et al., 1993; Carter et al., 1995; Shane et al., 1995, 1996), and which, over the last 20 k.y., has been input at rates up to one-third that of fluvial terrigenous sediment (L. Carter et al., 1996); and (5) terrigenous sediment that is derived from uplifting mountains in New Zealand, after the inception of the modern Alpine Fault plate boundary (i.e., Miocene-Holocene Otakou Group equivalents [Carter, 1988b]; cf. Fig. F4A) and transported into the path of the DWBC by turbidity currents traveling 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, 1997; Lewis, 1994). In contrast, little is known regarding the geologic record or history of the DWBC.
The available seismic records suggest that the DWBC has been active along the eastern New Zealand margin since at least the Miocene, and probably since the middle Oligocene (32 Ma) (Carter and McCave, 1994). Starting at ~24 Ma, abundant terrigenous material was shed from rising mountains along the Alpine Fault plate boundary (Vella, 1962; Norris et al., 1978) and fed through the eastern South Island shelf into the Solander, Bounty, and Hikurangi Channel systems. Sediment supply accelerated at ~6.5 Ma in the late Cenozoic, when collision increased along the plate boundary (Walcott, 1998; cf. Kennett, von der Borch, et al., 1986), and supply to the deep sea was probably enhanced again from the start of major glacial lowstands at ~2.6 Ma onward. Much of this sediment ultimately became entrained in the DWBC drift system, which carries it northward to be eventually subducted into the Kermadec Trench (L. Carter et al., 1996).
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 it extends for >450 km before discharging into the DWBC at Emerald Basin between Macquarie Ridge and the western side of Campbell Plateau (L. Carter et al., 1996; Carter and McCave, 1997; Schuur et al., 1998). The Hikurangi Fan has been termed a "fan-drift" by Carter and McCave (1994) 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 within a bathymetric embayment, has retained its fan morphology and has developed directly across the path of the DWBC, the only evidence of modern drift formation being scour of the upper fan and redeposition of material on the middle to lower fan as a series of small, discrete ridges (Carter and Carter, 1993, 1996). Compared to Hikurangi Fan-Drift, Bounty Fan has formed in a region where the DWBC is inferred to be slowed (1) by the lack of forcing by the ACC, which branches east across the Pacific just south of Bounty Trough; and (2) the loss of the topographic steering, and current acceleration, provided by the steep eastern slope of the Campbell Plateau, which ceases abruptly at Bollons Seamount, again at the southern edge of the Bounty Trough. However, 6 yr of satellite sea-surface temperature data, summarized by Carter et al. (1998), indicates that meanders from the ACC periodically affect the outer Bounty Trough, and the water motions that accompany them may also play a role in current-winnowing on the Bounty Fan.
During the later Cenozoic, the two described abyssal fans have been supplied with sediment by turbidites passing through the Bounty and Hikurangi Channels, each of which is over 1000 km long. 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 (Lewis, 1994). 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 may therefore be sea-level (i.e., climatically) controlled, with most sediment being fed into them during glacial lowstands, whereas in interglacials the same sediment stream is diverted along the inner shelf, some of it reaching the Hikurangi System via the Kaikoura Canyon (Carter and Herzer, 1979).
Eastern New Zealand is thus the site of a major recycling system, whereby sediment is shed from uplifting mountains along the Australian/Pacific plate boundary and provided to the deep sea via several major submarine channel systems. Once at abyssal depths, the sediment is re-entrained by the ACC and DWBC and passed north along the edge of the Campbell Plateau, around the tip of the Chatham Rise, northwestward along the foot of the Hikurangi Plateau, to finally arrive in the Hikurangi-Kermadec trench, where it is subducted, melted, and returned to the surface as juvenile volcanic rock. Remarkably, and largely because of the effects of the plate boundary, the two small islands of New Zealand supply roughly 2% of the world's sediment load to the oceans (Milliman and Syvitski, 1992); it is this sediment load that is then entrained in what L. Carter et al. (1996) have termed the Eastern New Zealand Oceanic Sedimentary System (ENZOSS) (Fig. F5).
Recent publications (Carter and Carter, 1993; Lewis, 1994; Carter and McCave, 1994; L. Carter et al., 1996) have delineated the ENZOSS region, between the Solander Trough and the Kermadec Trench, east of the modern Australian-Pacific plate boundary, as an integrated sediment source-transport-sink area. During the latter half of the Cenozoic, sediment from mountains along the New Zealand plate boundary was transported through deep-sea channel/fan systems, delivered into the path of the DWBC, entrained northward within this current system, and finally consumed by subduction at the same plate boundary after a transport path of up to 4500 km.
ODP Leg 181 targeted drill sites located in the eastern New Zealand region and the key Southwest Pacific gateway because
The Leg 181 drilling schedule included 51 days at sea with drilling operations at seven sites. We began by drilling shallow-water sediment drifts on the upper continental slope near South Island, New Zealand, moved south in difficult weather conditions to drill sites on the central Campbell Plateau, and, at its eastern foot, turned north to drill a deep hole through the levee sediments of the Bounty Fan. The leg finished with two holes through sediment drifts on the north side of the Chatham Rise and one into the shallow rise itself. Overall, we recovered 3600 m of core and made over a million shipboard measurements. The material collected on Leg 181 will lead to a better understanding of the history and evolution of the Pacific ACC-DWBC system and nearby oceanic fronts, and will highlight the important role they play in global ocean circulation and production. Finally, that the stratigraphic and paleontologic information retrieved on the cruise contained many surprises was itself predictable, given the paucity of previous drilling in the Southwest Pacific region. This information will serve as a vital database for the targeting of future drilling legs in the Southern Ocean.