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 (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 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 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), directly in the path of the evolving ACC-DWBC system (Fig. 3).

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