Leg 181 will drill sites located in the key Southwest Pacific Gateway because:

  1. The Pacific Deep Western Boundary Current (DWBC) is the largest single contributor today to the deep waters of the world's oceans (20 Sverddrups [1 Sv = 106 m3s-1]; Fig. 1), and deciphering its history is, therefore, of fundamental importance to global and Pacific ocean hydrography.
  2. The stratigraphic record of the eastern New Zealand Plateau and its abyssal margins is the best one available for deciphering the development history of Pacific Southern Ocean water masses, and of the sediment drifts they deposited. Recent publications (Carter and Carter, 1993, 1996; Lewis, 1994; Carter and McCave, 1994, in press; L. Carter et al., 1996; McCave and Carter, 1997) delineate the 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, termed the Eastern New Zealand Oceanic Sedimentary System (ENZOSS). Since ~10 Ma, sediment from mountains along the New Zealand plate boundary has been transported through deep-sea channel/fan systems, delivered into the path of the DWBC, entrained northwards within this current system and finally consumed by subduction at the same plate boundary after a transport path of up to 3500 km. The stratigraphic record from the ENZOSS, and in particular any new high-resolution, Neogene Ocean Drilling Program (ODP) sections from its deep-sea parts, are directly relevant to one of the most important unresolved problems of Cenozoic climatology, namely the timing and precise nature of the development of widespread glaciation on the Antarctic continent (Barrett, in press). In turn, the same glacial events that contribute source water to the DWBC and its companion flow, the Antarctic Circumpolar Current (ACC), force the boundary current south of 49S.
  3. The gateway region includes two major oceanic fronts, the Subtropical Convergence and the Subantarctic Front, and is proximal to a third, the Antarctic Convergence (Figs. 2, 3). Thus, the region is in a prime position to allow determination of the migration of these boundaries, the forcing processes that cause them to move, and the environmental response to their movement.

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