SEDIMENTARY RECORD OF THE ACC-DWBC
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; L. 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 Plateau, in water and paleowater depths between 300 and 5500 m (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; (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; (4) airfall rhyolitic and andesitic ash, which derives from explosive Miocene–Holocene explosive arc volcanism in New Zealand (Ninkovich, 1968; van der Lingen, 1968; Lewis and Kohn, 1973; Nelson et al., 1985; Froggatt et al., 1986; Froggatt and Lowe, 1990; Shane, 1990; Shane and Froggatt, 1991; 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 (Carter et al., 1995); 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; cf. Fig. 4A) 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.
To 181 Scientific Report: Eastern New Zealand Oceanic Sedimentary System (ENZOSS)