Site 1125 lies on the north slope of Chatham Rise, 600 km east of Kaikoura, South Island, at a water depth of 1360 m (Fig. F1). The site lies under the northern edge of the Subtropical Convergence (STC), a zone of high productivity, and the surface waters above are swept by the East Cape Current (Heath, 1985), which runs south along eastern North Island before turning east along the Chatham Rise. The STC zone is also supplied with water (and suspended sediment) by the Southland Current (Heath, 1972; Chiswell, 1996), which flows up the eastern South Island coast and then turns east in two branches, one north (derived through the Mernoo Saddle) and one south of the crest of Chatham Rise. The crest of Chatham Rise, therefore, is supplied with sediment from five quite different sources: (1) biopelagic snow, generated by high productivity within the vigorously mixing water masses along the STC (Bradford-Grieve et al., 1998); (2) suspended terrigenous sediment derived from river flooding in eastern North Island (East Cape Current); (3) similar terrigenous sediment from the South Island (Southland Current); and, intermittently, (4) direct airfall tephra from major explosive eruptions in the central North Island (e.g., Carter et al., 1995), or (5) occasional rafted iceberg debris (Cullen, 1965). Despite these potentially prolific sediment sources, the crest of the Chatham Rise is mostly shallower than 500 m, and vigorous currents (e.g., Chiswell, 1994) inhibit sediment deposition there. It is well established that only a thin discontinuous layer of upper Neogene sediment occurs on the crest of the rise, which is underlain at shallow subseafloor depths by Miocene chalks and glauconitic marls (Lewis et al., 1985; Wood et al., 1989). Phosphatized, glauconitized, and bored pebbles of Miocene limestone are commonly dredged from the Chatham Rise seafloor (Karns, 1976; Cullen, 1980).
Before Leg 181, the ultimate depocenter for material winnowed off the top of the Chatham Rise was unknown, but it seems likely that sediment becomes trapped in topographic lows in deeper waters on both the north and south flanks of the Rise. Site 1125 was drilled at the end of Leg 181 as an alternate site for which only an outdated, reconnaissance seismic line was available (Fig. F2). On the basis of this unsatisfactory seismic record, it was anticipated that the hole would traverse the shallowly dipping lower Miocene chalks that underlie the crest of Chatham Rise, penetrating beyond that to perhaps retrieve a sequence of unaltered upper Paleogene biopelagic sediments suitable for isotope analysis. In the event, Site 1125 proved to contain a thick sequence of rhythmic upper Neogene hemipelagic and biopelagic sediment, and undoubtedly represents a major rise-flank depocenter that has trapped sediment for at least the last 10 m.y. The site is a close counterpart to Deep Sea Drilling Project (DSDP) Site 594, located on the south side of the Chatham Rise at a similar depth (1204 m).
DSDP Site 594 and Ocean Drilling Program (ODP) Site 1125 both lie within lower Antarctic Intermediate Water (AAIW). In the North Atlantic Ocean, intermediate water has been shown to increase in depth range and speed during glaciations, concomitant with a decrease in North Atlantic Deep Water production in the Norwegian-Greenland Sea (Labeyrie et al., 1992). Analogously, in the Southern Ocean, Pudsey et al. (1992) have shown that Antarctic Bottom Water production also diminished during glaciations, in which case AAIW production may also have become greater at the same time. If the vigor of global deep circulation decreased during glaciations, then upper Circumpolar Deep Water from both the Indian and Pacific oceans may have become even more nutrient enriched and oxygen depleted than it is today. Material from Site 1125 will be used for oxygen isotope, carbon isotope, and trace-element analysis to reconstruct through time the changing paleochemistry and paleoproductivity of the site.
Recent paleoceanographic studies have suggested that the STC remained fixed in its general position along the crest of the Chatham Rise throughout the major glacial/interglacial fluctuations of the late Quaternary (Fenner et al., 1992; Weaver et al., 1998). If the STC was similarly stable during the last 10 m.y., then global ocean chemistry shifts, such as the late Miocene carbon isotope excursion at ~6.5 Ma (Loutit and Kennett, 1979) and changing CO2 fluxes (Compton and Mallinson, 1996), should be able to be sharply delineated and studied in stratigraphic context. The recovered upper Neogene sequence from Site 1125 will thus provide a record of AAIW paleohydrography and of the changing patterns of paleoproductivity that relate either to migrations of the position of the STC or to global ocean chemistry changes.
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