The sedimentary regime of the 500- to 1500-m-deep Campbell Plateau is not well known. Summerhayes (1969) ran a reconnaissance survey of surficial sediments, which were mainly foraminifer ooze. In the 1960s and early 1970s, the Eltanin surveyed the shallow structure and sediment thickness using a single-channel air gun system. Results from those surveys have been summarized by Davey (1977). Eltanin seismic profiles were used to select a series of holes for Deep Sea Drilling Project (DSDP) Leg 29 (Kennett, Houtz, et al., 1975). Two of the sites, 275 and 276, were located at the eastern and western margins of Campbell Plateau and were characterized by a series of pronounced disconformities, which Kennett, Houtz, et al. (1975) related to erosion under an extremely active, shallow western boundary current (nominally the northern edge of the ACC). Site 275, in particular, had a disconformity encompassing the entire Cenozoic and part of the Upper Cretaceous. Accordingly, Site 1120 was located toward the central plateau within a Cenozoic sedimentary sequence anticipated to be 700 m thick.
Cores from Site 1120 penetrated a succession of calcareous biogenic oozes. With the exception of one sample containing 85% calcium carbonate, all other ooze samples had more than 90% CaCO3, with two-thirds in the range of 94% to 96%. Despite this high degree of lithologic homogeneity, four basic units are recognized on the basis of subtle changes in the calcareous biogenic and noncarbonate components, along with variations in bedding and color (Table T2). The resulting lithologic logs, together with physical properties and biostratigraphic control, are summarized in Figure F3. Of note is the improved correlation, compared with Site 1119, between light reflectance and calcium carbonate percentage (Fig. F4). At Site 1119, a much poorer correlation was realized because the spectrophotometer was too high above the core surface, thereby allowing reflectance scatter. By altering the sensor height to as close to the sediment surface as possible, better results were obtained. These results are too preliminary to act as a proxy for calcium carbonate. This is in part caused by the limited color range associated with the sediment's high concentration and restricted range (>90%) of CaCO3.
The youngest lithostratigraphic unit occupies the top 4.6 m of Hole 1120C, where it rests above more lithified sediment; the contact is interpreted as an unconformity (Fig. F5). The younger sediments are thinly bedded with layers typically <0.5 m thick. Contacts are generally bioturbated and are disrupted further by one layer injecting into another--a process of soft-sediment deformation that probably was caused by drilling. Beds are distinguished by color variations of white and light gray. Colors alternate between light gray (N7) and various hues of white (N8, 5Y8/1, 5Y8/2). However, some hues of white are not covered by the Munsell color chart. Of note are layers with a brilliant white resembling fluorescent toothpaste for which the term "bright white" has been coined for use on the visual core description sheets. The surface of the unconformity below this banded sequence is white with a yellow-red hue (10YR 8/2). Sediments are dominated by calcareous nannofossils and foraminifers in differing proportions so that the lithology varies between foraminifer nannofossil ooze and nannofossil foraminifer ooze. Lithologies containing mainly nannofossils tend to be "bright white." Accessory components include pyrite, found frequently lining or infilling the chambers of foraminifer tests, quartzofeldspathic silt, skeletal debris of uncertain genesis, and trace amounts of heavy minerals. Quartzofeldspathic grains were present in the upper 3 m of the core, below which they occur in only trace amounts.
Bioturbation is pervasive, but the lack of contrast in the various lithologies, together with soft sediment deformation, have precluded reliable identification of ichnofossils, although possible Planolites traces are present.
The unit begins at the postulated unconformity at 4.6 mbsf (interval 1120C-1H-4, 10 cm). The sediment below is indurated, but not to the extent that it could be classified as chalk. However, further below the contact, the sediment again becomes softer, but individual beds are typically thicker than 1 m and are still discernible on the basis of color, although color differentiation is not as clear as in Unit I. There is a more substantial difference in composition. Unit II is mainly foraminifer nannofossil ooze, that is, the dominant biogenic component is nannofossils with foraminifers as a subordinate constituent, although still occasionally rivaling the nannofossils' abundance. Unit II also contains glauconite and rare pyritic concretions, both of which are absent in the overlying beds.
Bioturbation is again pervasive, but as the sediments are firmer than those of Unit I, burrows are better preserved and, in some instances, highlighted by pyrite stains (note that "pyrite" is used as a generic term for authigenic iron sulfides). Identified trace fossils include Chondrites.
A sequence of weakly bedded ooze extends from 10.2 to 54.9 mbsf. At this level, color differentiation is minimal and sediments range from white (N8) to white (5Y8/1). Bioturbation has ensured extensive but very subtle mottling of these differently hued whites. As in Unit II, foraminifer nannofossil ooze is the dominant lithology, but there the similarity ends. Glauconite disappears in Unit III, and its place is taken by siliceous sponge spicules. Pyrite continues to be a prominent accessory and, in two instances (Sections 181-1120C-4H-4 and 4H-5), it forms concretions with clusters of sponge spicules.
The color and lithologic similarity of the sediments in Unit III make identification of ichnofossils difficult, although some burrows are accentuated by dark gray pyrite staining. Identifiable forms are Zoophycos and Planolites.
Between 54.9 mbsf and the base of the core at 220.7 mbsf, the sediment exhibits a fairly uniform lithology. Sediment color varies mainly between two hues of white (N8 and 5Y8/1). Pervasive bioturbation results in a mottled white deposit, the monotony of which is interrupted by occasional ill-defined layers of light gray (N7) ooze, and dark gray stains associated with concentrations of pyrite. A rhythmic sequence of slightly dark bands (<1 cm wide) first appears in Section 181-1120B-18X-2 and is assumed to mark the onset of drilling biscuits. Biscuits have a rhythmic character and form across existing sedimentary features, such as primary bioturbation. Where unaffected by biscuiting and soft sediment deformation, the following trace fossils were identified: Zoophycos, Terebellina, Planolites, and Palaeophycus. A single echinoid spine was observed in Section 181-1120B-8H-5.
Compared to the younger units, Unit IV lithology is a nannofossil ooze with markedly reduced numbers of foraminifers (concentrations in the "Present" category), and a very reduced accessory assemblage. The noncarbonate mineralogy is dominated by pyrite, and quartzofeldspathic silt occurs only in trace amounts. Flakes of deformed mica, indicative of a probable metamorphic source, shards of colorless and brown glass, and diatoms all make sporadic appearances toward the base of the core below ~160 mbsf. Smear slides from the dominant lithology and a light gray layer (Section 181-1120D-6X-2) revealed the latter to have higher quantities of foraminifer tests ( "Site 1120 Smear Slides"). Of special note are small pebbles of gray, dense pumice in Section 181-1120B-16X-2 and of gray limestone in Section 181-1120D-8X-1.
A feature of the pelagic drape sampled at Site 1120 is its high content of calcium carbonate which averages 93%. Such a high amount is consistent with the site's isolation from terrigenous sources. New Zealand is 500 km to the northwest of the site, and Campbell Plateau is topographically isolated from the main sediment conduits leading from New Zealand, namely Solander Channel, and Bounty Channel with its attendant canyon system draining the eastern margin of the South Island (e.g., Carter et al., 1996a). Flow across the plateau is mainly from south to north, coming from a region with negligible sources of terrigenous material. Furthermore, Site 1120 is over 300 km south of the main pathway of aeolian dust carried east of New Zealand under the vigorous westerly wind regime (e.g., Thiede, 1979). Nevertheless, during periods of northwesterly wind, some aeolian detritus is likely to reach Campbell Plateau both from local sources and Australia (Hesse, 1994).
The lowest carbonate concentrations are within the upper Pleistocene sequence of alternating light and dark nannofossil and foraminifer oozes represented by Unit I (Fig. F3). Kasten cores from elsewhere on Campbell Plateau suggest that these alternations are interglacial (light beds)/glacial (dark beds) cycles (e.g., Carter et al., unpubl. data). The glacial periods were times of increased windiness, a response to enhanced thermal gradients associated with the equatorward migration of cold water (e.g., Stewart and Neall, 1984). Thus, the horizons of lower carbonate in Unit I may be caused by an increased aeolian input--a supposition that is supported by higher concentrations of angular quartzofeldspathic silt grains in this unit. Carbonate continues to increase downward through the upper core to reach a local peak at the base of Unit II. Below this it decreases to a minimum around the middle part of upper Miocene Unit III before climbing to high values (~92%-96%) that are more or less maintained to the base of the core. The cause of the reduced carbonate in upper Unit III is unknown. It may be related to the arrival of siliceous organisms, in particular sponges, which at times were also accompanied by diatoms. Such an incoming of siliceous species often heralds a change to cooler waters (e.g., Weaver et al., 1998), which in turn may influence carbonate production and preservation.
As discussed above, the alternations of light and dark colors in Unit I are likely to represent glacial/interglacial cyclicity as noted elsewhere off eastern New Zealand (e.g., Griggs et al., 1983; Nelson et al., 1986). However, the components responsible for the contrasts in plateau sediments differ slightly from other examples because the plateau is largely isolated from terrigenous sources. Light layers have a dominance of nannofossils that impart a "bright white" color to individual layers. By comparison, the darker glacial beds have a higher proportion of aeolian detritus and foraminifers, whose tests are rendered darker by the formation of pyrite within test chambers. Carbonate curves, magnetic susceptibility profiles, and stable isotope measurements made on kasten cores from the region (Neil, 1998; Carter et al., unpubl. data) demonstrate that the glacial/interglacial cyclicity commenced at isotope Stage 6 and continued through to the present Stage 1. If this cyclicity holds for Site 1120, and it is continuous through to the unconformity at 4.6 mbsf, then the oldest sediments in Unit I could be about Stage 21. However, the biostratigraphy indicates a break at ~3 mbsf. This level approximates isotope Stage 16 with a top age of 620 ka (again assuming there is no undetected break in cyclicity between 0-3 mbsf).