Site 1121 was planned to penetrate a sedimentary body deposited along the foot of the Campbell Plateau in water depths of 4400 to 4600 m. This deposit was initially delimited from a suite of single-channel air gun profiles run by the Eltanin in the 1960s and 1970s. One of these profiles, Eltanin line 43, was used to guide Site 276, drilled during DSDP Leg 29 (Kennett, Houtz, et al., 1975). This site was located at 4761 m depth within a scour moat eroded along the base of the plateau. There, the drill string penetrated a current-swept lag deposit of gravelly sand and recovered a few rock chips probably derived from underlying Paleogene siliceous siltstones (Kennett, Houtz, et al., 1975). A resume of the DSDP and Eltanin data sets by Carter and McCave (1997) concluded that the moat had been eroded by a fast-flowing core of the Deep Western Boundary Current (DWBC) in consort with the Antarctic Circumpolar Current (ACC). East of the moat, the seismic profiles were interpreted to show sediment deposited to form the Campbell "drift." This interpretation received some support from a Kasten core (NIWA Core Y11) collected during the site survey for Leg 181 onboard Tangaroa in 1996. The 2-m-long core recovered ferromanganese-stained, foraminiferal sand from the "drift" crest. The sediment was interpreted as a mixture of material reworked by the abyssal flow. Accordingly, the site was considered likely to contain a record of the abyssal circulation potentially back to the Oligocene.
This lithologic sequence is divided in two units based on the visual description of the sediment and estimates of the composition from smear slides (see "Site 1121 Smear Slides"). These data are also supported by measurements of calcium carbonate, physical properties, light reflectance, and bulk mineralogy, and are summarized in Figure F3. Lithostratigraphic Unit I extends from the core top to 32.7 mbsf and is composed predominantly of terrigenous, current-influenced sediment, and is of early Pleistocene to Neogene age. Unit II contains pelagic sediment with a high biogenic component that exhibits significant variations in the dominant nannofossil and diatom constituents. Sediment of Unit II is late to early Paleocene in age and extends to the base of the core at a depth of 139.7 mbsf.
Unit I represents terrigenous, current-influenced sedimentation and comprises Subunits IA and IB.
Subunit IA is composed of alternating sediment packages. One package is composed of silty or sandy clay that is generally well bioturbated and mottled in color (Fig. F4). The sediment color ranges from yellow (10YR 7/6) to yellowish brown (10YR 5/4) to brown (10YR 7/6 and 4/4). The other sediment package contains silt, silty sand, and sand beds that are grayish brown (10YR 5/2) and light yellowish brown (10YR 6/4). The sandy bed present at the top of the core contains abundant foraminifers and nannofossils, which are not present in the sandy beds deeper within the sediment column. There is a brownish yellow (10YR 6/6) sand at the bottom of Core 181-1121B-1H that has an extremely sharp contact with the underlying yellow (10YR 7/6) clay (Fig. F5). This sand bed exhibits no grading and is not thought to be a turbidite deposit. It may represent a period of intense winnowing. A number of large ferromanganese nodules (>1 cm in diameter) are present at the core top and at three different depth horizons in Subunit IA (Figs. F6, F7). The largest nodule recovered was 7 cm long by at least 6 cm wide (Fig. F7). Ferromanganese nodules form during times of low sedimentation, yet their burial in the sediment column implies periods of either relatively higher deposition, possibly resulting from times of less erosive current activity or reduced bioturbation (McCave, 1988). Both Subunits IA and IB (0 to 32.7 mbsf) contain authigenic clinoptilolite, which suggests a significant supply of silica to the interstitial water from silica-rich material such as diatoms, radiolarians, tephras, or clays.
There is a bioturbated contact between Subunits IA and IB. This contact is also marked by a downcore decrease in the natural gamma ray and an increase in the color reflectance. Subunit IB comprises pale yellow (2.5Y 7/4) and light yellowish brown (2.5Y 6/4) clay exhibiting faint lighter color banding and sparse black (N 1) smears, which are probably ferromanganese. Fragments of chert layers and nodules are present throughout and were responsible for the drilling difficulties, low recovery, and high core disturbance (Fig. F8). Flow-in occurs in Sections 181-1121B-2H-6 through 2H-CC and in Section 181-1121B-3H-2 to the base of the core. Cores 181-1121B-4X and 5X have poor recovery and are almost exclusively brecciated chert, which is most likely from chert layers broken by drilling and/or cave-in.
Unit II represents a period of almost entirely pelagic, biogenic sedimentation. This unit extends from 32.7 to 139.7 mbsf and is divided into Subunits IIA and IIB, according to the relative abundance of diatoms, nannofossils, and clay in the sediment.
Subunit IIA consists of white (10YR 8/2) nannofossil ooze that contains a subordinate amount of diatoms, radiolarians, sponge spicules, and silicoflagellates. These beds alternate with light greenish gray (5GY 7/1) nannofossil diatom ooze and greenish gray (5GY 5/2) and pale green (5G 6/2) highly diatomaceous ooze. The contact with the overlying Subunit IB was not observed because of drilling disturbance that extends through the upper part of Subunit IIA (32.7-34.2 mbsf). A single ferromanganese nodule found at the top of Core 181-1121B-6X had probably fallen downhole. Subunit IIA shows a decrease in natural gamma ray relative to Subunit IB. The top of Subunit IIA is younger than nannofossil Zone NP5 (57.5 to 58.4 Ma).
In Section 181-1121B-6X-1, at 35.07 mbsf, a sharp contact marks the change from white nannofossil ooze to light greenish gray (5GY 7/1) nannofossil diatom ooze. Until 119.4 mbsf, the sediment exhibits color variations from greenish gray (5GY 5/2), to light greenish gray (5GY 7/1), to pale green (5G 6/2) as gradational variations in Sections 181-1121B-6X-3 and 6X-4 (36.3 to 37.28 mbsf); as grayish green (5G 5/2) laminae of <1 cm thickness in Section 181-1121B-7X-5 (42.3 to 51.34 mbsf); and as color banding of greenish gray (5GY 5/2) and pale green (5G 6/2) in Section 181-1121B-8X-5 (52 to 60.06 mbsf). The lighter greenish color appears to correlate with a higher content of nannofossils while the darker green sediment represents a more diatomaceous ooze. In support, the carbonate content is substantially higher in the lighter intervals (50%) than in the darker green ooze (<10%). Bioturbation is common throughout the subunit and identified trace fossils include Planolites (in Sections 181-1121B-6X-3 and 6X-5), Thalassinoides, Palaeophycus, and Skolithos (in Sections 181-1121B-8X-5). The chert clasts and ferromanganese nodules that appear at the tops of Cores 181-1121B-6X, 7X, and 9X probably result from cave-in of the hole. Chert breccia and chert layers are common below 71.2 mbsf (Core 181-1121B-10X). This probably relates to the presence of diatom ooze in this depth interval. Pyrite stains are scattered throughout Cores 181-1121B-11X and 12X, and in Section 181-1121B-8X-6, gray laminae ~1 cm thick with pyritized bottom contacts are present. Moderate core disturbance is present throughout Subunit IIA and drilling biscuits appear below Core 181-1121B-11X. Recovery decreases markedly downhole with only few centimeters recovered in Cores 181-1121B-13X and 14X.
Another interval of light greenish gray (5GY 7/1) nannofossil ooze occurs between 181-1121B-15X-1 to 17X-2 (119.4-132.7 mbsf). This is interrupted by carbonate concretions and carbonate drilling breccia in Cores 181-1121-15X and 16X.
The contact with Subunit IIB occurs within an interstitial water sample, and so uncertainty exists about its character and exact depth. This subunit consists of greenish gray (5GY 5/1 and 6/1) nannofossil-bearing clay. The increase in clay content is supported additionally by an increase in the natural gamma signal.
Site 1121 is situated on a thick sedimentary deposit close to the Subantarctic Slope of the Campbell Plateau. Both the ACC and DWBC flow along the Subantarctic Slope and influence the sedimentation in this locality.
Many characteristics of the sediment in Unit I suggest sedimentation under the presence of a vigorous, but varying, current regime. The most obvious is the very low net sedimentation rate (deposition minus erosion) of Unit I. The youngest sediment recovered at this site is late Pleistocene in age (<0.9 Ma), whereas sediments of late Paleocene age are certainly present below 33 mbsf and may be as shallow as 19.4 mbsf in Subunit IB (no datums are available between 3.6 through 56 Ma), which provides an extremely low net linear sedimentation rate (~0.6 mm/k.y.). Evidence for intervals of less or no sedimentation, reflecting the strong current influence, is given by the occurrence of ferromanganese nodules in Subunit IA. Although the sandy layer in Subunit IA has a sharp basal contact, it is not normally graded and is not thought to be turbiditic in nature. Thus, this sand layer may also document an erosional imprint of high-energy current flows with intense winnowing. The silty clay and clayey silt intervals found in Subunit IA presumably represent sedimentation under low flow conditions. Burial of ferromanganese nodules at depth within the sediment column implies that there have been episodes of higher net sedimentation. The sediment input through Unit I is predominantly terrigenous and possibly represents reworked material from Campbell Plateau or sources to the south. While it is entirely possible that turbidity currents occurred, no typical Bouma-type turbidite sequences were found.
Subunit IB comprises a relatively homogeneous clay interval, suggesting stable and low-intensity flow depositional conditions. This unit is only interrupted by the occurrence of chert layers. The age of the sequence could be late Paleocene as indicated by radiolarians (but they could be reworked), whereas no nannofossil age was available at this depth. So the existence of a hiatus between Subunits IA and IB is very likely and makes it probable that the ACC-DWBC, which is suggested to have begun during the Oligocene (Kennett, 1977), is marked by this horizon. The alternative is that the onset of the ACC-DWBC is marked by the contact between Subunits IB and IIA following an initial high-velocity phase that eroded down to the Paleocene. Terrigenous material and biogenic silica subsequently accumulated under slower flow to form Subunit IB. Renewed current winnowing yielded Subunit IA. Distinguishing between these possibilities will require further micropaleontological work.
Unit II indicates that almost entirely pelagic, biogenic sedimentation occurred at this site throughout the early to late Paleocene. This unit has a composition similar to that inferred from nannofossil, and chert-bearing diatom ooze/fragments at DSDP Site 276 (Kennett, Houtz, et al., 1975). This succession is indicative of changes in the depth of the Carbonate Compensation Depth (CCD) through variations in presence of cold, corrosive bottom water. The top of Subunit IIA consists of nannofossil ooze with a minor amount of siliceous fossils, and was probably deposited above the paleo-CCD. The sharp contact with the underlying diatom-rich sediment, concurrent with a slight decrease in nannofossil content, was possibly caused by a shallowing of the CCD. The increased content of nannofossils in Cores 181-1121-15X to 17X suggests a relatively deeper CCD between ~60 and 62 Ma. Subunit IIB is mostly depleted of nannofossils, which may reflect deposition below the CCD. The high clay content of this unit is confirmed by the higher gamma-ray signal in this depth interval.
Although Hole 1121B initially penetrated the drift-type silty sediment characteristic of Unit I, the presence of Paleocene siliceous and calcareous pelagites in the rest of the sequence shows that the main sediment body is not a drift, as suggested by Carter and McCave (1997). Rather, it now appears to be the remnant of a pelagic apron that formed along the eastern margin of Campbell Plateau. Subsequent initiation of a strong abyssal circulation under the ACC and DWBC has extensively eroded the apron. Apart from Unit 1, the post-Paleocene succession has been either eroded or prevented from accumulating or both these processes have occurred. Furthermore, topographically induced scour along Campbell Plateau has effectively isolated the apron sediment to produce a linear sediment body on the floor of the adjacent Southwest Pacific Basin.
From a paleoceanographic perspective, there have been several bouts of current activity, the precise ages of which are yet to be delineated. The initial phase(s) of strong bottom flow resulted in erosion of the apron. Until more drill-hole information is available from the Oligocene through Miocene interval, we can only speculate that these phases coincided with the late Oligocene opening of the circum-Antarctic seaway and a late Miocene cooling event in Antarctica (e.g., Kennett, 1977). Both these events would have encouraged the production and passage of deep waters through the Southwest Pacific, and produced widespread unconformities, manifested by including seismic reflectors X and Y of Kennett, von der Borch, et al. (1986). In addition, the presence of at least three ferromanganese nodule/sand horizons in Subunit IA also suggests periods of strong current activity, occasionally punctuated with periods of quieter currents and therefore more sediment accumulation. Assuming the youngest sediments are late Pleistocene in age, it then appears that strong erosive currents have prevailed since that time. Assessments of the sedimentary regime of southern New Zealand (Carter and McCave, 1997) and physical oceanographic observations coupled with the output from a numerical model (Carter and Wilkin, in press) reveal that the dominant force behind abyssal erosion along the front of the Campbell Plateau is the ACC. Interaction of the current with the marked topography of Campbell Plateau intensifies the flow as well as forming large-scale eddies that extend from the surface to the seabed (Bryden and Heath, 1985; Morrow et al., 1992).
The discovery of extensive erosion off Campbell Plateau allows us to refine the model for the Eastern New Zealand Oceanic Sedimentary System (ENZOSS). When first proposed, it was suggested that some of the sediment transported north by the ACC-DWBC accumulated on what was interpreted as the Campbell "drift," with another portion moving east with the ACC (Carter et al., 1996). The cores recovered from Site 1121 show that very little sediment settled in this region. For example, the layer of drift sediment capping the oceanic Paleocene sediments is, at most, only 32.7 m thick, and it may be as little as 15.2 m. It would appear that the ENZOSS system to the south of Bounty Fan (i.e., within the path of the ACC) is dominated by erosion and transport. North of the fan, erosion is less pervasive and sediment accumulates as drifts (McCave and Carter, 1997).