As the northward-flowing DWBC passes the eastern end of Chatham Rise, it swings northwestward along a region of moderate to low relief. The effect of this weak western boundary is to decelerate the flow, thereby instigating sediment deposition (McCave and Carter, 1997). Part of this load has accumulated to form Rekohu Drift, which extends north from 41°S to 39°S. Apart from a few 3.5-kHz and single-channel seismic profiles, the only other control of the drift sediments is a kasten and a piston core collected at National Institute of Water and Atmospheric Research (NIWA) Station S931, just north of Site 1124. The barrel of the kasten core was bent during coring, presumably as a consequence of striking a consolidated tephra layer. The piston corer, however, penetrated 2.6 m and recovered a cyclic sequence of carbonate-bearing (interglacial) and carbonate-poor (glacial) mud, which, on the basis of the carbonate curve and two dated tephra layers--the Kawakawa Tephra of 22,590 radiocarbon years (Carter et al., 1995) and Omataroa Tephra of ~28,000 radiocarbon years (Froggatt and Lowe, 1990)--extended back to isotope Stage 5. Even though the 4001-m depth of Site 1124 approaches the regional Carbonate Compensation Depth (CCD) of ~4500 m, a reasonable carbonate record was obtained in piston core S931 with a maximum value of 73% (isotope Stage 5e). Hence, the site was anticipated to provide a carbonate-based record of Lower Circumpolar Deep Water entry into the Pacific.
Cores from Site 1124 showed that the sedimentary sequence is composed mainly of clay-bearing nannofossil ooze and chalk with interbeds of more terrigenous sediment and, in the top half of the core, frequent layers of tephra. A total of six lithostratigraphic units are recognized on the basis of the core visual descriptions supplemented with data from smear slides, carbonate analyses, X-ray diffraction (XRD), and physical properties including light reflectance. The generalized characteristics of the lithostratigraphic units are summarized in Figure F4 with a more specific set of logs, combining biostratigraphic and magnetostratigraphic data, presented in Figure F5.
With respect to reflectance, values at 550-nm wavelength were plotted as the dependent variable against CaCO3 percentage (Fig. F6). The resultant correlation was sufficiently robust to allow use of reflectance as a proxy for carbonate concentrations for most of Site 1124 cores. As reflectance measurements were made at 2-cm intervals, the data provided a detailed and accurate measure of carbonate cyclicity and fluxes (e.g., Mix et al., 1995) (Fig. F7). An exception was the dark brown mudstone of Unit IV, which had anomalously high reflectance relative to its 50% carbonate content. A downcore plot of reflectance as a ratio of 700/400 nm produced a profile that outlined the boundaries of the main lithostratigraphic units (Fig. F7).
Unit I encompasses a succession of interbedded ooze, chalk, and silty clay extending from the seafloor to 300 mbsf. Three subunits, IA to IC, are recognized (Fig. F4).
Commencing at the seafloor as an oxidized, light yellowish brown (10YR 6/4) nannofossil silty clay, Subunit IA continues downcore as a sequence of light and darker colored layers. The light layers are light greenish gray (5GY 7/1) to white (5Y 8/1) clay-bearing nannofossil ooze and nannofossil ooze, with a small component of foraminifers. These carbonate-dominant sediments are interpreted as representing interglacial periods. By comparison, darker layers are mainly greenish gray (5GY 5/1 to 5GY 6/1) silty clay to nannofossil-bearing silty clay, which are regarded as glacial period deposits. The darker layers are further distinguished by a small but significant biosiliceous component of radiolarians, diatoms, and spicules, as well as fewer foraminifers and less quartz/feldspar. Both foraminifers and biosiliceous elements help distinguish Subunit IA from the underlying subunits. Judging by the 500-nm reflectance profile (Fig. F7), there is a general increase in carbonate toward the base of the subunit. This change is accompanied by a small increase in quartz and feldspar indicating either higher terrigenous or volcanic fluxes or both of these inputs. Obviously, the carbonate flux outstripped the other increments, suggesting either higher production or lower dissolution (e.g., see Weaver et al., 1998).
Bioturbation is pervasive and is manifested in two ichnofacies. The upper part of Subunit IA has a Zoophycos facies of Zoophycos and Chondrites together with the less facies-specific Planolites and Palaeophycus. In contrast, the lower part of the subunit bears the Cruziana facies of Thalassinoides, Skolithos, and Cylindrichnus again with Planolites and Palaeophycus (Fig. F8).
Another feature is layers of typically pinkish gray (5YR 5/1 to 5YR 6/1) to light pinkish gray (5YR 7/1) macroscopic tephra up to 92 cm thick (181-1124C-3H-3 to 3H-4) (Fig. F9). However, this may not be the largest deposit encountered as Core 181-1124A-1H failed to fully penetrate a tephra of >81 cm thickness. A typical tephra layer has a sharp base, normal grading and, frequently, a bioturbated top (Fig. F10). Such attributes are consistent with both airfall deposits and turbidites, but the purity of the ash, the absence of Bouma-type sedimentary structures, and the location of the site on the crest of a ridge well above Hikurangi Channel collectively favor the airfall hypothesis. Sixty-eight tephra layers were recorded, mainly within four clusters throughout the subunit (Fig. F11). The total of 68 is, of course, a minimum value in light of unrecovered sediment, in particular the 8.15 m absent in Core 181-1124C-1X (Fig. F5). Furthermore, the total does not include the products of eruptions that produced smaller amounts of tephra. Carter et al. (1995) suggested that deposits <1 cm thick had a low rate of survival because of bioturbation. Certainly, layers <1 cm thick are few, whereas tephra-filled burrows are often present.
Dark green silty clay laminae are scattered throughout the core. Smear slides and XRD analyses from laminae indicate the presence of smectite and fragments of volcanic glass, suggesting formation by the alteration of thin layers of basic tephra. A similar conclusion was also reached by Gardner et al. (1986) and Nelson et al. (1986) for green layers at Deep Sea Drilling Project (DSDP) Site 593 on Challenger Plateau, west of New Zealand. Site 1124 green layers were excluded from the tephra count.
Subunit IB consists of 0.2- to 1.0-m-thick beds of greenish gray (5GY 5/1 to 5GY 6/1) and light greenish gray (5GY 7/1) silty clay grading to nannofossil silty clay intercalated with light greenish gray (5GY 7/1 to 5BG 7/1) and white (5Y 8/1) clayey nannofossil ooze grading to nannofossil ooze. This lithologic and color layering is similar to that of Subunit IA. Likewise, the dominance of the Zoophycos and Cruziana ichnofacies is common to both subunits, although the Subunit IB Cruziana facies may be more diverse with the addition of Teichichnus. Furthermore, Subunit IB contained 60 macroscopic tephra layers, which is only two less than Subunit IA. Despite the similarities, there remain sufficient differences to warrant a separate Subunit IB. The distinguishing features follow:
Subunit IC is heralded by a change from ooze to chalk. However, the sequence still retains the light and darker color banding characteristic of the overlying subunits, and it supposedly reflects a similar paleoclimatic cyclicity. At the top of the sequence, the differences between bands are subtle with light greenish gray (5GY 7/1) clay-bearing nannofossil chalk intercalated with greenish gray (5GY 6/1) clayey nannofossil chalk. Further downcore, the trend is toward less carbonate-rich sediments, although, as the carbonate curve shows, this trend is irregular (Fig. F5). At 226.5 mbsf, for example, a light/dark couplet comprises pale yellow (5Y 7/3) clayey nannofossil chalk and pale olive (5Y 6/3) nannofossil mudstone, and at 255 mbsf the dark couplet is replaced by mudstone. This downcore change is accompanied by an increase in the amounts of quartz, feldspar, and mica.
The subunit is further distinguished from its younger counterparts by a paucity of tephra layers. Only eight thin layers, with a total thickness of 29 cm, were detected in the upper part of Subunit IC. The oldest recognized tephra is at 207 mbsf and has an estimated age of 12 Ma. Since that time, it and any older tephra have altered, mainly to clay.
The ichnofacies are again alternations of Zoophycos and Cruziana, the latter having slightly different assemblages comprised of Thalassinoides, Skolithos, and Cylindrichnus in the uppermost 30 m of Subunit IC and Teichichnus and Skolithos in the basal 52 m (Fig. F8).
The top of Unit II is positioned near the base of Core 181-1124C-31X on the basis of abrupt changes in sediment characteristics recorded by the wireline logging tools (see "Downhole Measurements"). Core 181-1124C-31X was the first section below a zone of very poor recovery in interval 181-1124C-29X to 30X, in which there is an abrupt change in lithology. By comparison, the base of the unit is well constrained by what we interpret to be the Marshall Paraconformity. The actual boundary is present in Core 181-1124C-43X. It appears as a well-defined contact separating light greenish gray (5BG 7/1) clay-bearing nannofossil chalk from underlying white (5Y 8/1) nannofossil chalk. Planolites and Zoophycos burrows mix sediment across the contact (Fig. F12).
The upper part of Unit II is white (5Y 8/1) to light greenish gray (5GY 7/1) nannofossil chalk with interbeds and laminae of greenish gray (5G 6/1) clay-bearing nannofossil chalk that grades downcore through a nannofossil-bearing mudstone to a plain mudstone. These greenish gray beds have a conspicuous biosiliceous component of radiolarians, diatoms, and spicules, the last component sometimes having high enough concentrations to warrant the term "spicule bearing." Of note is the presence of finely laminated, flaser-like beds, which is the first evidence of bottom current activity in the unit (Fig. F13).
Regularly alternating zones characterized by the Cruziana and Zoophycos ichnofacies occur through Unit II. The Zoophycos ichnofacies is particularly well developed (Fig. F14) and is further distinguished by the presence of Helminthoida.
Unit III is composed of a uniform white (5Y 8/1) clayey nannofossil chalk. The white color belies the presence of a significant clay content, which may be as high as 45% as suggested by a single carbonate determination (Fig. F5). Smear slides reveal a dominance of nannofossils with abundant clay and accessory amounts of feldspar, mica, and foraminifers. The unit is massive and is probably bioturbated, although the lack of color differentiation precludes identification of individual trace fossils.
The top of the unit is bounded by the Marshall Paraconformity, whereas the base is a sharp contact with the multihued mudstones of Unit IV.
Unit IV marks an abrupt change to mudstone in various hues of reddish brown (5YR 6/6 - 6/5), pink (5YR 8/3), yellowish brown (10YR 6/4-5/4), and dark brown (10YR 4/3) toward the unit's base. Contacts between the various colored layers tend to be gradational, and bioturbation is pervasive with a Zoophycos ichnofacies and a strong representation by Planolites and Palaeophycus. Smear slides reveal a dominance of clay and quartzofeldspathic silt, with the latter tending to decrease toward the dark brown mudstone at the unit's base. Nannofossils also decrease toward the dark brown mudstone, whereas zeolites tend to increase downcore and become ~25% of the sediment. X-ray diffraction of the mudstone reveals a high content of expandable, dioctahedral clay minerals possibly Na-montmorillonite. No chlorite was found, but traces of illite and small amounts of quartz and silica were detected. The clinoptilolite is the most likely candidate.
The main lithology in Unit V is a nannofossil-bearing mudstone that, near the top of the section, is in various shades of light brown (light brownish gray = 10YR 6/2; pale brown = 10YR 6/3 to 7/3). Isolated beds of nannofossil chalk also occur near the top of the section, whereas the lower section includes pinkish white (7.5YR 8/2) mudstone and nannofossil mudstone laminae. The contacts between the light brown beds and light laminae are often knife-edge sharp and appear to represent chemical fronts whose brownish hue may be related to manganese in the sediments. The basal sedimentary rock is reddish brown (5YR 6/4) mudstone with chert layers or lenses. Smear slides indicate that the light brown mudstone is composed mainly of clay, nannofossils, and needle-like crystals of zeolite that are tentatively identified as clinoptilolite on the basis of a single XRD analysis. Foraminifers are also present and help differentiate Unit V from underlying rocks.
Differences in sediment color at bed and laminae contacts highlight the trace fossils, which mainly belong to the Zoophycos ichnofacies containing Chondrites and the ubiquitous Planolites and Palaeophycus. Some burrow fillings are dark pink to red (2.5YR 4/8), suggesting the possible the onset of chert formation.
The base of Unit V is the Cretaceous/Tertiary (KT) boundary which, on the basis of the biostratigraphy, is positioned between Cores 181-1124C-48X-5 and 49X-1. The inferred position of the boundary coincides with a strong spike in the wireline resistivity logs (see "Downhole Measurements").
Rocks of Unit VI are nannofossil-bearing mudstones in various colors that include (passing downcore) brown, gray, white, and pink. Munsell notations and detailed distribution of the different colored beds and laminae are summarized in the appropriate barrel sheet. The main components are clay, zeolites (clinoptilolite?), and nannofossils. Foraminifers are absent.
Bioturbation is present throughout with trace fossils best seen near the contacts of differently colored beds. Burrows appear to be vertically compressed but the fossils are still recognizable. Compared to Unit V, Unit VI has a Cruziana ichnofacies of Teichichnus and Skolithos, together with Palaeophycus and Planolites (Fig. F8). Some burrows have a distinct reddish brown color (2.5YR 6/4) that presumably highlights the presence of chert.
The sedimentary record postdating the Marshall Paraconformity encompasses the last 27 m.y. Apart from a 5-m.y. hiatus in the lower Miocene, this record is expected to provide a long-term history of deep-water sedimentation beneath the DWBC. The following is a brief interpretation of the paleoceanographic events beginning from the base of the Site 1124 sequence.
Over the past 27 m.y., sedimentation at Site 1124 has exhibited marked cyclicity as shown in the 500-nm reflectance proxy for calcium carbonate (Fig. F7). Furthermore, the cycles are superimposed on longer term changes exemplified by the general increase in reflectance/carbonate up through the lower Pliocene (Fig. F7). Whether or not the cycles have Milankovich frequencies must await an astronomically tuned time scale for the site. Whatever the case, the cycles display changes in sediment composition that may be interpreted in terms of oceanographic variability, tectonism, and volcanism.
The Marshall Paraconformity is a regional feature formed by widespread erosion of the ocean floor around 32 Ma (e.g., Kennett et al., 1985; Carter, 1985; Fulthorpe et al., 1996). In the case of Site 1124, it probably heralds the inflow of the DWBC to the Southwest Pacific Ocean (Kennett, 1977; Carter and McCave, 1994). Waning of the flow following its initial incursion may be recorded in the sediments directly above the paraconformity. Basal sediments of Unit II have elevated terrigenous contents that coincide with the appearance of the Cruziana ichnofacies. This assemblage, according to Pemberton and MacEachern (1995), is indicative of an energetic, well-ventilated, deep-water environment. Furthermore, the appearance of a small but diverse biosiliceous component of radiolarians, diatoms, and spicules indicate cold-water conditions that would be consistent with an inflow from Antarctic reaches. This cold inflow appears to diminish in the late Oligocene, as manifested by the prevalence of nannofossil ooze (chalk) and by the occurrence of the Zoophycos ichnofacies, which indicates a quiet, deep-water environment. Further cycles of a terrigenous, energetic setting and a pelagic, quiet-water setting occur through Unit II, but in a more subdued form compared to the cycle near the Marshall Paraconformity.
Immediately following the erosional event responsible for the early Miocene (19-23.8 Ma) unconformity at the top of Unit II, the basal beds of Subunit IC reveal an increased input of terrigenous sediment that is coupled with a switch to the energetic Cruziana ichnofacies. Like Unit II, the terrigenous component decreases and nannofossil ooze (chalk) increases further upsection in consort with a change to the Zoophycos ichnofacies. Nevertheless, a higher frequency cyclicity, as identified from the reflectance profiles, is expressed in the carbonate content. The causes of these fluctuations are unclear. It may simply be a response to variations in terrigenous load carried by the DWBC. Significantly, this load contains mica. This mineral is interpreted to be an indicator of a southern source, in particular, the Bounty Trough, which receives sediment from the mica-rich, metamorphic terrain of the South Island (Carter and Mitchell, 1987). This would mean that the Bounty Channel was actively injecting sediment into the DWBC in early Miocene times. It is unlikely that the other main sediment supplier, Hikurangi Channel, was contributing to the DWBC and Rekohu Drift, because the channel had probably not yet diverted from its original course toward Kermadec Trench (Lewis, 1994, 1998; this report, see "Background and Objectives").
Carbonate cyclicity may also have been influenced by dissolution, as suggested by the paucity and poor preservation of the nannofossil and planktonic foraminiferal fauna. Dissolution may be related to changes in the regional CCD. Site 1124 is only 500 m above the modern CCD and is thus exposed to relatively small fluctuations of this boundary. Such changes may be related to large-scale incursions of cold water carried north by the DWBC and/or changes in the global CCD. But direct evidence for the postulated incursions is equivocal. Biosiliceous microfossils, indicators of cold water masses, are few at Site 1124. Their paucity, however, may be related to corrosion, as suggested by the poor condition of the few specimens observed (see "Biostratigraphy"). This poor preservation of microfossils extends through Subunit IC.
For the lower part of Subunit 1C, incursions of cold corrosive waters is at odds with the paleoclimatology, which reveals a climatic optimum from 19.5 to 16.5 Ma in high southern latitudes (Kennett and von der Borch, 1986). However, following this event there appears to have been a major expansion of the East Antarctic ice sheet and cooling of bottom waters, which presumably extended into the Southwest Pacific under the DWBC.
Terrigenous sediment increases towards the top of Subunit IC and on to the middle of Subunit IB at the base of the Pliocene (~5.3 Ma). The trend is consistent with the increased tempo of uplift onshore that followed instigation of a collisional plate boundary through the New Zealand region (e.g., Rait et al., 1991). Although the long-term trend is one of terrigenous increase, it is superimposed on a cyclicity that is well displayed in color reflectance and other physical parameters (Fig. F5; also see "Composite Depths") as well as by the alternations of light-colored nannofossil ooze and darker colored silty clay. In view of the prominence of the terrigenous signal, such cyclicity would have been strongly influenced by changes in sediment supply from south of Chatham Rise. Supply may be affected by bouts of erosion under the DWBC and Antarctic Circumpolar Current (ACC), as indicated by the intensely eroded sediment cover at Site 1121, or by variations in the sediment supply via Bounty Channel (e.g., Carter and Carter, 1993), or by both these processes. Furthermore, there is a suggestion that the cycles may be climatically driven. Dark layers have a biosiliceous component of radiolarians, diatoms, and spicules that is typical of glacial periods, whereas the light-colored nannofossil oozes are more interglacial in character (e.g., Griggs et al., 1983; Weaver et al., 1998).
The Pliocene sediments display a reversal in the earlier compositional trend, with a gradual increase in carbonate content extending from 5.3 to ~2.3 Ma (Fig. F7). This trend is consistent with a mid-Pliocene increase in biogenic carbonate productivity that resulted from a general warming of Southwest Pacific surface waters, concomitant with increased upwelling (Kennett and von der Borch, 1985). From 2.3 Ma to the late Quaternary, the compositional trend inferred from the 550-nm reflectance data suggests a slight change back to a more terrigenous sediment (Fig. F7). This change is not clear in the carbonate record (Fig. F5), but this record is based on widely scattered data points and is not definitive. Again, a series of cycles are superimposed on the general trend, and these are regarded as glacial/interglacial cycles on the basis of their microfossil contents and lithology. The overall change to more terrigenous and, in the case of glacial period sediments, to more biosiliceous sediments, reflects the combined influence of large paleoclimatic fluctuations, increased uplift along the New Zealand plate boundary, and increased volcanism. Specifically,