The crest of Chatham Rise marks the position of the STC, separating warm subtropical waters in the north from cooler, more nutrient-rich subantarctic waters in the south. Although designated a convergence, the STC is a region of intense mixing, eddy generation, and upwelling. The result is enhanced productivity of plankton, which become an important contributor to the sediment flux (Bradford-Grieve et al., 1998; Nodder, 1998). However, the proximity of Chatham Rise to New Zealand, and the presence of two current systems to transport the terrigenous load to the Rise (the East Cape Current flowing east along the northern flanks of the Rise and the Southland Current passing along the southern Rise), have ensured that a supply of terrigenous material reached Site 1125 (Mitchell et al., 1989; Neil, 1998).
Apart from an old (1972) industry seismic line (Mobil vessel Fred H. Moore, line 72-21) and a single 3.5-kHz profile (National Institute of Water and Atmospheric Research [NIWA] cruise 3011), the only other information on the sediments is provided by a single kasten core from NIWA station R657 at 1408 m depth. This is immediately downslope of Site 1125. Stable isotopes and calcium carbonate profiles, together with foraminiferal assemblage analyses and flux estimates for R657, reveal changes in intermediate-depth water masses and changes in fluxes that are a response to paleoclimatic cycles going back to isotope Stage 6 (Neil, 1998; Weaver et al., 1998; Carter et al., unpubl. data). These data assisted in the choice of Site 1125 as providing a historical record of AAIW and sediment fluxes, in particular the paleoproductivity record associated with the STC. Furthermore, Site 1125 and DSDP Site 594 on the south side of Chatham Rise, provide control points with which to evaluate the long-term position of the STC (e.g., Nelson, 1986).
Cores from Site 1125 recovered a succession of clay-rich nannofossil ooze and chalk with interbeds of more terrigenous silty-clay. The sedimentary sequence is divided into two basic lithologic units that are recognized on the basis of changes in the calcareous biogenic and noncarbonate components along with variations in bedding and color. The division of the lithologic units is supported by estimates of core composition from smear slides (see the "Core Descriptions" contents list), together with shipboard measurements of calcium carbonate, physical properties, light reflectance, and bulk mineralogy using X-ray diffraction. The generalized characteristics of the lithostratigraphic units are summarized in Figure F3, and a more specific set of logs, combining biostratigraphic and magnetostratigraphic data, is presented in Figure F4.
Light reflectance at 550-nm wavelength (Fig. F5) is presented as a proxy of CaCO3, the relationship being based on data collected at Sites 1120 to 1124. Calcium carbonate data specific to Site 1125 are still required.
Unit I extends from the present seafloor to 245.2 mbsf and represents an interglacial/glacial cyclic sedimentation pattern of alternating nannofossil ooze and silty clay that can be divided into Subunits IA and IB on the basis of lithology and color.
Subunit IA, extending from 0 to ~75 mbsf, is a sequence of light and darker colored layers consisting of light gray (5Y 7/2) to white (5Y 8/1) clayey nannofossil ooze interbedded with light olive-gray (5Y 6/2) to olive-gray (5Y 5/2) nannofossil-bearing silty clay. Beds are distinguished by color variations with layers typically between 0.5 and 1.5 m thick. Contacts are generally bioturbated.
The accessory components of the clayey nannofossil oozes, which are regarded as interglacial deposits, include foraminifers in "Present" to "Common" abundance, as well as a small component of sponge spicules and biogenic siliceous fragments. The darker olive-gray nannofossil-bearing silty clay layers are distinguished by an increased abundance of quartz/feldspar and a larger biosiliceous component of radiolarians, diatoms, and spicules together with fewer foraminifers. These layers are interpreted as representing glacial periods. Below ~30 mbsf (Core 181-1125A-4H), pyrite is found as smears or frequently as aureoles around and infilling burrows. Glauconite is locally a significant constituent of the sediment.
Pervasive bioturbation exists throughout Subunit IA and identified ichnofauna include Zoophycus, Chondrites, Planolites, Thalassinoides, and Skolithos (Fig. F6). This assemblage shows a succession from a dominant Cruziana facies in the upper ~37 m of the sequence to an alternating and finally dominant Zoophycos facies below ~50 m.
Numerous macroscopic tephra layers are present in Subunit IA (Table T2; Fig. F7), ranging in thickness from <1 cm to a maximum of ~20 cm. Tephra layers are typically pinkish gray (5YR 5/1 to 5YR 6/1) to light pinkish gray (5YR 7/1) and are darkened occasionally by the presence of authigenic pyrite. As with Sites 1122, 1123, and 1124, layers commonly have sharp bases, normal grading, and bioturbated upper contacts. Below basal contacts, reworked tephra often infills burrows, particularly those of Thalassinoides. The composition of the tephra layers (dominantly fresh glass and phenocrysts of plagioclase), the absence of Bouma-sequence sedimentary structures, particularly ripples, and the location of the site downwind of the Central Volcanic Region source, indicate the tephra accumulated from airfall rather than turbidity currents. A total of 26 tephra layers were recorded in Subunit IA. In addition, dark green, silty clay laminae are dispersed throughout the subunit and probably represent alteration of thin basic tephras, as previously described by Gardner et al. (1986) and Nelson et al. (1986).
Core disturbance is minimal through Subunit IA, with only a small amount of flow-in present in Cores 181-1125A-5H and 7H.
Subunit IB extends from 74.8 to 245.2 mbsf. It consists of massive beds (up to ~8 m thick beds) of greenish gray (5GY 5/1 to 5GY 6/1) nannofossil-bearing silty clay intercalated with equally thick light greenish gray (5GY 7/1 to 5BG 7/1) clay-bearing nannofossil ooze. Below ~206 mbsf (Core 181-1125B-23X), the sediments become sufficiently indurated to be classified as nannofossil-bearing mudstone and clay-bearing nannofossil chalk, respectively.
The Zoophycos ichnofacies, seen toward the base of Subunit IA, continues downcore through Subunit IB. However, a reappearance of Cruziana assemblage occurs between ~220 to 222 mbsf, albeit with slightly different trace fossils, which include Palaeophycus, Teichichnus, and Skolithos (Fig. F6).
A noticeable difference between Subunits IA and IB is the paucity of tephra in the latter. Only 12 macroscopic tephra beds (1-20 cm thickness) were detected, all of which occur in the unlithified section of Subunit IB. Furthermore, only a single green lamina is noted in Subunit IB (Section 181-1125B-23X-5).
The change from APC to XCB coring at around 189 mbsf (Core 181-1125B-21X) suggests the sediment becomes lithified through the formation of carbonate cement at that level, some 17 m below the lithologic determination of the change from ooze to chalk. This transition is also marked by a decrease in the foraminiferal abundance and the inception of drilling biscuits.
Unit II consists of mainly clayey nannofossil chalk and extends from 245.2 mbsf through to the base of Hole 1125B at 552.10 mbsf. The unit is divided into Subunits IIA and IIB on the basis of lithology and color.
The sedimentary sequence between 245.2 and 331.5 mbsf is a monotonous, very light gray (5Y 7-7.5/1) clayey nannofossil chalk. Subunit IIA is recorded as an increase in light reflectance (Fig. F5), which is most likely associated with higher carbonate percentages. Glauconite is dispersed throughout the sequence and is locally concentrated in sandy beds. The site is well above the modern regional carbonate compensation depth (~4500 m), and so the carbonate-rich sediments are well preserved. Bioturbation is common through this subunit and, for the most part, is dominated by a Zoophycos ichnofacies containing Chondrites, Planolites, and Zoophycos. The pervasive occurrence of Chondrites is highlighted by the presence of pyrite, which commonly fills these burrows. Only below ~312 mbsf does the sporadic appearance of Teichichnus and Skolithos suggest an alternation with Cruziana ichnofacies (Fig. F6).
A total of three macroscopic tephra layers, ranging in thickness from ~1 to 2.5 cm, are present in Subunit IIA. All show the typical form described previously.
Subunit IIB has a dominant lithology of clayey nannofossil chalk, in various shades of gray (light gray 5Y 7/1 and light greenish gray 5GY 7/1 to greenish gray 5GY 6/1). This subunit is distinguished from its overlying counterpart by an abundance of pale yellow (5Y 7/3) zones that are interpreted as infilled burrows. This fill consists of clayey nannofossil chalk with a conspicuous biosiliceous component of radiolarians, diatoms, and spicules. Contacts are generally bioturbated, but the abundant biscuiting and brecciation makes the positioning of many contacts subjective. Cyclicity in the color layering is intermittently present but in general it is too subtle for visual identification in split core.
The pale yellow burrow fills highlight the trace fossils, which show notable changes downcore. Initially, fossil assemblages belong to the Zoophycos ichnofacies containing Zoophycos, Chondrites, and the widespread Planolites and Palaeophycus. Between ~432 and 466 mbsf, alternating zones of Zoophycos and Cruziana facies are present, whereas below 466 mbsf Cruziana dominates with robust assemblages containing Skolithos, Teichichnus, Thalassinoides, and Terebellina. Although this facies is placed in the Cruziana category, it is close to being a Skolithos assemblage and is therefore indicative of an even more energetic and well-ventilated water mass. From ~508 mbsf to the base of Hole 1125B, the dominant high-energy Cruziana ichnofacies is interspersed with zones of Zoophycos.
Another feature is the abundant tephra layers; a total of 20 are recognized in Subunit IIB (Fig. F7). Some layers have been disturbed by drilling biscuits, and their estimated thicknesses range from 1 to 10 cm. Of note in these tephra layers is the presence of fine planar laminae, which is probably evidence of bottom-current activity in the unit. This is consistent with the ichnofauna assemblages described above.
Like other sites whose sediments are predominantly a varying mixture of terrigenous and calcareous biogenic components, sediments of Site 1125 have a cyclicity that is well displayed in the 550-nm reflectance profiles (Fig. F5). Such cycles are a response to changes in the burial flux of the two main sediment components, and, on the basis of data from nearby piston core R657, these changes are directly correlatable to isotope stages (at least for Stages 1-6 measured in R657 by Weaver et al., 1998). Therefore, it is tempting to relate the cycles to Milankovitch frequencies, but such a correlation must await development of an astronomically tuned time scale for the site. These cycles are superimposed on longer term changes. Of particular note is the gradual, long-term increase in reflectance/carbonate in the late Miocene (10.5 to 5.5 Ma), followed by a sharp reduction to a level that is more or less maintained through the late Miocene to Holocene (Fig. F5). These data, together with lithologic and faunal information, suggest that Site 1125 sediments evolved as follows.
Around 10.5 Ma, northern Chatham Rise received nannofossil ooze (now chalk) and a lesser amount of terrigenous sediment. The biogenic siliceous component was relatively minor. Up to 6 Ma, the rate of accumulation decreased gradually in response to a reduction in the terrigenous supply, as suggested by a concomitant decrease in reflectance values and the lack of visually identifiable sedimentary cycles (note the spectrophotometer-detected color cyclicity was invisible to the naked eye). A high-energy environment prevailed for the first half of Subunit IIB as suggested by the dominance of a Cruziana ichnofacies (see Pemberton and MacEachern, 1995). Bioturbation was extensive and produced the strongly mottled nannochalk with its characteristic large, pale yellow burrows. However, the upper reaches of Subunit IIB marked a change to the Zoophycos ichnofacies, heralding a less energetic setting.
Around 6 Ma, sedimentation rates increased markedly (see "Age Models and Sedimentation Rates"). The continued increase in the already-dominant nannofossil ooze is consistent with the enhanced production of biogenic carbonate caused by warmer ocean temperatures and increased upwelling at the STC (Kennett and von der Borch, 1986). Higher sedimentation rates, together with fluctuating benthic conditions, may also have influenced the change in bioturbation style seen in Subunit IIA (i.e., the replacement of large, pale yellow burrows by smaller less conspicuous traces belonging to ichnofacies that alternated between Zoophycos and Cruziana).
The change to Unit I occurred around 5.5 Ma and is proximal to an abrupt decrease in reflectance values at 241 and 256 mbsf (Fig. F5). This decrease marks the first appearance of terrigenous mud and of visually obvious color cycles (as opposed to cycles measured only by reflectance). There is also a change in the overall sediment color from light gray in Unit II to green gray in Unit I. The enhanced terrigenous supply followed a major reorientation of the motion of the New Zealand plate boundary. Around 6.4 Ma, the plate motion changed from predominantly strike slip to one with a major compressive component (Walcott, 1998). As a result, uplift increased, erosion accelerated, and more sediment was introduced to the eastern South Island. The reason for the suddenness of the terrigenous influx at Site 1125, 0.9 m.y. after the change in plate motion, is not clear. Biostratigraphic data provide no evidence for a hiatus between Units I and II. However, an abrupt reduction of the diatom flora as a result of corrosion suggests a change in oceanographic conditions, that is, inflow of waters undersaturated in silica (see "Biostratigraphy"). Thus, the influx of terrigenous sediment may reflect a change in the circulation, in particular, in the path of the Southland Current.
Reflectance/carbonate increased slightly in the early Pliocene but did not reach the levels recorded in Unit I. However, the increase was short lived. Around 5 Ma, carbonate reduced to a general level that remained fairly constant until Holocene times. The reduction in carbonate was accompanied by a gradual reduction of the overall sedimentation rate, suggesting a cause-and-effect mechanism. This may be the case, but the overall sedimentation rate continued to decline gradually into the Pleistocene at a time when the terrigenous influx from New Zealand increased as a result of greater tectonic uplift along the plate boundary, progressive exposure of readily erodible rocks, and increasing severity of glaciations (e.g., Carter and Carter, 1993). Furthermore, there was a significant contribution from tephra, with macroscopic tephra contributing up to 2.7% of Unit IA's thickness. It would appear that following the burst of deposition between 5 and 6 Ma, northern Chatham Rise either became increasingly isolated from the terrigenous input from New Zealand or conditions on the Rise became less conducive to deposition. This is inferred from a change to more energetic conditions suggested by the Cruziana ichnofacies in the Pleistocene. However, the presence of well-formed grains of glauconite in Unit I indicates that the crest of Chatham Rise, a site of extensive greensand deposits (Cullen, 1987), provided detritus to Site 1125. Presumably, such interchanges occurred mainly during lowstands of sea level when reaches of the crest would be affected by storm waves and currents.
Superimposed on the general reduction in sedimentation are prominent cycles (Fig. F5) which, on the basis of data from Neil (1998) and Weaver et al. (1998), are probably related to glacial/interglacial periods. Their results reveal that the carbonate flux increased in interglacial periods, whereas the terrigenous-biogenic silica flux dominated during glaciations. In core R657, the greatest difference between fluxes occurred during Stage 2 with terrigenous detritus increasing markedly and the carbonate input decreasing to near zero. Such a difference was in part caused by dissolution of planktonic foraminifers, possibly by corrosive AAIW (Weaver et al., 1998). In contrast, Stages 4-6 had carbonate and terrigenous-biogenic silica fluxes that were similar, both being within a fairly narrow range of 0.5-1.0 g/cm2/ky. However, further dissolution events were detected sporadically in core-catcher samples from the Pleistocene section of Site 1124.