Sediments recovered during Leg 182 record carbonate deposition in a mid- and high-latitude setting against the background of an evolving Southern Ocean and northward drift of the Australian continent. Sediments from the upper continental rise, in 3875 m of water at the toe of the slope (Site 1128), chronicle the change from early Paleogene time, when a humid onshore climate flushed large amounts of terrigenous clastic sediment into the deep sea, to Neogene time, when increasing continental aridity promoted mostly marine carbonate deposition. Green Eocene siliciclastic sands and silts that accumulated largely below the carbonate compensation depth (CCD) in a poorly oxygenated, bathyal setting become finer grained upward, with much of the deep-water late Eocene represented by clay deposition (Fig. F3). Initiation of the contemporary Southern Ocean circulation system, and therefore the modern global ocean, is signaled by a gradual change to lower Oligocene brown clay and carbonate, as this part of the seafloor became ventilated and the CCD deepened. The deep-water Neogene record is one of pink pelagic carbonate ooze punctuated by white planktonic foraminiferal turbidites. Late Oligocene-middle Miocene time is represented by a major hiatus and sediment gravity-flow deposits.
Most drilling took place on the upper slope and outermost shelf, in 202-784 m of water, through a mainly carbonate succession (Figs. F3, F4). Two distinct groups of strata, Eocene-late middle Miocene and late Pliocene-Quaternary in age, form the upper part of the continental margin separated by a thin upper middle Miocene-lower Pliocene interval characterized by slumps, sediment gravity-flow deposits, and/or unconformities. The older succession is stratigraphically equivalent to and roughly coeval with most of the Eucla Group, exposed onshore in the Eucla Basin beneath the Nullarbor Plain. Sediments are a package of Eocene shallow-water terrigenous sands and carbonates that deepens upward into Oligocene and lower-middle Miocene pelagic ooze and chalk (Fig. F3). Although recovery was generally poor through the Oligocene and Miocene interval because of silicification (Site 1134 is an exception for the Miocene), recovered carbonates are characterized by stained hardgrounds and numerous omission surfaces. The younger, wholly Neogene succession is a large seaward-dipping wedge of carbonate sediment that downlaps onto the older sediments and has been prograding seaward onto the Eyre Terrace since late Miocene time. The contact between the two successions is represented, particularly in the upper Miocene and especially the Pliocene, by slumps, sediment gravity-flow deposits, or unconformities. Such erosion, corrosion, and/or mass-wasting and redeposition processes reflect periods of margin instability, seismicity, or lowered sea level.
An important finding of Leg 182 was that the huge wedge of slope sediment prograding onto the Eyre Terrace is nearly entirely Quaternary in age (Figs. F3, F4). This deposit, formed by carbonate produced on the outer shelf and upper slope and swept seaward, is more than 550 m thick in the center (Fig. F4), where rates of accumulation exceed 40 cm/k.y., equivalent to many shallow-water tropical carbonates and twice the rate of Bahamian slope sedimentation (Eberli, Swart, Malone, et al., 1997). The green and gray material is surprisingly uniform in composition, made up of fine carbonate sand and silt that was reworked in place by generations of burrowing organisms, leading to multitiered trace fossil assemblages. Particles are all skeletal fragments, mainly delicate bryozoans, ostracodes, benthic and planktonic foraminifer tests, tunicate sclerites, nannofossils, and siliceous sponge spicules. The deposit grades from mud and sand at the top, downward to partially lithified sediment in the middle, to hard limestone at the base. Sediment below 400 meters below seafloor (mbsf) is usually neomorphosed with numerous fossil molds or partially altered to sucrosic dolomite.
The facies transition upslope into shallower water is marked by the presence of numerous bryozoan-rich buildups (Figs. F3, F4). These mounds, in water depths of ~200-350 m, are dominantly muddy and characterized by the prolific growth of numerous and diverse bryozoans. The mounds in particular have seafloor relief of as much as 50 m and extend laterally many hundreds of meters. These are among the first modern analogs to similar mounds that were an important part of the carbonate depositional systems in earlier Phanerozoic time.
The Cenozoic sequence penetrated during Leg 182 mainly represents the Quaternary-Eocene. Biostratigraphic data indicate that these Cenozoic successions are mostly hiatus bound (Fig. F5), with the mid-Pliocene, lower Pliocene-upper Miocene, and upper part of the middle Miocene successions either missing or highly condensed. Other intervals of the Miocene and Oligocene are also missing, although these hiatuses are not present at all sites.
A greatly expanded Quaternary-uppermost Pliocene section was recovered at shallow-water Sites 1129, 1131, and 1127 (the "eastern transect"). The longest section at Site 1129 exceeds 555 m in thickness, with an average accumulation rate of more than 400 m/m.y. (Fig. F6). In contrast, the Quaternary-uppermost Pliocene sections only extend to ~260 mbsf at Site 1130 and ~240 mbsf at Site 1132. The Quaternary-upper Pliocene section is underlain at most sites by a thin lower Pliocene-upper Miocene interval, which in turn disconformably overlies a unit of mainly early middle Miocene age. The absence of many lower Pliocene to upper middle Miocene biozones from Sites 1129 and 1131 signifies a hiatus of at least ~7-11 m.y. This major unconformity was represented at other sites as two or three shorter hiatuses of various durations at the middle/upper Miocene, upper Miocene/lower Pliocene, and lower/upper Pliocene boundaries. They are most clearly recorded as disconformities capping the upper Miocene section at Sites 1126, 1134, and 1133 in intermediate water depths, and at shallow-water Sites 1130 and 1132 (Fig. F5). Thus, it is reasonable to speculate that the unconformity at the Quaternary-uppermost Pliocene/middle Miocene contact probably resulted from at least two major events that subsequently almost completely erased the sedimentation record of the upper middle Miocene-Pliocene.
The upper Miocene succession is better represented at Site 1130 and at deep-water sites. The middle Miocene, mainly the lower part, is present at all sites except Sites 1128 and 1130, and a thin lower Mio-cene unit was encountered at the base of Hole 1131A (~590-610 mbsf). An expanded middle Miocene section at Site 1132 was accompanied by sedimentation rates ranging from 14 to 54 m/m.y. (Fig. F6). A similar rate of 50 m/m.y. was recorded for the middle Miocene at Site 1133. The absence of middle-lower Miocene biofacies from Sites 1130 and 1128 is interesting because these two sites respectively represent intermediate and deepest water sites along the depth transect. At Site 1130 (488 meters below sea level [mbsl]), the upper Miocene section is disconformably underlain by sediments of mainly early-middle Oligocene age, indicating a depositional gap of ~15 m.y. At Site 1128 (3875 mbsl), the entire middle Miocene-upper Oligocene interval corresponds to a ~15-m-thick debrite with mixed calcareous nannofossil and planktonic foraminiferal assemblages of early-late Miocene age, suggesting an unconformity totaling ~13 m.y. in duration. Sites 1126 and 1134, two intermediate-water sites, exhibit generally similar assemblage compositions, ages of recovered sediments, and position and number of hiatuses. These are the only sites yielding definite lower Miocene calcareous nannofossils and planktonic and benthic foraminifers.
Sediments of Oligocene age and older were recovered at Sites 1126, 1128, 1130, 1132, and 1134 (the "western transect"). At Sites 1130 and 1132, the Oligocene section probably overlies Eocene sediments disconformably, and a hiatus of ~2 m.y. is indicated by biostratigraphic data. However, the zonal succession of nannofossils and planktonic foraminifers is largely continuous across the Oligocene/Eocene boundary, suggesting a conformable succession at Sites 1126, 1128, and 1134. Mainly lower Oligocene assemblages are present at Sites 1128, 1130, and 1132 in the west, and an expanded lower Oligocene section is indicated by nannofossils at Site 1128. Biostratigraphic resolution below 70 mbsf at Site 1128 was largely achieved by calcareous nannofossils because planktonic foraminifers were rare or barren in sediments dominated by siliceous oozes and packstones.
Holes 1126D and 1134A are the only holes containing rich calcareous microfossils of Eocene age. Poor preservation, however, impaired proper recognition of species and biozones in some intervals. Although present at all western sites, the middle Eocene is represented by poorly preserved, impoverished assemblages in various poorly recovered lithologies. The Eocene is represented by a siltstone (~150 m thick) with sporadic nannofossils at Site 1128; a calcareous packstone (~30 m thick) with moderately preserved nannofossils and planktonic and benthic foraminifers at Site 1126; and a dark, iron-stained sand (30 m thick) with rare and poorly preserved microfossil assemblages at Site 1134. A calcareous sandstone at Site 1130 and a bioclastic limestone at Site 1132 both contain shallow-water associations of planktonic and benthic foraminifers. No calcareous nannofossils or foraminifers were discovered in the dark green sandstone at the base of Hole 1126D, which may correspond to Cretaceous synrift sediments along the southern Australian margin.
The succession of microfossil assemblages from Leg 182 provides an excellent record of Eocene-Holocene sea-level and circulation changes in the Great Australian Bight. Expanded Quaternary successions at Sites 1127, 1129, 1130, 1131, and 1132 contain upper bathyal benthic foraminiferal assemblages with a redeposited neritic component. Apparent fluctuations in the relative proportions of neritic taxa and upper bathyal taxa indicate that downslope transport periodically varied in intensity during the Quaternary. These changes, together with the periodic intrusion of warm-water species into temperate planktonic foraminiferal assemblages, indicate responses to changing climate and oceanic circulation. Calcareous nannofossil assemblages dominated by Braarudosphaera bigelowii indicate that a major ecological crisis affected the planktonic ecosystem near the base of the Quaternary-uppermost Pliocene.
At three shallow-water sites (Sites 1129, 1131, and 1132), diversified, extremely well preserved benthic foraminiferal assemblages, including unusually large specimens (>1 mm), are found together with abundant and well-preserved bryozoan fragments (Fig. F5). These assemblages were probably part of a highly dynamic ecosystem that became established at the seafloor during the Quaternary, coincident with bryozoan buildups in paleodepths of 200-300 m. A shallowing-upward trend from middle bathyal paleodepths in the middle and late Miocene to upper bathyal paleodepths in the Quaternary is recorded at Sites 1129, 1130, and 1132 (Fig. F5). Middle bathyal faunas at these sites reflect middle and late Miocene transgressive pulses that are also expressed onshore in southern Australia by thin transgressive tracts.
Middle bathyal assemblages from upper Eocene-upper Miocene successions show major changes in composition at intermediate-water Sites 1126 and 1134. The most severe changes in the early Miocene probably relate to global changes in deep- and intermediate-water circulation affecting the distribution of bathyal benthic foraminifers worldwide. The composition of abyssal assemblages at Site 1128 indicates deposition above the CCD during the Quaternary-latest Pliocene, close to the lysocline in the late Oligocene-middle Eocene, and below the CCD in the early-middle Eocene in the deep basin (Fig. F5). Assemblage boundaries appear to be coeval in the Eocene, Oligocene, and lower Miocene of intermediate- and deep-water Sites 1126, 1128, and 1134, suggesting that benthic foraminifer distribution was controlled by major environmental changes during these periods. Discrepancies between assemblage boundaries of deep-, intermediate-, and shallow-water sites in the middle Miocene-Quaternary sequence relate to stratigraphic or lithologic differences and reflect distinct depositional and paleoceanographic regimes in the region.
Previous carbonate legs have proved very unsatisfactory for paleomagnetists because the magnetic intensity of the sediments was so low that remanence measurements were essentially unreliable. However, with the change to direct-current superconducting quantum interference devices in the new instrument, it was possible to measure the magnetization at most sites during Leg 182. Nevertheless, the natural remanent magnetization (NRM) of sediments from Sites 1126 and 1134 was so weak that, after a coring overprint was removed, the magnetization of much of the sediment was at the noise level of the instrument.
During Leg 182, an experimental nonmagnetic cutting shoe was used in advanced hydraulic piston corer (APC) coring (see "Appendix: Magnetics Experiment"). The shoe and other components of the bottom-hole assembly were used to investigate anomalous magnetization observed in cores. Comparisons were made between cores taken with (1) standard core-barrel assemblies, (2) standard core-barrel assemblies and the nonmagnetic shoe, and (3) nonmagnetic core barrels and shoe. On some occasions, it was clear that the nonmagnetic shoe and assembly greatly improved the paleomagnetic record. On other occasions, the shoe alone improved the record compared with the standard core-barrel assemblies. However, there were also sites where neither the nonmagnetic shoe nor the entire nonmagnetic core-barrel assembly had any obvious effect. This is consistent with the intermittent nature of the anomalous magnetization experienced during past legs. Postcruise work will focus on trying to isolate other sediment properties such as grain size, porosity, or shear strength, which may control the distribution of coring contamination.
A common feature of the rock magnetism in the upper sections of Leg 182 holes was the extremely high ratio of anhysteretic remanent magnetization to isothermal remanent magnetization (ARM/IRM). This is a measure of the degree to which single-domain particles dominate among the magnetic phases present in the samples. This ratio generally decreased downhole and was accompanied by a decrease in intensity of magnetization and coercivity. This decrease in magnetization intensity is generally interpreted as dissolution of the finest grained magnetic phases in the sediment. IRM acquisition suggests that magnetite and magnetic sulfides are the principal remanence carriers. The observation of a downhole decrease in ARM/IRM and intensity is consistent with diagenetic models involving organic matter oxidation and sulfate reduction as the principal processes that regulate the preservation of ferromagnetic phases.
Long-core and discrete sample measurements yielded a record of inclination from which a Pliocene-Quaternary magnetostratigraphy was interpreted. Overall, deeper water records are of good to fair quality, yielding accurate estimates of the expected inclination for the present latitude. In contrast, lower intensities in the shallow-water sites result in shallow inclinations with large scatter. The Brunhes/Matuyama boundary was identified in five high-sedimentation-rate sites (Fig. F7; Table T1). The Jaramillo Subchron (C1r1n) and the top of the Olduvai Chron (C2n) were also found at three sites. The interpretation of magnetic polarity in Figure F7 implies that sedimentation rates in the early Quaternary-latest Pliocene were considerably lower than during the Brunhes Chron. In addition, variations in the intensity of the remanence after partial demagnetization were found to oscillate on time scales comparable with those of the geocentric axial dipole. Although the intensities have not been normalized to account for variations in concentration, these sediments may provide a valuable relative paleointensity record that can be used for high-precision correlation between sites. For Sites 1126 and 1134, magnetostratigraphic data were obtained for parts of the Miocene. Site 1128 also contains a long record of the early Oligocene polarity sequence.
The
drilling of nine sites off the Eucla margin provided a
unique insight into the fluid dynamics along a continental
margin dominated by cool-water carbonates. The most
significant discovery was the presence of a brine, varying
in salinity between 80 and 105, that was present in and
underlying seven of the nine sites (the exceptions being
Site 1128, drilled in a water depth of 3884 m, and Site
1133). At Site 1133 (located in 1043 m of water) fluids were
encountered with a salinity of 40, but recovery problems
precluded the measurement of the salinities in deeper
samples. The brine was present at relatively shallow depths
at the deeper sites (1134 and 1126), whereas maximum
salinities were not encountered until depths 
400
mbsf at the shallower sites (Sites 1127, 1129, 1130, 1131,
and 1132). The impact of the brine is more readily apparent
along the eastern transect (Sites 1127, 1129, and 1131; see
Fig. F4).
Similar processes appear to occur at Sites 1130 and 1132,
although they are reduced in nature.
At Sites 1127, 1129, and 1131, a gradual increase in salinity was encountered, reaching a maximum value of ~100 at Site 1127 (farthest seaward) and 92 at Site 1129 (closest to the shelf). Pore fluids in these Quaternary portion of the sediments from these sites also possess a Na+/Cl- ratio exceeding that of seawater, suggesting that the fluids in the sediments had been involved in the dissolution of NaCl. Although the origin of the fluids has not yet been established, a probable explanation is that they formed during the Quaternary when the Eucla shelf was exposed numerous times during sea-level lowstands. We suggest that large hypsersaline lagoons developed during these episodes of low sea level. Because of the greater hydrostatic head, fluids with high salinity were forced into underlying strata and out onto the adjacent continental slope. During sea-level highstands, high-salinity fluids then diffused upward as additional sediment was deposited, and the profiles of individual ions were modified by diagenetic reactions in the sediments. This sequence of events probably occurred a number of times as sea level fluctuated during the Quaternary. This hypothesis is supported by an examination of the Cl- distribution at the three sites (Fig. F8), which suggests that the top of the brine has a common depth below sea level and, therefore, crosscuts sequence boundaries.
All three sites exhibited high concentrations of H2S and methane, combined with high values of alkalinity. The high concentrations of H2S are derived from the oxidation of organic material by sulfate-reducing bacteria. The relatively low concentrations of iron in carbonates, in contrast to siliciclastic-rich environments, means that the H2S is not sequestered as iron sulfides. Consequently, concentrations of H2S are able to reach high levels (>150,000 ppm at Site 1131). As a result of the high rate of sedimentation (>200 m/m.y.) and the position of the sites close to the continental shelf, these sites contained an initially high concentration of organic material. Under normal conditions, organic material would be oxidized first by oxygen and then by sulfate-utilizing bacteria, thereby creating alkalinity and H2S. Although this process also takes place on the Eucla margin, the high-salinity brines underlying and within the Quaternary succession provide as much as three times the normal seawater sulfate concentrations; therefore, with sufficient organic material, significantly higher amounts of H2S can be formed. As the sediments are buried, the concentration of sulfate is depleted in the sediment and a gradient is established between the sediments, the overlying seawater, and the underlying brine. As a result of the high sulfate concentration of the brine, the flux of sulfate diffusing into the zone of organic material remineralization is significantly greater than in a normal marine sediment, resulting in greater than normal sulfate reduction and higher alkalinity.
In addition to H2S, high concentrations of methane and the presence of other higher molecular-weight hydrocarbons were discovered in the sediments at Sites 1127, 1129, and 1131. At Site 1127, gas pockets in the core were found to contain as much as 73% methane and 365 ppm ethane. We suggest that these gases, including the higher molecular-weight hydrocarbons, are bacterial in origin and generated in situ. Higher molecular-weight alkanes are generally only believed to form through thermogenic processes. However, the concentrations of ethane, propane, and butane did not increase with depth, but instead reached a maximum associated with the maximum in methane, indicating that these gases also originated within this portion of the sediment and were not thermogenic in origin (Fig. F9).
The oxidation of organic material also has an important influence on carbonate recrystallization. The sediments at all sites cored during Leg 182 initially contained a mixture of aragonite, HMC, and LMC. Of these minerals, aragonite and HMC are metastable and alter to the more stable LMC and dolomite. This process is evident at all sites drilled, and is greatly accelerated by the processes of sulfate reduction and consequent formation of H2S. During the oxidation of organic material, two important chemical reactions take place that alter the carbonate chemistry in the pore waters: (1) sulfate reduction creates two moles of alkalinity for every mole of sulfate that is consumed, and (2) one mole of H2S is produced (2CH2O + SO42- = H2S + 2HCO3-). The H2S dissociates in pore water, causing the pH to be reduced and the speciation of carbonate in the pore waters to move from being dominated by HCO3- to H2CO3. This sequence of events produces pore fluids that are undersaturated with respect to the metastable carbonate minerals. The most thermodynamically soluble form of calcium carbonate, HMC, dissolves first, followed by aragonite and then LMC. This trend is observed at all sites. The high-alkalinity environment creates a thermodynamic regime favorable for the formation of dolomite. Dolomitization consumes Mg2+ from pore waters, setting up a strong diffusive gradient into the reaction zone from the overlying seawater and underlying brine. The formation of dolomite at Sites 1127, 1129, and 1131 is well illustrated in Figure F10, which shows concentrations of ~5% at Sites 1127 and 20% at Sites 1129 and 1131.
Although the high-salinity pore fluids were also present at most other sites (Sites 1130, 1132, 1126, and 1134), these sites showed lower concentrations of H2S and methane and consequently lower amounts of carbonate diagenesis. The controlling factor at these sites is the supply of organic material combined with the sedimentation rate.
Downhole logging data were collected at eight of nine sites drilled during Leg 182. To complement these data, an extensive program of sediment physical properties measurements was undertaken. The presence of chert, particularly in the pre-Quaternary sedimentary sequences, resulted in low recovery. Analysis of downhole logging data will provide the information necessary to fill in recovery gaps, refine the placement of lithostratigraphic boundaries, and enable interpretation of sedimentary facies, composition, and structure within missing intervals. Check-shot surveys conducted at five sites will facilitate correlation of the drilled strata to the regional grid of high-resolution seismic data.
The downhole logging and physical properties measurement program undertaken during Leg 182 provided the opportunity to investigate early and postdepositional diagenesis in a nontropical carbonate system. A significant result of Leg 182 was the determination that nontropical carbonates display diagenetically driven mineralogical, geochemical, and textural changes that are similar to those occurring in tropical carbonates, which is contrary to the notion that cool-water carbonates have relatively low diagenetic potential because of their dominantly LMC mineralogy. Nevertheless, our analyses indicated that sediment alteration and cementation were not as extensive as seen in warm-water carbonates. In tropical carbonates, diagenesis results in distinctive, widely ranging physical properties measurements: P-wave velocity (~1.5-6 km/s), porosity (~10%-80%), and bulk density (~1.5-2.8 g/cm3) (Eberli, Swart, Malone, et al., 1997). Petrophysical investigations during Leg 182 show that equivalent variations within cool-water carbonate sediments are much less dramatic: P-wave velocity (~1.5-2.2 km/s), porosity (~30%-60%), and bulk density (~1.5-2.0 g/cm3).
An important finding of Leg 182 was the discovery of distinct, possibly Milankovitch cyclicity recorded in gamma-ray and density data from the thick Quaternary sediment wedge drilled at numerous upper slope/shelf edge sites (e.g., Fig. F11). The high sedimentation rates within this sequence, in excess of 40 cm/k.y., will enable high-resolution frequency analysis of the logging and physical properties datasets that will assist in the refinement of the biostratigraphic ages within this time interval. Formation MicroScanner (FMS) data collected at six of the logged holes will permit detailed investigation of depositional facies and sedimentary structure within the sediment cycles, particularly in those intervals where recovery was poor. Correlation of the cyclicity present in the temperate-water carbonate wedge to that in other Quaternary carbonate and noncarbonate continental margin systems will provide important information on differences in eustatically influenced sedimentation patterns.
