PRINCIPAL RESULTS
Site 1128
Site 1128, the deep-water component of the Leg 182 shelf-to-basin transect, is located on the upper continental rise in 3874.6 m of water. The primary goals of this site were to (1) recover pelagic ooze from the upper continental rise to compile a paleoceanographic record of the Cenozoic opening of the Southern Ocean and development of the Circum-Antarctic Current, (2) determine the history of Cenozoic and Late Cretaceous CCD fluctuations and deep-water mass variations during the evolution of the Southern Ocean, and (3) determine depositional and diagenetic facies on the upper continental rise.
The sedimentary succession intersected at Site 1128 was divided into four major lithostratigraphic units. Unit I (0–95.6 mbsf) is a pink to brown, bioturbated, calcareous nannofossil ooze punctuated by numerous thin glauconite and planktonic foraminiferal sand calciturbidites and conglomeratic sediment gravity-flow deposits. A chaotic zone of debrites and slumped sediment (54.4–70.0 mbsf) separates this succession into three subunits containing variable proportions of resedimented material. Unit II (95.6–281.9 mbsf) is a thick section of uniform, green, variably calcareous clay and claystone that is locally interrupted, particularly in the upper parts, by several centimeter-thick turbidites composed of planktonic foraminiferal and nannofossil ooze. The relatively few carbonate particles in the sediment are etched and corroded, suggesting accumulation near the carbonate lysocline. The lower parts of this unit contain numerous well-preserved trace fossils (Planolites, Chondrites, Zoophycos, Terebellina, and Thalassinoides), and numerous thin chert horizons. A thin unit of green, glauconitic sand-to carbonate nannofossil ooze turbidites forms Unit III (281.9–284.0 mbsf) and marks a sharp change in sedimentation to the coarser, green, carbonate-free silts and clays of Unit IV (284.0–452.6 mbsf). Unit IV was subdivided into an upper Subunit IVA (284.0–358.4 mbsf) composed of bioturbated clayey siltstone with minor chert and glauconitic sand turbidites, and Subunit IVB (358.4–452.6 mbsf) made up of highly burrowed sandy siltstone that grades to green silty sandstone at the base. The well-preserved siliceous microfossils and almost total lack of calcareous microfossils indicates accumulation below the CCD. The burrowed green sediment points to accumulation in dysoxic bottom waters.
Two major biostratigraphic successions were recovered at Site 1128, dated by calcareous nannofossils and planktonic foraminifers as late Miocene–Pleistocene (0–55 mbsf) and early Eocene–early Oligocene (72–427 mbsf). These intervals are separated by the debrite interval (55–72 mbsf) containing mixed upper Paleogene and Miocene nannofossils and planktonic foraminifers. Calcareous nannofossil data indicate four possible hiatuses within the Neogene succession, and a major unconformity spanning the Miocene/Oligocene boundary at ~72 mbsf. Both nannofossil and planktonic foraminifer data show that this unconformity spans a gap of at least 5 m.y., as the entire upper Oligocene is missing. Below 72 mbsf, nannofossils and planktonic foraminifers show increasing signs of dissolution and are absent at some levels, suggesting deposition near the lysocline and CCD. Sedimentation rates were 10–12 m/m.y. through the Neogene, 50–60 m/m.y. in most of the early Oligocene, and 4 m/m.y. in the late Eocene. Although characterized by poor core recovery and barren intervals, datum levels from the remainder of Eocene sediments suggest a sedimentation rate of 40–45 m/m.y. Five benthic foraminiferal assemblages are distinguished: (1) a diversified Pleistocene and Pliocene assemblage indicating abyssal paleodepths above the CCD; (2) a mixed assemblage of displaced faunas in the debrites; (3) an impoverished early Oligocene–early-late Eocene assemblage indicating lower bathyal to abyssal paleodepths close to the lysocline; (4) an impoverished early-late to late-middle Eocene calcareous assemblage indicating lower bathyal to abyssal paleodepths close to the lysocline; and (5) an impoverished late-middle to late-early Eocene agglutinated assemblage indicating deposition below the CCD.
Long-core measurements established a magnetostratigraphy for the uppermost 40 mbsf, spanning the Brunhes Chron, Matuyama Chron, and part of the Gauss Chron. Below this, the interval characterized by debris flows disrupts the record and it is not possible to recognize a magnetostratigraphy until a depth of ~60 mbsf, where the record was correlated with the early Oligocene magnetic polarity time scale. A nonmagnetic APC coring assembly was used at this site, and produced significant improvement in the declination record in shallow APC cores. Excellent records were obtained both with and without the nonmagnetic assembly toward the bottom of the intervals cored by APC.
Construction of a composite depth section from Holes 1128B and 1128C indicates that a complete Pleistocene through upper Miocene sedimentary record was recovered at Site 1128. A break in the record occurs at ~68 mcd, corresponding to a 15-m-thick debris flow separating upper Miocene from lower Oligocene sediments. The numerous turbidites in the section aided correlation. The primary lithologic parameters used to create the composite section were magnetic susceptibility, gamma-ray attenuation (GRA) wet bulk density, natural gamma emissions data acquired by the multisensor track on whole cores, and color reflectance data (400 nm) measured on split cores.
Only low concentrations of methane were detected (maximum of 6.2 ppm), with most samples having less than 3 ppm. Calcium carbonate content has a bimodal pattern with generally high values (75–90 wt%) present in the upper 50 mbsf, followed by a decrease to a low of 2.1 wt% at 138 mbsf. Between 138 and 240 mbsf, carbonate content increases to a second mode with a maximum value of 68 wt%, and then decreases downhole with very low values (0–1 wt%) below 353 mbsf. Organic carbon concentrations are generally less than 0.16 wt% down to the base of Hole 1128B. Because organic carbon values are 0 wt% from 200 to 276 mbsf, and headspace methane concentrations in this depth range are barely above background, no samples in Hole 1128D were analyzed for organic carbon. Sulfur is present in only five samples in Hole 1128B, with values ranging from 0.05 to 0.21 wt%. Nitrogen is not detectable in any sample. Sulfur and nitrogen were not analyzed in samples from Hole 1128D.
Site 1128 does not appear to be influenced by the high-salinity pore fluids observed at Sites 1126 and 1127. With the exception of the lower part of the cored interval, salinities are close to normal seawater values throughout. As a consequence of slow rates of deposition and low concentrations of organic material, pore fluids are not significantly depleted in sulfate in the upper sediments, and rates of carbonate recrystallization are relatively low. Two significant changes in pore-water chemistry occur at Site 1128, the first at 20–40 mbsf, and the second between 236.8 and 253.3 mbsf. The change in pore-water chemistry at the shallow depth is manifest as an abrupt decrease in silica concentration from values of ~570 to 350 mM, corresponding to a slump unit. Seismic imagery indicates that this unit outcrops at the seafloor, so it is possible that bottom water infiltrates the formation to cause this effect. The second major change in pore-water chemistry, accompanied by a decrease in salinity, is evident in most of the major and minor cations and occurs between the transition from lithostratigraphic Unit II to Unit IV. Concentrations of magnesium, potassium, sulfate, and sodium show a marked decrease, whereas lithium, alkalinity, and calcium increase. Although the change in salinity is accompanied by a decrease in chlorinity, the chloride decrease is not of the magnitude expected from the salinity decrease alone, indicating that a portion of the salinity decrease is a result of the removal of cations and anions through precipitation and adsorption. Downhole logs indicate the presence of numerous relatively impermeable layers in the interval between lithostratigraphic Units II and IV, which would effectively limit vertical diffusion between these two units and allow them to geochemically evolve relatively independently of each other. The sediments in lithostratigraphic Unit IV contain very low concentrations of carbonate minerals, and reactions leading to the enrichment and depletions in the pore-water constituents principally involve clay minerals.
Sediment physical properties data at Site 1128 closely reflect lithologic variations observed in the recovered sediments and provide essential data for core-log correlation. Physical properties data were subdivided into five units on the basis of trends in the measured parameters. Unit 1 (0–70 mbsf) is characterized by high variability in all datasets, corresponding to the lithologic sequence of turbidites and debris flows interbedded with nannofossil ooze. The base of this unit is marked by abrupt shifts in all parameters measured. Unit 2 (70–139 mbsf) is characterized by low variability in all datasets, punctuated by a number of distinct data offsets (1–5 m thick) corresponding to sharply bounded intervals of redeposited nannofossil ooze. Unit 3 (139–231 mbsf) is an interval characterized by relatively low and nearly constant values for all parameters, corresponding to a sequence of monotonous clays. An increase in NGR, magnetic susceptibility (MS), GRA, discrete P-wave velocity, and a decrease in porosity is observed at the upper boundary of Unit 4 (231–363 mbsf), after which all parameters display increased variability reflecting alternations of indurated and nonindurated sediments. The upper boundary of Unit 5 (363–452 mbsf) is marked by a negative shift in GRA, MS, and NGR, corresponding to the transition into lithostratigraphic Subunit IVB. Other than porosity, which decreases through the unit, the other parameters measured show constant downhole trends for the rest of the recovered interval.
Site 1128 was successfully logged with the triple combo and Sonic/FMS tools. Because of time constraints, the GHMT and WST tools were not run. Downhole logging data were divided into four units, with trends closely correlating with lithologic and sediment physical properties data. Unit 1 (0–242 mbsf) is characterized by uniform values, punctuated by sharp decreases in gamma-ray values and increases in density and resistivity apparently corresponding to intervals of redeposited nannofossil ooze. Unit 2 (242–295 mbsf) is characterized by high variability in all datasets measured, corresponding to an interval of high variability in sediment physical properties data and an interval of variable lithification in cores. The lower part of this unit corresponds to lithostratigraphic Unit III and consists of nannofossil ooze interbedded with variably indurated sandstone turbidites. These interbedded sediments are reflected in logging data as alternations of low gamma-ray, high-density, and resistive intervals corresponding to calcite-rich layers, and high gamma-ray, low-density, and conductive intervals corresponding to sandstones. Logging Unit 3 (295–362 mbsf) has low variability and nearly constant values on all logs except the gamma ray, which increases downcore. Increased concentrations of thorium and potassium correspond to an increased concentration of terrigenous sediment. Unit 4 (363–414 mbsf) has minimal variability in most parameters, with the exception of cyclic variations in gamma-ray values. Downhole measurements from Unit 5 indicate the presence of clays and quartz through the unit as shown by the recovered sediment.
Site 1128 provided the first opportunity to sample and establish a seismic stratigraphy for this thick (containing over 10 km of sediment) and laterally extensive continental rise basin. Seismic imagery shows that the thin (72 m), relatively condensed Neogene section (largely corresponding to lithostratigraphic Unit I) was deposited in a small perched sub-basin lying within a thick Paleogene and Mesozoic succession and that the Neogene sequence is thin or absent over much of the basin. Exposed Mesozoic sediments on the middle and lower slope are the presumed source of lithified material commonly present within resedimented intervals. Although not intersected because of operational time constraints, a prominent angular unconformity ~150 m below total depth at Site 1128 almost certainly marks the base of the Cenozoic. Apart from hiatuses between the lower and middle Eocene and Oligocene and Miocene, Site 1128 results indicate that the upper part of this basin contains a thick, almost continuous biosiliceous record of Southern Ocean development through the Paleogene.