PRINCIPAL RESULTS
Site 1127
Site 1127 was the first and most seaward site of a planned three-site transect through a spectacular set of late Neogene clinoforms immediately seaward of the present-day shelf edge. Site 1127, located on the upper slope in 480.6 m of water, intersects an expanded record of the youngest clinoforms as well as the lowest, more condensed portion of the clinoform sequence. The principal objective of this transect was to collect detailed, high-resolution profiles through a late Neogene shelf-edge (high energy) to upper slope (low energy) succession deposited within a cool-water carbonate environment to determine the response of this type of depositional system to Pliocene–Pleistocene sea-level fluctuations.
At Site 1127, we recovered a 510.7-m-thick monotonous succession (Table 2) that is dominated by very fine- to fine-grained, heavily bioturbated, unlithified to partially lithified, greenish gray wackestones to packstones, made up of three units. Unit I (0–6.0 mbsf) consists of two subunits, an upper nannofossil ooze and a lower foraminiferal ooze. Unit II (6.0–464.5 mbsf) consists of alternating bioclastic wackestone- and packstone-dominated packages with thin grainstone beds, divisible into five subunits. These subunits are characterized by a lower portion composed of wackestones to packstones, grading up into wackestones, and dominated at the top by packstones with thin capping grainstones. Shallower water fauna are most abundant near the top of each subunit. The lowermost subunit (~420–464.5 mbsf) has a slumped base with abundant clasts, including bryozoan and large skeletal fragments within an ooze matrix. Unit III (464.5–510.7 mbsf) consists of an upper nannofossil ooze and a lower glauconite-rich bioclastic packstone, with minor capping grainstone beds with intraclasts.
Calcareous nannofossils and planktonic foraminifers indicate an extraordinarily expanded Pleistocene sequence of more than 450 m, with a hiatus of over 3 m.y. between basal Pleistocene (Zone NN19) and underlying Miocene sediments. Based on preliminary biostratigraphic datums, average sedimentation rates were 240 m/m.y. in the Pleistocene, and 2–8 m/m.y. in the Miocene. The thin Miocene section contains a strongly mixed planktonic foraminiferal assemblage. A well preserved benthic Pleistocene foraminiferal assemblage is recognized down to 414.5 m. Between 414.5 and 501.1 mbsf, two benthic assemblages were identified that contain mainly upper bathyal taxa but include large numbers of abraded and corroded tests, indicating extensive mixing or reworking. Calcareous nannofossil assemblages throughout most of Zone NN19 are dominated by small forms, suggesting some sorting. Similarly, Pleistocene benthic foraminiferal assemblages display a uniformity in test size and lack of medium to large tests that is consistent with grain-size sorting and downslope redeposition. The cool temperate Pleistocene fauna is accompanied by warm-water species at various intervals, probably reflecting a combination of global climatic fluctuations and regional paleoceanographic variations, especially in relation to flow of the Leeuwin Current.
Paleomagnetic measurements revealed a long section of normal polarity to 343.4 mbsf, which we interpret as the Brunhes Chron. The mean inclination was –49.9°, with a standard deviation of 22°. It was only possible to locate a very approximate onset (380 mbsf) and end (395 mbsf) of the Jaramillo Subchron. We found intensity fluctuations within the Brunhes Chron in the NRM after 20 mT demagnetization that appear to correlate with standard records of geomagnetic field fluctuations back to 400,000 yr. The calculated sedimentation rate for the whole Brunhes Chron is 480 m/m.y., whereas the rate for the first 400,000 yr based upon the intensity estimation is 277 m/m.y. Rock magnetism analysis revealed that the dominant magnetic phase was chemically unstable, so that the saturation remanence decayed by as much as 60% within 48 hr. This behavior is typical of the ferromagnetic sulfide greigite, which inverts sluggishly to paramagnetic pyrite. The high ARM/IRM suggests a dominant single-domain grain size and a strong magnetotactic bacteria input. However, if this suggestion is correct, the magnetotactic bacteria synthesize greigite rather than magnetite.
The most striking organic geochemical results at Site 1127 are the very high concentrations of methane and hydrogen sulfide present throughout the section. Gas pockets within the core were common from 47 to 312 mbsf, and these were sampled directly through the core liner with a gas-tight syringe (vacutainer). Methane concentrations in these gas pockets range from 165,000 to 730,000 ppm, with most values between 400,000 and 600,000 ppm. Hydrogen sulfide ranges from ~60,000 to 138,000 ppm. Methane is present at lower and highly variable concentrations in headspace samples, with the lowest values at the top and bottom of the drilled interval. Methane/ethane ratios in vacutainer samples decrease downhole from 2728 to 1309, with a decrease in the ratio to 174 in headspace samples. Carbonate content values predominantly range between 85 and 92 wt%, with a trend toward lower values with increasing depth. Organic carbon values are primarily in the range of 0.1–0.6 wt% down to ~250 mbsf, and then vary from 0.5 to 1.0 wt% to the bottom of the hole.
Although similarly influenced by high-salinity pore fluids (up to 100 in Pliocene and older portions of the section), pore-water geochemistry at Site 1127 was fundamentally different than at Site 1126 because of an extended sulfate reduction zone. Because of the more inshore location, Site 1127 not only had a higher initial organic matter content compared to Site 1126, but more of the organic material was preserved as a result of higher sedimentation rates. Because of the high sulfate concentration in the deeper, higher salinity fluids, the sulfate reduction zone is enlarged and the consequent production of hydrogen sulfide is much higher than in normal organic-rich sediments. In addition, the relatively iron-poor nature of these sediments precludes the formation of iron sulfide phases that would normally sequester hydrogen sulfide. The greatest amount of carbonate recrystallization appears to occur at the lower interface between the sulfate-depleted zone (~180 mbsf) and the underlying sediments, where sulfate is actively diffusing upward from the higher salinity fluids below. This process produces pore fluids undersaturated with respect to metastable minerals, causing the precipitation of low-Mg calcite and dolomite. Striking variations in the amount of low-Mg calcite, high-Mg calcite, aragonite and dolomite also occur throughout the succession, with presumed sea-level lowstands characterized by increased high-Mg calcite and the presence of dolomite.
Because of disruption of the sediments during degassing, laboratory physical properties measurements are of limited value for developing a downhole stratigraphy. However, the NGR measurements were only affected to a minor extent, and can be used to define three PP units: PP Unit 1, from 0 to 130 mbsf, is characterized by three high-amplitude cycles superimposed on a rising trend, and corresponds to seismic subsequence 2a. PP Unit 2 is from 130 mbsf to the Pliocene/Pleistocene boundary at 467 mbsf, which is marked by a sudden decrease in NGR values. This unit is characterized by low-amplitude, high-frequency cyclicity in NGR values and corresponds to seismic subsequences 2b and 2c. PP Unit 3 corresponds to the Pliocene sediments below the Sequence 2 boundary, which are also significantly denser than the Pleistocene sediments. Magnetic susceptibility was uniformly very low, and the data were shown to be nonreproducible at the core-section scale.
Hole conditions were excellent for logging, with most caliper readings within 10 cm of bit size. Hole 1127B was logged with three tool strings: the triple combo and FMS/Sonic to the mudline, and the GHMT to 59 mbsf. The triple combo was run without the nuclear source in the hostile environment lithodensity sonde (HLDS) because of safety concerns, and consequently density logs are not available for this site. During and after logging, close examination showed that there was no obvious damage to either the logging cable or tools from the limited H2S exposure. Cycles and surfaces seen in the high-quality FMS data correlate well with conventional logs (gamma ray [GR], sonic, and resistivity) and indicate the presence of high-frequency cycles (10–20 m thick) in the lower part of the Pleistocene succession. Simultaneous peaks on GR, resistivity, sonic, and susceptibility logs may reflect changes in sediment lithification throughout the logged section. Because of the high quality and uniform response of the sonic log, the WST was not deployed at this site. We anticipate that a checkshot survey at one of the other sites in this transect will provide appropriate drift control on the integrated sonic log.