PRINCIPAL RESULTS (continued)Site 1167: Continental Slope (Trough Mouth Fan)
Hole 1167A was cored with the advanced hydraulic piston corer (APC) system to refusal at 39.7 mbsf. Coring then proceeded with the extended core barrel (XCB) system to a total depth of 447.5 mbsf. Planned drilling time at the site was shortened by 42 hr because of icebergs and a ship schedule change, and the target depth of 620 mbsf (base of the Prydz Trough Mouth Fan) was not achieved. Four icebergs approached to within 0.1 nmi of the drill site, causing a total of 27 hr delay.
The sedimentary section at Site 1167 comprises a 447.5-m-thick sequence of clayey silty sands with dispersed rock clasts with minor beds of coarse sands, clays, and sandy clays. Two lithostratigraphic units are identified (Fig. 18).
Unit I (0 to 5.17 mbsf) is composed of olive and reddish brown clay and sandy clay with minor admixtures of biogenic components (e.g., as much as 2% diatoms and 1% sponge spicules). There are isolated beds of fine sand and rare lonestones. Diffuse reddish brown color bands are present in several thin intervals. The transition to Unit II is gradational. Unit I records a period of hemipelagic deposition during the last interglacial, when fine particles, biogenic material, and IRD settled out of the water column.
Unit II (5.17-447.5 mbsf) makes up the majority of the section at Site 1167 and is composed of one major facies (II-1) and three minor facies. Facies II-1 is composed of interbedded, poorly sorted dark gray sandy silt, silty sand, clayey sand, and clast-poor diamicton. Numerous color alternations of dark gray and dark reddish gray with sharp contacts occur between 64 and 98 mbsf. Some decimeter- to meter-scale successions of clast-poor diamicton and gravel beds are noted. Lonestones are common, with variable lithologies including granite, granite gneiss, garnet-bearing gneiss, metaquartzite, and sandstone. Dolerite, schist, conglomerate, and rare carbonized wood are also present. Sandstone and granite components vary systematically in the hole, with sandstone lonestones common below 200 mbsf and granite lonestones common above 200 mbsf. Facies II-2 is composed of gray, moderately sorted coarse sand. Grains are subrounded and predominately quartz, K-feldspar, and mafic minerals. The first occurrence of Facies II-2 downcore is at 179 mbsf. Facies II-3 is composed of dark gray clay with silt laminations, rare sand grains, and no lonestones. Sharp contacts mark the top and base of this facies. Some silt laminae converge and indicate cross-bedding. Facies II-4 is composed of green gray clay with dispersed clasts, abundant foraminifers, and few nannofossils. The upper contact is sharp, and the lower contact is gradational to sharp.
Unit II (Facies II-1 and II-2) records deposition by mass transport,
probably massive debris flows as evidenced by poor sorting, abundant
floating clasts, little visible grading, and a lack of biogenic components.
The debris flows most likely represent deposition during glacial periods
when ice extended to the shelf break and could deliver large volumes of
sediment to the upper continental slope. Individual flows cannot be
identified visually. The thin intervals of fine-grained sediment (Facies
II-3 and II-4) are similar in appearance and composition to muddy
contourites observed at Site 1165 and, hence, may denote times when
contour currents were active on the fan. The silt laminae and bioturbation
in Facies II-3 are not consistent with turbidite deposition. Facies II-4
may record short intervals, possibly interglacials, when pelagic
Sixteen lithologic varieties of lonestones were cataloged, and they generally vary randomly in size, with only a small size increase downhole to 200 mbsf. Below ~160 mbsf, the number of lonestones per meter remains fairly consistent, except for three intervals (160 to 210, 300 to 320, and 410 to 420 mbsf) where there are downward increases. Systematic variations in concentration of sandstone and granite clasts (noted above) suggest that possibly two different source areas delivered material to Site 1167 at different times (Fig. 19).
X-ray diffraction analyses show that the total clay mineral content is relatively constant throughout the hole. XRD analyses of clay types give mixed results for Units I and II, with smectite more common in Unit I and at depths below 382 mbsf than elsewhere and illite found in all samples. Further detailed analyses are likely to clarify whether changes in illite-smectite ratios indeed relate to times of glacial advances.
Chronostratigraphy at Site 1167 is poorly controlled because of the unexpected paucity of siliceous microfossils; however, dates in Unit I are younger than 0.66 Ma, and a sample from ~215 mbsf seems to be of early or middle Pleistocene age. Foraminifers are present consistently throughout the section and include pelagic foraminifer shelf faunas in diamictons and in situ midbathyal faunas in a few samples. Changes in foraminifer faunas match closely changes detected in various lithological parameters. Age control at this time is not adequate to determine average sedimentation rates.
Magnetostratigraphic analyses identified the Matuyama/Brunhes boundary between 30 and 34 mbsf (Fig. 20). The magnetic polarity below 34 mbsf remains mainly reversed, possibly including the Jaramillo and Olduvai Subchrons. The concentration-dependent magnetic parameters (susceptibility and anhysteretic and isothermal remanent magnetization) indicate that magnetite concentrations have large-scale cyclic (tens to hundreds of meters) variations, not commonly seen (Fig. 20). The values increase abruptly uphole at ~208 mbsf, between 113 and 151.2 mbsf, and between 55 and 78.5 mbsf, followed by a nearly linear uphole decrease. Superimposed on the large-scale cycles are small-scale variations. The anhysteretic over isothermal remanent magnetization ratio indicates that the magnetic grain size changes uphole from finer to coarser at 217 mbsf. The origin of the large-scale cycles is not yet understood but is likely related to systematic changes in sediment provenance caused by changes in the volume of ice from different sources and the location of areas of maximum erosion during glacial periods.
Interstitial water profiles document downhole sediment diagenesis, mixing of distinct subsurface interstitial water intervals, and diffusional exchange with modern bottom seawater. From 0 to 20 mbsf, chlorinity and sulfate increase by ~3% over seafloor values, suggesting that high-salinity, last-glacial-maximum seawater is preserved (Fig. 21). Sulfate decreases downhole from the seafloor (30 mM) to 433 mbsf (24 mM) in a stepped profile, raising the possibility that a number of "fossil" sulfate reduction zones may also be preserved (Fig. 21). Dissolved manganese increases downhole from 15 to 20 mM between the seafloor and ~25 mbsf. Alkalinity decreases downhole from 3 to 1.3 mM between the seafloor and 40 mbsf before steadily increasing to 2 mM at 433 mbsf. From 0 to ~60 mbsf, dissolved downhole profiles of calcium (10 to 25 mM), magnesium (56 to 42 mM), potassium (12 to 2 mM) and lithium (30 to 5 mM), all suggest diagenetic silicate-clay reactions are occurring. Below 5 mbsf, dissolved silica concentrations are enriched slightly over modern bottom waters (~300 vs. ~220 mM), reflecting the absence of biogenic opal within the sediments. Calcium carbonate is a minor component in the matrix sediments throughout the hole and is slightly higher in lithostratigraphic Unit II than in Unit I.
The concentration of hydrocarbon gases was at background levels (4-10 ppmv) for methane, and ethane was present at detection limits only in a few cores from deeper than 350 mbsf. The organic carbon content averages ~0.4 wt%, with no apparent trend with depth. Organic matter characterization by Rock-Eval pyrolysis indicates that all samples contain predominantly recycled and degraded thermally mature organic matter.
Sediment water content and void ratio decrease sharply with depth in lithostratigraphic Unit I, reflecting normal compaction. But, within Unit II, these properties were relatively uniform, except for a downhole decrease at 210 mbsf, where grain density and magnetic susceptibility values also decrease abruptly. P-wave velocities increase at this depth. Undrained shear strength values increase uniformly throughout the hole at a lower than typical rate, possibly because of the clay mineralogy combined with the high proportions of the silt and sand fraction of the sediment. There is no evidence of sediment overcompaction.
Wireline logging operations in Hole 1167A were attempted with the triple combo tool string. The tool string was lowered to 151 mbsf, where an obstruction halted the tool. A conglomerate interval was noted in the cores at this depth. Log data were collected from this depth to the base of pipe at 86.9 mbsf, covering an interval of 66 m. Time constraints, poor hole conditions, and problems encountered with the lockable flapper valve, resulted in a decision to switch to LWD in a new hole. Excellent spectral gamma ray and resistivity data were recorded to 261.8 mbsf before time ran out. Resistivity data show several clay and gravel-rich beds, with high gamma-ray values for a red bed interval at 60-90 mbsf, and low values between 90-120 mbsf and 215-255 mbsf. The change to low values may be due to a reduced concentration of granitic clasts or a change from a clay-rich to a sandier matrix.
Site 1167 is the first drill site to directly sample the sedimentary fans that are common on the upper continental slope around Antarctica, seaward of glacially carved sections of the continental shelf. The site reveals previously unknown large-scale (20 m to more than 200 m thick) cycles in magnetic susceptibility and other properties that are not yet fully explained but are likely due to systematic changes in the Lambert Glacier ice-drainage basin during Pleistocene and late Pliocene(?) time. Within the large cycles are likely many separate debris flows and interbedded hemipelagic muds that indicate times of individual advances and retreats of the ice front to, or near, the continental shelf edge. The debris flows are well represented in the cores, but the mud intervals are sparse and may either have not been recovered or have been removed by younger flows. Because of few age control points, it is not yet possible to determine sedimentation rates at Site 1167. If the rates are high, as we suspect from the limited available age dates, then almost all sediment during the latest Neogene glacial intervals sampled at Site 1167 were deposited as debris flows on the trough mouth fan and are not getting to the Wild Drift (Site 1165), where sediment rates are low. Alternatively, some of the fine component of the latest Neogene glacial sediment is being carried away by deep ocean currents.
Principal Results-Site 1165 | Table of Contents