We drilled Site 1173 in the trench outer margin (Fig. F9) in order to provide a reference for the predeformation status of geological and geochemical characteristics of the incoming sedimentary section. We recognized five lithostratigraphic units (Figs. F8, F12): Unit I (0 to 102 mbsf) is Quaternary in age and composed of sandy to muddy turbidites and hemipelagic mud of the outer Nankai trench-wedge facies. Unit II (102 to 344 mbsf) is Quaternary to Pliocene in age and made up of hemipelagic mud with abundant interbeds of volcanic ash that were probably derived from the Kyushu and/or Honshu volcanic arcs (upper Shikoku Basin facies; Fig. F13). Unit III (344 to 688 mbsf) consists of Pliocene to middle Miocene bioturbated silty claystone (lower Shikoku Basin facies). The boundary between Units II and III is controlled, in part, by diagenesis. Unequivocal ash beds are abruptly lost and are replaced downsection by siliceous claystones (Fig. F14). Unit IV (688 to 725 mbsf) is of probable middle Miocene age and is composed of variegated siliceous claystone and silty claystone (volcaniclastic facies); Unit V (725 mbsf) is middle Miocene basalt.
Biostratigraphic ages provided by calcareous nannofossils indicated a total of 25 biostratigraphic events. The continuous sedimentary section spans the time interval from the Pleistocene (Subzone NN21b) through the middle Miocene (Zone NN5). Magnetostratigraphy clearly identified the Bruhnes/Matuyama boundary (0.78 Ma), Matuyama/Gauss boundary (2.581 Ma), the Gauss/Gilbert boundary (3.58 Ma), and the termination of the Gilbert Chron (5.894 Ma). Paleomagnetic and bio-stratigraphic ages indicate high sedimentation rates (450-650 m/m.y.) for the turbidite deposits, decreasing rates for the upper Shikoku Basin section (72-77 m/m.y.), and lowest rates for the lower Shikoku Basin section (27-37 m/m.y.).
Deformation structures at Site 1173 are sparse, as expected from a reference site oceanward of the prism. The section above 375 mbsf is characterized by horizontal bedding with occasional steeper dips and microfaults between 250 and 275 mbsf. Bedding dips exceeding 30°, perhaps due to lateral extension associated with normal faulting, occur abruptly at 375 mbsf and continue down to 550 mbsf, below which they occur sporadically to the bottom of the hole. A 30-cm zone of foliated breccia indicates somewhat greater deformation around 440 mbsf. Deeper cores contain rare mineralized veins; a few possible dewatering structures such as thin, sediment-filled veins reflect early compaction processes.
Variations in physical properties correlate well with the lithostratigraphic units. High variability characterizes the turbidites of the outer Nankai Trench wedge, and porosities decrease with depth. Porosity increases at the boundary between the outer Nankai Trench wedge and the upper Shikoku Basin facies and continues to increase slightly with depth. These elevated porosities deviate from a typical compaction profile. An increase in P-wave velocities within this interval of increasing porosity suggests that there may be slight cementation. At the boundary between the upper and lower Shikoku Basin facies (~340 mbsf), grain densities increase slightly and porosities decrease sharply. This porosity decrease is accompanied by increasing thermal conductivity, P-wave velocity, and resistivity. A gas-probe permeameter showed that ash bands in the upper Shikoku Basin sediments are several orders of magnitude more permeable than the hemipelagites, although the contrast disappears in the lower Shikoku Basin section.
Seven reliable determinations of downhole temperatures were made at depths of 35 to 284 mbsf in Hole 1173A, using the advanced hydraulic piston corer (APC) temperature tool, water-sampling temperature probe, and Davis-Villinger temperature probe. The measured temperatures closely define a linear gradient of 0.183°C/m in the upper 300 m, where the average measured thermal conductivity is ~1.0 W/(m·°C); this yields a conductive heat flow of ~180 mW/m2 at Site 1173. Deeper than 300 m, thermal conductivities increase by 30%-50%, so the gradient should decrease proportionally, and in situ temperatures of ~110°C are estimated for the bottom of the hole—similar to the basement temperature estimated for Site 808. The heat flow value is somewhat higher than prior determinations of high heat flow near the site and greater than the predicted heat transfer for the 15-m.y. crustal age.
A high-resolution pore fluid concentration-depth profile shows that the pore fluid chemistry has been extensively modified from seawater by both microbially mediated reactions and by abiological, inorganic fluid-rock reactions. The chemical modifications from the microbially mediated reactions provide crucial independent information on the depth range, intensity, and nature of microbial activity in the deep subsurface. Each inorganically controlled dissolved species analyzed (i.e., Cl, Ca, Mg, SiO2, K, and Na) shows a distinct to sharp discontinuity at 340 mbsf, which corresponds to the lithologic boundary between Units II and III. Furthermore, minima in Cl and Na concentrations and significant inflections in the Mg and Ca profiles occur at ~380-390 mbsf. These features suggest that this horizon may be hydrologically active. A broad ~350-m-thick low-Cl zone within Unit III that has ~9% dilution relative to seawater requires a source of low-Cl fluid. The geometric similarities between this low-Cl zone and that at ODP Site 808 are striking, except that at Site 808 the dilution relative to seawater is more than twice that observed at this site.
Total organic carbon (TOC) values are low (average = 0.35 wt%) and decrease with depth (0.85 to 0.20 wt%). The C/N ratios indicate the presence of marine organic matter throughout the hole and show a slight increase in the lower ~200 m. The low sulfate and high methane concentrations in the upper section below the sulfate reduction zone are consistent with a bacterial origin. The increase in sulfate concentrations from ~400 to 700 mbsf coupled with the low concentrations of methane may indicate that sulfate is inhibiting production of hydrocarbons that were more abundant at Site 808. The presence of low concentrations of light hydrocarbons (ethane and propane) below 300 m to total depth may be due to some in situ thermal maturation of kerogen in the sediments. The low concentrations of methane at depth and the lack of evidence for any migration of hydrocarbons from above the facies transition (347.3 mbsf) support these conclusions. The microbes observed (~480 m) at temperatures above 90°C in the presence of elevated sulfate concentrations suggest that methanogenesis due to microbial activity is not completely inhibited, although at these temperatures, thermogenic hydrocarbons are likely being produced.
Samples taken for bacterial enumeration show that bacteria are present in all samples to 500 mbsf and thereafter are absent (the detection limit is 4.75 × 105). The population profile generally follows the average line obtained from other ODP sites for the upper 250 m of the hole with a rapid decrease in population size in the upper few meters as the sulfate was depleted. Between 43 and 80 mbsf, there is a significant (sevenfold) increase in bacterial numbers coincident with elevated methane concentrations. At 250 mbsf there is both a temperature boundary for bacteria (45°-50°C, the change from mesophilic to thermophilic populations) and significant differences in interstitial water (IW) chemistry, which complicates interpretation. Further changes occur in IW chemistry at a lithologic boundary at 343 mbsf. Between 250 and 460 mbsf, bacterial numbers are lower than average. Another microbiological temperature boundary (thermophilic to hyperthermophilic populations) occurs at 460 mbsf as temperatures exceed 80°C. One positive enumeration was made in this zone at 500 mbsf and ~85°C, where populations increase by a factor of 13. At this depth there were relatively high concentrations of organic carbon plus increasing concentrations of sulfate, methane, and hydrogen that could support a deep hyperthermophilic population of sulfate-reducing bacteria.
Tracer tests were successfully carried out on two APC cores and two extended core barrel (XCB) cores with both perfluorocarbon tracer (PFT) and fluorescent microspheres. PFT was detected in the center and midway between the center and the outside in some APC core sections; however, PFT was absent from the center of the XCB core sections. In contrast, microspheres were generally absent in samples taken midway between the center and the outside of the core in both APC cores and one of the XCB cores and were only present in the centers of some of the XCB core sections. These results suggest that intrusion of microspheres into the center of the cores was a result of postrecovery handling and not diffusion of drilling fluid during coring. This is the first time fluorescent microsphere tracers have been used during the collection of cores with the XCB.
Hole 1173A was logged with both the triple combination logging string (spectral gamma ray, dual-induction resistivity, lithodensity, and neutron porosity tools) and the Formation MicroScanner (FMS) and dipole sonic imager (DSI) tools. The interval from 65 to 373 mbsf was logged in two passes, and high-quality compressional and shear travel time data and FMS images were acquired. During the second pass, a new low-frequency (<1 kHz) dipole source was used on the DSI and produced excellent shear waveforms despite the very low formation velocity. Logging results are generally consistent with the homogeneous hemipelagic core lithology, with few identifiable lithologic boundaries in the logged interval. Density is low from 97 to 336 mbsf, with a slight gradual decrease with depth, then sharply higher in the 358-440 mbsf interval. Compressional and shear wave velocities are nearly constant with depth to 225 mbsf, then increase with depth. Velocities decrease sharply in the short logged interval below 348 mbsf, corresponding to the Unit II/III boundary. Numerous ash layers and other sedimentologic and diagenetic features observed in the cores were well imaged by both FMS passes, which should permit high-resolution core-log integration.
Site 1174 (ENT-03A) is located in the protothrust zone of the Nankai accretionary prism (Fig. F9) and was designed to sample a zone of incipient prism deformation. When combined with our reference Site 1173 (~11 km seaward) and Site 808 (~2 km landward at the frontal thrust), Site 1174 provides a transect of stratigraphy, structural data, physical properties, and geochemical gradients across the deformation front of the accretionary prism.
We recognized five lithostratigraphic units and three subunits at Site 1174 (Figs. F8, F15). Unit I (slope-apron facies) is Quaternary in age and extends from the seafloor to a sub-bottom depth of 4.00 mbsf. This facies is composed mostly of mud that was deposited on the lowermost trench slope by hemipelagic settling. Unit II (trench-wedge facies) is Quaternary in age and includes three subunits. Subunit IIA (axial trench-wedge facies) extends from 4.00 to 314.55 mbsf and is characterized by thick sand turbidites, silt turbidites, and hemipelagic mud (Fig. F16). The lithologies of Subunit IIB (314.55-431.55 mbsf) are limited to silt turbidites and hemipelagic mud, whereas Subunit IIC (431.55-483.23 mbsf) is composed of hemipelagic mud, volcanic ash, and silt turbidites. The gradual transformation in facies character downsection is consistent with a change in depositional environment from the outer trench wedge to abyssal floor. Unit III (upper Shikoku Basin facies) is Quaternary to Pliocene in age and extends from 483.23 to 660.99 mbsf. Lithologies within this unit include hemipelagic mudstone and volcanic ash; the lower unit boundary coincides with the deepest identifiable bed of vitric tuff. In contrast, Unit IV (lower Shikoku Basin facies) contains mostly bioturbated mudstone with sporadic interbeds and nodules of carbonate-cemented claystone and siliceous claystone. Replacement of glass shards by smectite and zeolites (clinoptilolite or heulandite) increases gradually with depth and is more extreme in finer-grained deposits. As a consequence, both ash to bentonite diagenesis and temporal changes in pyroclastic influx govern the lithologic distinction between the Upper and lower Shikoku Basin facies. The unit boundary shifts upsection as Shikoku Basin deposits migrate toward the Nankai deformation front and become increasingly affected by rapid burial and heating beneath the trench wedge. The lowermost stratigraphic unit at Site 1174, Unit V, begins at a depth of 1102.45 mbsf. We drilled only 8.86 m of variegated claystone in this middle Miocene volcaniclastic facies.
Deformation bands are well developed between 218 and 306 mbsf (Fig. F17) and are concentrated in two oppositely inclined sets striking at 033° with the acute bisectrix inclined 10°NW from vertical (Fig. F18). They occur immediately above a narrow but abruptly sheared interval that, with indications of reverse movement and a paleomagnetically restored southeast dip, may be a backthrust. Between 470 and 506 mbsf, fractured and markedly steepened bedding may represent a thrust; no significant deformation was seen in the cores equivalent to the thrust apparent on the seismic profile at 550 mbsf. Narrow, widely spaced zones of fractures and brecciation characterize the interval between 688 and 807 mbsf. Between 807.6 and 840.20 mbsf an irregular downward increase in intensity of inclined fractures and fineness of brecciation defines the décollement, which is thicker and more heterogeneous than at Site 808 and more thoroughly comminuted in its lower part (Figs. F19, F20). The underthrust sediments show little tectonic deformation; however, zones of significant bed steepening were noted between 950 and 1000 mbsf and around 1020 mbsf, the latter accompanied by evidence for shear deformation.
Nannofossil assemblages are indicative of the Pleistocene (Subzone NN21b) to middle Miocene (Zone NN6) ages. Twenty-three biostratigraphic events are recognized. Nannofossils are common and generally moderately preserved in the Pleistocene, whereas Pliocene and Miocene nannofossils are rare and mostly poorly preserved. Sedimentation rates based on biostratigraphy are 630-770 m/m.y. for the late Quaternary and are significantly lower (11-125 m/m.y.) for deposits older than 0.8 Ma.
Paleomagnetic results indicate that the Brunhes Chron (0-0.78 Ma) ranges from 0 to 544.70 mbsf and extends through the trench-wedge turbidites. The Matuyama Chron occurs from 544.70 to 685.95 mbsf, the Gauss Chron, from 685.95 to 727.85 mbsf, and the Gilbert Chron, from 727.85 to 802.07 mbsf. High magnetic intensities occur from 0 to ~550 mbsf, below which they drop to low values to the bottom of the hole.
The main characteristics of the interstitial water concentration-depth profiles at Site 1174 are similar to those at Site 808. There is an intense, very shallow, sulfate reduction zone, and alkalinity and ammonium concentrations peak in the uppermost 200 m of the section. The solutes that are controlled by fluid-rock reactions, such as Cl, Na, and Si, have sharp changes in their gradients at a depth that corresponds to the boundary between the trench wedge and Shikoku Basin facies (lithostratigraphic Units II/III boundary). At the depth that corresponds to the thrust intersection (~470 ± 5 mbsf), there are also significant excursions, most distinctly exhibited in the Cl and Si concentrations, that may indicate hydrologic activity. The chemical changes across the major tectonic feature, the décollement, are subtler, but a high-resolution record of pore fluid chemistry was recovered across and within the Nankai Trough décollement for the first time. A local Cl maximum of 496 mM within the décollement decreases smoothly to ~485 mM ~50 m above the décollement zone, whereas it decreases very sharply (~10 mM) in the 10 m below the structure. The cause of the Cl maximum in the décollement is as yet unclear. A low-Cl zone in the 200-m interval below the décollement, with minimum concentrations that are ~17% diluted relative to seawater, occurs at an almost identical distance below the décollement at Site 808. The dilution, however, is ~21% at Site 808, ~17% at Site 1174, and considerably less (~9%) at reference Site 1173. In the lowermost ~100 m of the underthrust section, Cl concentrations increase, approaching seawater concentration at 1110 mbsf. Hydration reactions in the lower volcaniclastic or an underlying upper basement fluid flow system may be responsible for the increase in the Cl concentrations.
Dissolved silica concentrations appear to be controlled by biogenic silica dissolution in the trench-wedge sediments, by volcanic ash diagenesis in the upper Shikoku Basin facies, and by the low-Cl source plus in situ silicate reactions at >70° to ~130° in the lower Shikoku Basin facies. Dissolved sulfate increases below the sulfate reduction zone, 1-2 mM below the upper and lower Shikoku Basin facies boundary sediments, at ~660 mbsf, reaching 8-10 mM below the depth interval of the Cl minimum and remaining constant to the bottom of the section. At Site 1173 the first sulfate increase below the sulfate reduction zone is observed at a much shallower burial depth, ~400 m shallower than at Site 1174. The sulfate distributions at these sites may reflect a dynamic relationship among sedimentation rates, temperature, and microbial sulfate reduction rates.
Organic matter decreases with depth and low TOC values are low (0.90 to 0.11 wt%; average = ~0.38 wt%) in the core. The C/N ratios indicate the presence of marine organic matter with only a slight increase in the upper trench-wedge facies (~200 mbsf) and in the lower Shikoku Basin facies below the décollement (~1000 mbsf). Discrete intervals of elevated methane concentrations are present between 225 and 700 mbsf. Minor amounts of ethane (200-800 mbsf) and propane (400-650 and 950-1110 mbsf) are probably attributable to some in situ thermal maturation of organic matter.
Microorganisms were enumerated in 40 samples collected from the surface to 1100 mbsf at Site 1174. With the exception of two samples with low abundances (~1.8 × 106 cells/cm3) in the sandy layers at 26 and 66 mbsf, abundances from the surface to 400 mbsf were close to values predicted based on data from previous ODP sites. Abundances were lower than predicted below 400 mbsf. The decrease may relate to the relatively high temperature gradient at Site 1174. Cell counts dropped below the detection limit at 528 mbsf and remained so until just above the décollement. Abundances at 778 and 789 mbsf were 4.8 and 4.2 × 106 cells/cm3, respectively; no cells were detected below these depths. Nineteen whole-round samples were used to inoculate anaerobic growth media and were maintained at the estimated in situ temperature. Samples were chosen from the surface through the known hypothermophilic region (113°C) (Blöchl et al., 1997), and subsamples at five depths were targeted for incubation at in situ pressure and temperature.
Porosities within the axial and outer trench-wedge facies (Subunits IIA and IIB) are characterized by high variability and generally decrease with depth. Porosity decreases across the boundary between the outer trench wedge and trench to basin transition facies (Subunit IIB/IIC boundary). Within the transitional facies, porosities are less scattered and decrease slightly with depth. The upper Shikoku Basin facies (Unit III) is characterized by nearly constant porosities, which is a deviation from normal compaction trends. Surprisingly, a high velocity interval between 510 and 520 mbsf is associated with an interval of elevated porosity. At the top of the lower Shikoku basin facies (Unit IV; ~660 mbsf), another high-velocity interval occurs. Porosities within the lower Shikoku Basin facies resume a compaction trend of decreasing porosity with depth. Porosities increase sharply by 2%-4% at the top of the underthrust sequence. This porosity increase is accompanied by a decrease in velocity and increase of electrical conductivity. However, the anisotropy of electrical conductivity is higher in the underthrust sediments than above the décollement zone. Porosities and velocities increase with depth within the underthrust sediments, whereas electrical conductivities decrease. In contrast to Site 808, porosities within the décollement are not significantly lower than above and below it, although values are somewhat scattered.
Uncalibrated gas permeameter measurements were made throughout the section. Shallower than 600 mbsf, silt-rich and ash horizons showed higher values than the silty clays. The axial trench-wedge sands gave the highest values, and the lowermost silty clays recovered gave the lowest.
In situ temperature measurements to a depth of 65.5 mbsf and laboratory thermal conductivity measurements indicate a near-surface heat flow of 180 mW/m2. If heat flow is purely conductive and steady state, a temperature of ~140°C is projected for the bottom of the hole.
Site 1175 (ENT-07A) was designed to penetrate the slope sediments that cover the large thrust-slice zone just landward of a major OOST (Fig. F11). Investigation of the age and lithologic characteristics would provide information on (1) the history of accretion, uplift, and deformation of the prism and (2) sedimentation within a trench-slope basin.
We cored three lithostratigraphic units at Site 1175 (Figs. F8B, F21). Unit I (upper slope-basin facies) begins at the seafloor and ends at a sub-bottom depth of 224.75 mbsf. Lithologies include nannofossil-rich hemipelagic mud, volcanic ash, and thin turbidites that range in texture from sand to silty sand, clayey sand, and silt. The most characteristic feature of Unit I is the common occurrence of contorted stratification (Fig. F22). There are eight discrete zones of soft-sediment deformation. Typical manifestations include variably inclined bedding, small-scale folding, and, in extreme cases, stratal fragmentation. The disruption was probably caused by submarine slumps and debris flows. Unit II (middle slope-basin facies) extends from 224.75 to 301.64 mbsf. Lithologies include hemipelagic mud, poorly sorted muddy sand to sandy mud, sporadic interbeds of volcanic ash, and rare occurrences of thin sand or silt turbidites. The unusual lithology of muddy sand is diagnostic of Unit II and probably was transported downslope by sandy debris flows or mudflows. Unit III (slope to prism transition) begins at 301.64 mbsf and ends at 435.40 mbsf. This unit is typified by carbonate-poor hemipelagic mud with numerous interbeds of silt and silty sand turbidites. The most striking lithology, however, is gravel to pebbly mudstone (Fig. F23). Its characteristics include disorganized and poorly sorted clast fabric, lack of internal stratification, partial to complete support of clasts by a matrix of clayey silt, and subrounded to rounded clasts up to 5.5 cm in size. A polymictic clast population was transported downslope by debris flows. The boundary between the lowermost slope sediment and the top of the accretionary prism cannot be defined with certainty using lithologic criteria, but it probably occurs within the upper 25-30 m of Unit III.
Site 1175 exhibits little evidence for tectonic deformation. However, the upper 205 m shows intervals of recumbent, isoclinal slump folding and disaggregated sediment interlayered with subhorizontal intact bedding. Fold orientations suggest the slumping was northward directed. Below 220 mbsf, bedding is subhorizontal, except for localized chaotic zones between 350 and 388 mbsf and dips up to 21° at 400 mbsf. Core-scale faults, probably compaction related, occur from 298 to 302 mbsf and sporadically from 340 to 435 mbsf. Possible web structure occurs in sands at 406.9 and 425.8 mbsf; near the bottom of the hole, an indurated sand contains several low-angle small faults.
Biostratigraphic age control was provided by calcareous nannofossils that are well preserved and abundant throughout the section. A total of ten biostratigraphic events were identified within the nannofossil assemblages. The continuous sedimentary record spans the time interval from the Pliocene (Zone NN18) through the Pleistocene (Subzone NN21b). Based on the biostratigraphic ages, sedimentation rates for the upper sedimentary units show high sedimentation rates (0.52 m/k.y.) for the upper to middle slope-basin deposits, with decreasing rates for the slope to prism transition (0.13 m/k.y.).
Hole 1175A inclination data after alternating-field (AF) demagnetization at 30 mT allowed interpretation of geomagnetic polarity changes from late Pliocene to Pleistocene. The 0.78 Ma Brunhes/Matuyama boundary is interpreted to occur at 298.80 mbsf (interval 32X-5, 80 cm). Seven short reversal events were observed in the Brunhes Chron and may represent geomagnetic excursions.
In Hole 1175A, pore fluids are less intensively modified from seawater than the pore fluids in Holes 1173A and 1174A. The main characteristics of the pore fluid concentration-depth profiles indicate that the intense microbially mediated reactions occur in the top <200 mbsf of the section. Microbial sulfate reduction is complete at ~15 mbsf. The alkalinity maximum also occurs at this depth. Only relatively small changes in the chemical gradients occur throughout the section and across the major lithologic boundaries in the abiogenic components. Volcanic ash alteration is insignificant because of the rather low geothermal gradient of 54°C/km. Instead of ash alteration, as indicated by the Ca, Mg, and alkalinity concentration-depth profiles, carbonate, particularly dolomite, diagenesis is the dominant diagenetic reaction. Dolomite forms both by direct precipitation of authigenic dolomite and by replacement of precursor biogenic calcite, which is abundant in this section. Carbonate diagenesis should influence some of the index physical properties such as porosity and density. An unidentified silicate reaction occurring below the drilled section controls the concentration profiles of K, Na, Si, and alkalinity below ~300 mbsf, corresponding to lithostratigraphic Unit III. The inferred diffusion of lower chlorinity interglacial water into the pore fluids at Sites 1173 and 1174 is absent at this site. One possible explanation is that the signal has been erased by widespread slumping in Unit I.
The sediments at Site 1175 contain low inorganic carbon (~0.11-4.59 wt%), and carbonate contents range up to 40 wt%, resulting in very immature organic matter and low hydrocarbon abundances. The low sulfate and high methane concentrations in sediments below the sulfate reduction zone and throughout Hole 1175A are consistent with a bacterial origin.
Bacterial abundance was enumerated in 18 samples obtained at Site 1175. The abundance near the surface is 6.97 × 107 cells/cm3 and declines rapidly, which is consistent with the decrease in sulfate concentrations. Abundances increase below 14.6 mbsf and are consistent with increases in methane concentrations. The sample at 50.8 mbsf is notable in that it contains 7.28 × 107 cells/cm3 (i.e., slightly more bacteria than the near-surface sample). This is followed immediately with almost the lowest population enumerated of 3.71 × 105 cells/cm3 at 59 mbsf. The deepest sample is 400 mbsf with 3.59 × 105 cells/cm3, equivalent to 0.5% of the near-surface population. Estimates of drilling fluid intrusion into the interior of the cores examined at this site range from below detection to 0.02 µL/g. In addition to the onboard assays, 17 whole-round cores were taken for shipboard enrichment cultures, cell viability, and shore-based microbiological analysis to measure potential bacterial activities, culture microorganisms, characterize nucleic acids, and investigate fatty acid biomarkers.
Porosities within the upper slope-basin facies (Unit I) are characterized by high variability and decrease slightly with depth from values of 62%-70% at the mudline to 61%-68% at ~100 mbsf. Porosities decrease abruptly at ~100 mbsf to values of 57%-61% and then decrease gradually to the transition between the upper and middle slope-basin facies (220 mbsf). Below 220 mbsf (within the middle and lower slope-basin facies), porosity decreases more rapidly with depth than in the upper slope-basin facies, reaching values of 38%-47% at 400 mbsf. The rapid decrease in porosity below 220 mbsf coincides with increasing P-wave velocity. There is no clear change in porosity, bulk density, or grain density at the depth of the middle slope-basin/lower slope-basin facies boundary (301 mbsf; Units II and III). A spike of high velocity and impedance 20 m above this transition may correspond to a seismic reflector. The depth of this spike coincides with the depth of the upper unconformity on the depth converted seismic profile. Four successful in situ temperature measurements at Site 1175 indicated a thermal gradient of 0.054°C/m.
Gas-probe permeameter measurements illustrate the huge influence of lithology. Uniformly low values are given by the hemipelagic clays that dominate the section, whereas a coarse, friable black ash at 23 mbsf gave a measurement six orders of magnitude higher. Turbiditic sands between 60 and 90 mbsf also yielded exceptionally high values. Thin bands of white-gray ashes also give relatively high values, in agreement with shallow, unaltered ashes at the other sites.
This site revealed that the age of accretion of the large thrust-slice zone is very young (<2 Ma). The young age of the accretion indicates rapid growth of the frontal part of the Nankai accretionary prism, ~40 km oceanward growth in 2 m.y. This rate of growth provides a significant revision to first order constraints for kinematic, structural, and hydrogeologic modeling of the prism.
The objective of Site 1176 was to determine the nature of accreted sediments of the large thrust-slice zone as well as to understand deformation and potential fluid flow related to a major out-of-sequence thrust (Fig. F11). The OOST itself, however, was not penetrated.
We recognized three lithostratigraphic units at Site 1176 (Figs. F8B, F24). Unit I (upper slope-basin facies) extends from the seafloor to 195.79 mbsf. Its lithologies include nannofossil-rich mud, volcanic ash, and sand to silt turbidites. The principal processes of sedimentation for Unit I were hemipelagic settling and turbidity currents, with occasional volcanic ash falls and remobilization by slumping. Unit II (middle slope-basin facies) extends from 195.97 to 223.54 mbsf. In addition to typical hemipelagic mud, Unit II contains sandy mudstone and rare beds of volcanic ash. Deposition of this facies occurred from muddy debris flows, routine settling of suspended sediment, and occasional ash falls. Unit III was cored to a depth of 440.36 mbsf and contains abundant interbeds of sand to silt turbidites, carbonate-poor hemipelagic mudstone, pebbly mudstone, gravel (Fig. F25), and rare volcanic ash. The primary depositional environment for this unit was probably a trench-fan system fed by a transverse submarine canyon, and depletion of carbonate supports the idea of deposition below the carbonate compensation depth (CCD). The petrographic compositions of sands and gravels, rich in sedimentary lithic fragments and quartz, show that their provenance is southwest Japan as typically represented by the Shimanto Belt. The junction between the lowermost slope sediment and the top of the accretionary prism probably coincides with the boundary between Units II and III.
Site 1176 can be divided into two structural domains: slope basin and accretionary prism. Deformation of the slope-basin sediments (0-224 mbsf) is characterized by inclined bedding intervals in which slump folds together with contorted and chaotically mixed bedding are locally developed. These features are interpreted to record the effects of active tilting and uplifting of the slope basin. Small faults are thought to result from extensional response to this tilting and uplift of the basin and/or burial compactional strains. In contrast, deformation structures are almost absent in the accreted sediments (below 224 mbsf), although core recovery was very poor. However, the apparently consistent near-horizontal bedding may reflect the flat part of a hanging-wall anticline formed in association with an underlying thrust.
Biostratigraphic age control was provided by calcareous nannofossils. Nannofossil assemblages are of Pliocene (Zone NN16) to Pleistocene age (Subzone NN21b) according to nine recognized biostratigraphic events. Although nannofossils are common and generally moderately preserved in the upper Pleistocene, nannofossils from sediments older than 1 Ma are rare and poorly preserved. Age models based on biostratigraphy indicate sedimentation rates of ~0.07-0.26 m/k.y.
Inclination data of Hole 1176A after AF demagnetization at 30 mT provided useful information for interpretation of geomagnetic polarity changes from the late Pliocene to the Pleistocene. The Brunhes/Matuyama boundary (0.78 Ma) is interpreted to occur at 199.55 mbsf. Seven short reversal events were observed in the Brunhes Chron and may represent geomagnetic excursions in this chron.
The most intense microbially mediated reactions occur in the top <100 mbsf of the section. Microbial sulfate reduction is complete at ~20 mbsf. In the top half of this zone, the sulfate reduction rate decreases linearly with depth, whereas in the lower half, maximum sulfate reduction occurs at the base of the zone; the alkalinity produced is involved in carbonate reactions, and the ammonium produced is involved in clay ion exchange reactions.
The alkalinity maximum and Ca and Mg minima coincide with the depth of the base of the sulfate reduction zone; thus, this depth interval is also characterized by intense carbonate diagenesis. The Ca and Mg concentration profiles indicate that in the sulfate reduction zone both authigenic dolomite precipitation and replacement of a precursor biogenic calcite occur. Deeper, however, through the upper and middle slope-basin section, replacement of a precursor calcite is the only dolomitization reaction.
Volcanic ash or other silicate diagenetic reactions are minimal because of the low geothermal gradient of 56°C/km. Diatom dissolution may control the pore fluid silica concentration. At the base of the section, pore fluids have a composition close to seawater, as indicated by the return to seawater concentrations of all abiogenic components, except for K. The Cl concentration profile is consistent with diffusion between a low-Cl zone at greater depth and the seafloor. Because of poor recovery, the location of the low-Cl zone is poorly defined but is constrained to be between 240 and 320 mbsf. The low-Cl fluid is enriched in Ca and depleted in Na, K, and Mg. A chemically similar fluid was identified at Site 1174.
Diffusion of low-chlorinity interglacial seawater into the sediment section is not observed at this site. As at Site 1175, this may be the result of repeated slope-failure events and sediment reworking. At greater depths, the residual signal from the glacial ocean seawater may have been overprinted by diffusion.
The total carbon content for the sediments examined between 200 and 401.6 mbsf at Site 1176 ranged from 0.05 to 2.25 wt%. The highest carbon value (2.25 wt% at 340 m) was dominated by a terrestrial component likely derived from fan debris flow to the trench sediments. The sulfur content showed a trend similar to TOC, with the highest values of sulfur (1.05 and 2.07 wt%) coincident with the highest TOC values (0.86 and 2.25 wt%). The inorganic carbon (~0.05-2.6 wt%) and high carbonate content (up to ~35 wt%) are similar to values observed at Site 1175. Methane concentrations in sediments below the sulfate reduction zone (~9.5 m) are consistent with a bacterial origin. Methane dominates the composition of the hydrocarbons measured throughout Hole 1176A.
Bacterial abundance was enumerated in 18 samples obtained at Site 1176. Abundance at the surface was 6.67 × 108 cells/cm3. The deepest sample is 363.49 mbsf with 1.71 × 106 cells/cm3, representing 0.25% of the surface population. The rapid decline of bacterial populations from the surface is consistent with the decrease in sulfate concentrations to near zero at 14.6 mbsf. The decline in bacterial abundance with depth follows the predicted depth/population size relationship very closely. In addition to the onboard assays, 11 whole-round cores were taken for shipboard enrichment cultures, cell viability, and shore-based microbiological analysis to measure potential bacterial activities, culture microorganisms, characterize nucleic acids, and investigate fatty acid biomarkers.
Porosities decrease gradually with depth in the upper slope-basin facies (Unit I), from values of ~65%-73% at the mudline to 55%-60% at 200 mbsf. Within the upper slope-basin facies, there is considerable scatter in porosity, with values ranging from 51% to 73%. This scatter may be related to the inferred deposition of this unit by slope failure processes. No clear changes in index properties occur at the boundary between the upper and middle slope-basin facies (Units I and II). Within the middle slope-basin facies (Unit II), porosity continues to decrease gradually with depth, following the same trend as observed for the upper slope-basin facies. Changes in index properties correlate with the boundary between the middle slope-basin and accretionary prism facies (Units II and III) at 225 mbsf. Porosity decreases from 53%-57% to 48%-54% across this boundary. Velocity and formation factor also increase at the top of Unit III. Within Unit III, porosities decrease with depth, reaching ~40%-47% by ~310 mbsf. From this depth to 405 mbsf, porosities remain constant, with values ranging from ~40%-47%.
Five successful in situ temperature measurements indicate a thermal gradient of 0.056°C/m.
Results from the gas permeameter at Site 1176 are slightly different from those from other sites in that the range is even greater and there is no general decrease with depth. Throughout the hole the hemipelagic clays show low values with greater scatter than elsewhere, and in the upper half of the section (above 200 mbsf), sands, gravels, and especially ashes gave much higher values. In the lower half of the hole, sands and gravels give high measurements, even at the bottom of the hole.
This site provided information regarding the nature of accreted sediments that compose the large thrust-slice zone including the slope-basin transition. Coarse clastic sediments of Outer Zone origin, perhaps transported through a transverse canyon, are the dominant lithology of the accreted sediments. The accreted section of this zone is, thus, very different from the axially transported, volcaniclastic-rich trench sediments at Sites 1173, 1174, and 808. The age of the prism is probably younger than 2 Ma, and slope-basin development initiated <1 Ma, suggesting extraordinarily rapid growth of the prism.
The science objective of Site 1177 was to study the stratigraphic, geochemical, and physical properties framework of a reference site along the Ashizuri Transect (Fig. F10). This transect includes DSDP Sites 297, 298, 582, and 583.
We recognized five lithostratigraphic units at Site 1177 (Figs. F8A, F26). Unit I (upper Shikoku Basin facies) is Pliocene in age (300.20 to 401.76 mbsf) and consists mainly of weakly indurated hemipelagic mud interbedded with fresh volcanic ash. Unit II (lower Shikoku hemipelagic facies) is late Miocene in age (401.76 to 449.30 mbsf) and is composed almost entirely of a more strongly indurated hemipelagic mudstone. Unit III (lower Shikoku turbidite facies) is early to late Miocene in age (449.30 to 748.35 mbsf) and consists of turbidite sand, silty sand, gravel, mudstone-clast conglomerate, and hemipelagic mudstone, plus a few thin layers of carbonate-cemented claystone and siliceous claystone. There are four sand-rich packets within this facies, and most of the siliciclastic sands contain abundant woody plant fragments (Fig. F27). Mudstones display vivid color contrasts due to variations in clay mineralogy. Sediment dispersal evidently occurred through a broad system of coalescing submarine fans. Unit IV (volcaniclastic-rich facies) is early Miocene in age (748.35 to 831.08 mbsf). This unit consists of variegated mudstone to claystone, volcanic ash (Fig. F28), and silt turbidites with both volcaniclastic and siliciclastic compositions. Many of the mudstone beds are enriched in expandable clay minerals. Unit V is basaltic basement (831.08 to 832.13 mbsf) and is probably early Miocene in age. The basalt contains one pillow structure, and an intrusive contact with overlying sediment is highly altered (Fig. F29).
Deformation structures at this site, oceanward of the prism, are very sparse, more so than at the reference site at the Muroto Transect. This near absence at Site 1177 of structures and bedding dips >10° may result from slower rates of sedimentation, slight differences in lithology, or differences in topography of the substrate. Early soft-sediment compaction-related structures are present between 748 and 831 mbsf; the main tectonic structure is a faulted and diagenetically altered interval between 579.45 and 581.10 mbsf. The basalt at the bottom of Hole 1177A, at 831 mbsf, exhibits glassy rinds at its contact with the overlying sediment and networks of veins bearing calcite and/or chlorite in complex interrelationships.
Biostratigraphic age control was provided by calcareous nannofossils, although their abundance and states of preservation were generally poor throughout the sequence; major intervals are barren of nannofossils. A total of 11 biostratigraphic events were identified. The continuous sedimentary section spans the time interval from the Pliocene (Zone NN18) through the early Miocene (Zones NN4-NN2). The biostratigraphic age estimates indicate an average sedimentation rate for the late Pliocene of 87 m/m.y. and a lower sedimentation rate of 28.7 m/m.y. for the lower Miocene to Pliocene sediments.
Magnetic inclination data of Hole 1177A after AF demagnetization at 30 mT were useful for interpretation of geomagnetic polarity changes from the early Miocene to Pliocene. The Brunhes/Matuyama boundary is expected to occur above the initial coring depth of 300 mbsf. The Reunion Event (2.14 Ma) during the Matuyama Chron is interpreted to occur at 301.85 mbsf. The Matuyama/Gauss (2.581 Ma) and Gauss/Gilbert (3.58 Ma) boundaries are interpreted to occur at 328.55 and 384.25 mbsf, respectively. The beginning of Chron C3A (5.894 Ma) is identified to occur at 427.45 mbsf.
Sharp chemical discontinuities between and within lithostratigraphic units, particularly intense in the Cl, Na, K, sulfate, and alkalinity concentrations, and a high-sulfate turbidite unit in the middle half of the section are outstanding characteristics of Site 1177 pore fluid concentration-depth profiles. Discontinuities within lithostratigraphic units are unique for this site and were not observed at Sites 1173, 1174, or 808. The chemical discontinuities correspond to discontinuities in physical properties, suggesting that solute and fluid transport out of specific sediment intervals may be retarded. Alternatively, these zones may reflect the recent onset of diagenetic reactions in compositionally distinct layers, such as ashes. The sharpest discontinuity occurs at ~410 mbsf, at the boundary between lithostratigraphic Units I and II. The concentration-depth profiles are therefore only continuous in Units I and II but show unusual variance in Units III and IV. The most conspicuous interval of this character was identified close to the bottom of the sediment section (775-805 mbsf).
Within Unit III there is a general decrease in Cl concentration; the minimum value is ~7% fresher than modern seawater concentration. This freshening is most plausibly produced by in situ smectite dehydration in combination with Cl uptake by an authigenic hydrous silicate. Na and K profiles show similar trends, but to a lesser extent. Sulfate reduction is complete in Units I and IV, driven by microbial activity. In Unit III, sulfate concentration is high, ~86% of the modern seawater value, indicating that since burial little microbial activity has occurred. This is probably the result of the very low content of nonwoody, labile organic matter available for microbial activity in the turbidites. Most of the labile organic matter was microbially oxidized when the turbidites were at or close to the seafloor and sulfate diffused into this section.
The important diagenetic reactions are ash alteration, particularly reflected in the Ca, Na, and K profiles, carbonate formation as reflected in the Ca and alkalinity profiles, and opal-A dissolution as reflected in the Si profile.
The TOC contents ranged from 0.03 to 1.62 wt%, with an average value of 0.45 wt%. The highest carbon values were measured in the Shikoku turbidite facies sediments (Unit III), which contained a terrestrial component characterized by plant detritus and pieces of wood. The sulfur content ranged from 0 to 0.81 wt%, with the highest concentrations occurring between 400-520 mbsf and 650-770 mbsf. The C/N ratios indicated that a mixture of both marine and terrigenous sources were contributing to the overall sediment composition. Unlike Sites 1175 and 1176, the inorganic carbon (~0.78 wt%) and carbonate contents (~2.7 wt%) were low with the exception of some thin-bedded carbonate-cemented layers (up to 65 wt%) in the Shikoku turbidite facies (Unit III).
Methane concentrations in sediments below the sulfate reduction zone (~4.5 m down to 734 mbsf) are consistent with a bacterial origin. The C1/(C1+C2) ratio for hydrocarbons in sediments below 750 mbsf plot within the mixing zone, suggesting that more than one source of hydrocarbons may be present.
Microorganisms were enumerated in 23 samples collected from 300 to 830 mbsf. Bacteria are present in all but two samples (687 and 830 mbsf) at abundances that are generally lower than expected based on results from previous ODP sites. A small, but statistically significant increase in bacterial populations occurs from 380 to ~740 mbsf that correlates with elevated sulfate concentrations in the interstitial water between these depths. The continued presence of sulfate, unexpected when bacteria are present at ~106 cells/cm3, may be related to very low organic carbon concentrations in the sediment preventing significant amounts of bacterial sulfate reduction. A total of 21 whole-round cores were taken for shipboard enrichment cultures, cell viability, and shore-based microbiological analysis to measure potential bacterial activities, culture microorganisms, characterize nucleic acids, and investigate fatty acid biomarkers.
Variations in physical properties at Site 1177 correlate well with the lithostratigraphic units. Units I and II are both characterized by low scatter in porosity. Unit I maintains a nearly constant porosity of 60%-65%. At the top of Unit II (402 mbsf), porosities begin to decrease rapidly with depth, decreasing to 45%-53% by 450 mbsf. Unit III is characterized by a gradual decrease in porosity with depth and by increased scatter that may be due to lithologic variations in this turbidite-rich sequence. Unit IV exhibits significant scatter and shows no clear trend with depth. An excursion to lower porosity (~40%) at 475-510 mbsf within Unit III occurs in a sandy section. Anomalously high porosity (~8%-15% higher than in surrounding sediments) within Unit IV occurs in a 30-m-thick zone between 765 and 795 mbsf. Low vertical P-wave velocities and formation factors also characterize this zone.
Most gas permeameter determinations at Site 1177 range around the values given by the background hemipelagites. Carbonate-cemented claystones at 540 and 591 mbsf give slightly higher values, as do the altered ashes of Unit IV, but the increase is small. The upper sands in Unit III account for all of the high measurements at the site. Numerous wood-bearing silty sands were measured in the lower part of Unit III, but most give identical results to the background hemipelagites, suggesting blockage of the pore connections, perhaps by smectite.
In addition to serving as the reference site for the Ashizuri Transect, Site 1177 provides a comparison to the Muroto reference site (Site 1173). Comparison of the two sites will aid our understanding of the evolution of the Nankai Trough accretionary prism in two different geologic settings characterized by differing angles of prism taper.
The science objective of Site 1178 included sampling of slope sediments and underlying LDRZ (Fig. F11) in order to clarify the structural evolution of the prism.
We recognized two fundamental lithostratigraphic units at Site 1178 (Figs. F8B, F30). Both are divided into three subunits. Interpretations of the lithostratigraphy are hampered by complexities in biostratigraphy and structural deformation. Subunit IA (upper slope-apron facies) is Quaternary to Pliocene in age and extends from the seafloor to a depth of 94.40 mbsf. Lithologies consist of hemipelagic mud, sandy mud, and volcanic ash. Subunit IB is Pliocene in age and extends from 94.40 to 127.00 mbsf. In addition to the normal hemipelagic mud, this subunit also contains abundant silt-sand turbidites, and minor mud-supported gravel. Subunit IC is Pliocene to late Miocene in age and extends from 127.00 to 199.20 mbsf. Lithologies in Subunit IC consist of hemipelagic mud with variable amounts of intermixed sand, rare volcanic ash beds, and rare mixed volcanic lapilli and gravel-sized mud clasts. Strata within Unit I have been subjected to significant amounts of displacement along a submarine slide surface. Below the dislocation surface, more highly deformed strata of Unit II are late Miocene in age and almost certainly part of the Nankai accretionary prism. Subunit IIA (411.00-199.20 mbsf) contains abundant sand and silt turbidites with interbeds of carbonate-poor mudstone. Similarities are striking between their lithofacies associations and those of the axial trench-wedge environment. Subunit IIB (411.00-563.95 mbsf) contains sporadic silt to sandy silt turbidites and a greater proportion of carbonate-poor mudstone, similar in all respects to the outer trench-wedge facies at Sites 1173 and 1174. The axial trench-wedge facies is repeated below 563.95 mbsf (Subunit IIC) and extends to the bottom of Hole 1178B. This repetition of facies confirms the occurrence of one of the imbricate thrust faults within the accretionary prism. The low carbonate content throughout Unit II indicates deposition below the CCD.
Structurally, Site 1178 consists of four domains. Domain I, from 0 to 200 mbsf, comprises the slope sediments, with discrete slump-folded packages and east-west-striking bedding. Domain II, from 200 to 400 mbsf, consists of accreted sediments but with only small-scale deformation features and gentle to moderate bedding dips. In contrast, Domain III extends from 400 to 506 mbsf and is characterized by marked deformation throughout (Figs. F31, F32, F33). The deformation has four chief elements: bedding dips ranging up to 55°, bedding-oblique foliation, bedding-parallel fissility (Fig. F31), and fracture sets that brecciate the sediment into roughly trapezoidal fragments and postdate the foliation/fissility. Toward the base of this 106-m zone of shearing, scaly surfaces with downdip slickenlines probably indicate a major prism thrust fault. Domain IV, from 506 mbsf to the base of the hole, is characterized by generally weaker deformation, although moderate bedding dips are common. Steeper dips and increased deformation around 550 mbsf and between 633 mbsf and the hole bottom presumably represent additional minor thrust faults. Thus Domain IV contains several thrust slices, each internally deformed much less than the sheet overlying the major thrust at the base of Domain III but probably contributing to biostratigraphic repetitions and thickening of the section.
Biostratigraphic age control was provided by calcareous nannofossils although their abundance and states of preservation varied throughout the sequence. The interval from 199.05 to 673.17 mbsf yields assemblages especially poor in preservation and low in abundance, making zonal identification problematic. Deformation of the sediments leads to a repetition of biostratigraphic events, resulting in a disturbed biostratigraphic secession. The sedimentary section spans the time interval from the late Miocene (Zones NN11-NN10) through the Pleistocene (Subzone NN21a).
Paleomagnetic measurements of magnetic inclination and intensity in Holes 1178A and 1178B show two hiatuses at 8.5 mbsf and ~400 mbsf. Based on the results of biostratigraphy, inclination changes from the top to the bottom of Holes 1178A and 1178B are identified as two different geomagnetic polarity intervals. Normal polarity is identified from 0 to 8.5 mbsf in Hole 1178A within the Brunhes Chron (0-0.78 Ma). Inclination changes from 8.5 to ~400 mbsf are considered to be geomagnetic polarity changes from Pliocene to late Miocene, including the Gauss (2.581-3.580 Ma), Gilbert (3.580-5.894 Ma), and C3A (5.894-6.935 Ma) Chrons. Continuous steep inclinations below 400 mbsf may be considered to be a repeat of the C4r Subchron (8.072-8.699 Ma).
The Cl concentration-depth profile exhibits a steep, continuous trend of freshening of up to 3%-4% relative to seawater Cl concentration. Superimposed on this background dilution profile are numerous smaller Cl minima. The largest ones occur at ~200 mbsf, corresponding to >6% dilution, and above the BSR at ~400 mbsf, corresponding to ~7% dilution. Based on measured core temperatures on the catwalk (a minimum of -0.5°C at 200 mbsf), the associated elevated methane concentration, and the observation that other dissolved components such as Si and Ca have similar dilution minima, we suggest that disseminated methane hydrate is widespread at this site, increasing in abundance from ~90 mbsf to the depth of the BSR. Hydrate is probably not evenly distributed within the sediment and seems more abundant in coarser-grained horizons.
The Ca, Mg, alkalinity, and sulfate concentration profiles are intimately coupled in the top 35 mbsf, with primary dolomite formation and dolomitization of biogenic calcite the most active reactions. The inverse relation between Ca and Mg below this depth suggests that they are involved in distinct reactions—Mg in silicate reactions below the depth drilled and Ca in ash dissolution and alteration plus probably carbonate reactions linked or associated with microbially mediated reactions at the BSR.
Similar to the deep-water Sites 1173 and 1174 but unlike the shallow water Sites 1175 and 1176, an increase in Cl concentration with depth in the top 35 mbsf is a trend consistent with diffusion of lower chlorinity interglacial water into the sediment.
The TOC content for the sediment samples examined at Site 1178 ranges from 0.57 to 1.03 wt% over the first 383.6 mbsf, with an average value of 0.73 wt%, the highest TOC values measured for Nankai sediments during Leg 190. Sulfur concentrations track the TOC values in this interval and range from 0.24 to 1.45 wt%, with the highest values occurring at 200 and 350 mbsf coincident with the highest TOC values. The moderate to low concentrations of methane throughout Holes 1178A and 1178B are attributed to the high concentrations of light hydrocarbons from ethane to hexane, indicative of older, more mature organic matter within the sediments (diagenesis) or migration (thermogenesis) of hydrocarbons from deeper depths. Overall, the concentrations of light hydrocarbon ethane to hexane reflect the thermal evolution and maturity of the sedimentary organic matter at Site 1178. The Bernard ratio (C1/[C2+C3]) for the hydrocarbons at Site 1178 also indicates that some contribution of the lighter hydrocarbons (ethane to hexane) in these sediments were produced from more mature organic matter present in situ mixed with thermogenic hydrocarbons that have migrated in from a more mature source at depth. Significant faulting has occurred over the lower 300 m of Hole 1178B that could facilitate fluid migration of more mature hydrocarbons buried at deeper depths to shallower sediments.
Microorganisms were enumerated in 30 samples collected from the surface to 633 mbsf at Site 1178. Bacteria are present in near-surface sediments at low, but close to expected abundances. This was probably related to high IW sulfate concentrations to at least 18.3 mbsf. However, bacterial populations decline rapidly to barely detectable at 272 mbsf. This is a much greater rate of decrease than was observed at other sites during this leg. A small but statistically significant decrease from the general trend that is associated with the presence of a small amount of gas hydrate occurs at 210 mbsf. Below 272 mbsf, population sizes generally vary between not detectable and barely detectable, except for a zone between 374 and 497 mbsf, where populations increased up to a maximum of 6 × 105 cells/cm3. These populations were not only locally statistically significant but were larger than those encountered at some of the more shallow depths at this site. No relationship was observed between bacterial populations and either the IW sulfate concentration or the methane concentration; therefore, the reasons for such a rapid rate of decrease in numbers remains unclear. Seventeen whole-round cores were taken for shipboard enrichment cultures, cell viability, and shore-based microbiological analysis to measure potential bacterial activities, culture microorganisms, characterize nucleic acids, and investigate fatty acid biomarkers.
There are no obvious differences in physical properties between the slope-apron deposits of Unit I and the underlying accreted sediments of Unit II. In general, porosities at Site 1178 decrease with depth, following a typical compaction profile. Deviations from the compaction trend occur at 70-100 mbsf, 140-160 mbsf, and ~200 mbsf. In addition, porosity values within lithostratigraphic Subunits IB and IC are more scattered than in Subunit IA and Unit II, and they probably reflect lithologic variation in this sandier part of the stratigraphic section or deposition by slope-failure processes. Velocities and formation factors increase with depth and are highly variable. Uncalibrated gas permeability measurements show a range of values similar to other Leg 190 sites, again due to differences in lithology. High values are given by volcanic ashes high in the section and by beds of silt and sand, including those down to 600 mbsf. The background hemipelagites gave uniformly low measurements.
Two in situ temperature measurements indicate a thermal gradient of 0.046°C/m.
Site 1178 drilling revealed that the LDRZ is composed of steeply dipping, pervasively foliated, and partly brecciated upper Miocene accreted sediments. This result will contribute significantly to our understanding of the tectonic evolution of the Nankai accretionary prism.