SITE 1151

Site 1151 is located 48 km south of Site 1150, in the deep-sea terrace of the Japan Trench, a similar geological setting as at Site 1150. The main objective at this site was to establish another borehole geophysical observatory to monitor active processes in a plate subduction zone with strain, tilt, and seismic sensors. A key difference is that this area is above an aseismic portion of the seismogenic zone. The strainmeter at this site measures volumetric strain changes. This second seafloor borehole geophysical observatory (NEREID-2) was successfully installed in Hole 1151B with much less downtime than for the first one. The sensor string was set in a section with a density of ~1.9 g/cm3 and P-wave velocity of ~2000 m/s. The target depth is slightly shallower at 1095 mbsf for the sensor string bottom, but the physical properties data suggest a more competent rock environment than at Site 1150. The bottom of the open hole was filled with cement up to ~50 m into the cased hole section. The observatory sites were visited by the Dolphin 3K of JAMSTEC between 2 and 10 September to start and check the system.

At Site 1151, the sedimentary section from 0 to 1113 mbsf was cored with APC (Holes 1151C and 1151D) and RCB (Hole 1151A) coring systems. The recovered sequence ranges from Holocene to middle Miocene age. The average recovery was 68% for Hole 1151A and ~100% for Holes 1151C and 1151D. The common major lithology at this site is diatomaceous silty clay with intercalations of minor lithologies such as volcaniclastic ash, pumice, silt, and sand. Brittle deformational structures dominate below 400 mbsf. Bioturbation can be seen in most cores below 300 mbsf. Detrital glauconite occurs as sand-sized grains distributed throughout the section. Authigenic glauconite, found in both the major and minor lithologies, is of fine silt size. The detrital glauconite is always found in association with the finer grained authigenic glauconite, but the latter may occur in the absence of the former. Alteration to limonite locally occurs. The following lithologic units were identified:

Unit I (0-295 mbsf; 0-4.5 Ma) consists of diatomaceous silty clay. Minor lithologies (sand, silt, ash, and pumice) occur frequently except in the range 78-106 mbsf. The biogenic components increase to their peak value.

Unit II (295-411 mbsf; 4.5-5.5 Ma) consists of diatomaceous spicule-bearing silty clay. Ash and pumice are rare throughout the unit.

Unit III (411-817 mbsf; 5.5-7.5 Ma) consists of various combinations of diatom-, glass-, and spicule-bearing silty clay. Brittle deformational structures occur first and reach their peak within this unit. Siliciclastic components compose >50% of the sediments.

Unit IV (817-1007 mbsf; 7.5-11.5 Ma) consists of diatom- and spicule-bearing claystone. Minor lithologies are rare. Unit V (1007-1113 mbsf; 11.5-16 Ma) consists of combinations of glassy or glass-bearing silty claystone, locally spicule bearing. The diatoms compose <10% of the sediment.

As at Site 1150, diatom datums provide the main age constraints, with diatoms being few to abundant throughout the sequence, except for some ash and dolomite layers. Overall, we identified 24 diatom datums from core-catcher samples, with the lowest being in the middle Miocene (<16.3 Ma). Diatom assemblages from all samples consist almost entirely of oceanic species, mainly from the subarctic North Pacific Ocean. Calcareous nannofossils are generally barren to abundant, with variable preservation. Similar to Site 1150, we identified 11 nannofossil datums, and seven of these gave ages younger than indicated by the diatom datums.

Sedimentation rates at Site 1151 were estimated using a combination of biostratigraphy and magnetostratigraphy. The upper 200 m has a relatively low rate (20 to 152 m/m.y.). It increases between 200 and 450 mbsf, reaching ~240 m/m.y., and remains at that level down to 800 mbsf, below which the rate gradually decreases. At 1027 mbsf, there is a hiatus of >0.2 m.y., and the rate then gradually increases downhole to 43 m/m.y. Site 1151 had high rates in the latest Miocene and low rates before and after this, a pattern similar to that at Site 1150. The intervals of low rate correspond to the early late Miocene (before 8.5 Ma) and the early to mid-Pleistocene (2.0-0.78 Ma).

Gas analyses at Site 1151 indicate that methane concentrations are between 0.4% to 5% with an average concentration of ~2%. Ethane concentrations fluctuate between 1 and 12 ppmv and typically are of ~4 ppmv. Methane/ethane ratios are ~4400 throughout. Other hydrocarbon gases are below the detection limit.

No trend is present in the distribution of carbonate abundances with depth. Abundances range from 0.08 to 79 wt%, with an average value of 3.3 wt%. Most values, however fall between 2 and 4 wt%, with excursions having peak values as high as 15 wt%. Low carbonate abundances are in agreement with low occurrences of calcareous fossils in the sediments.

Sediments exhibit relatively high abundances of organic matter with characteristic low (<10) C/N ratios. Abundance of organic carbon (Corg) fluctuates between 0.2 and 1.4 wt%, with an average value of ~0.9 wt%. Total sulfur abundances irregularly fluctuate between 0.35 to 1.5 wt%, with an average value of 0.85 wt%.

Several geochemical parameters exhibit similar distributions with depth. K+, Na+, Mg2+, chlorinity, and salinity show a characteristic decreasing trend with depth. Salinity gradually decreases with depth from a value of ~32 at the top of the borehole to a value of 18 at ~900 mbsf. Below this depth, salinity remains constant at 18 to the final depth. Chlorinity concentrations remain constant at ~500 mM in the upper 200 m of the borehole and then steadily decrease to 320 mM at the final depth.

Alkalinity values gradually decrease downhole in the upper 200 m from 31 to 17 mM and then increase to 25 mM at ~450 mbsf. Below this depth, values decrease steadily downhole to 2 mM at the bottom. Dissolved sulfate concentrations exhibit values lower than 2 mM throughout.

Concentrations of dissolved calcium (Ca2+) in pore waters slightly increase downhole from ~3 mM at the top to 18 mM at the bottom. Concentrations of dissolved strontium (Sr2+) fluctuate between 100 and 130 M down to 550 mbsf and then increase to reach a maximum value of 232 M at the bottom. Concentrations of dissolved lithium (Li+) gradually increase from 20 to 480 M in the upper 850 m and then decrease to ~280 M at the bottom.

The gamma-ray attenuation bulk density in Hole 1151A ranges from ~1.3 to 2.0 g/cm3. Lithologic Unit II is characterized by rather constant values, averaging ~1.4 g/cm3. Density varies from 1.3 to 1.7 g/cm3 along an oscillating trend in Unit III. Similar oscillating trends increase in magnitude in Units IV (1.3-1.8 g/cm3) and V (1.4-2.0 g/cm3). Such an oscillating trend is also observed in natural gamma-ray activity. The average thermal gradient, which was obtained from three measurements, is 35.9C/km.

In Hole 1151A, P-wave velocity (horizontal) ranges from 1540 to 5290 m/s, with most values being <2150 m/s. The highest velocities were measured in thin beds of carbonate-rich sediments (i.e., dolomite layers or dolomite concretions). The maximum velocity in Hole 1151A of 5290 m/s was measured on a dolomite concretion at 1108 mbsf.

The ranges of porosity, bulk density, and grain density in Hole 1151A are 10%-77%, 1.32 to 2.42 g/cm3, and 2.09 to 3.91 g/cm3, respectively. The section from 78 to 295 mbsf (Unit I) has scattered, but slightly inverse trends of porosity and bulk density: porosity and bulk density generally range from 57% to 77% and from 1.32 to 1.59 g/cm3, respectively. The top of Unit IV (825 mbsf) coincides with a shift to higher porosity and lower bulk density values. Yet another significant shift to higher porosity, by ~13%, occurs from 963 to 977 mbsf. The corresponding decrease in bulk density is ~0.22 g/cm3. Apart from these two shifts, index properties show normal trends across Units IV and V. At the base of Hole 1151A, porosity, bulk density and grain density are 49%, 1.75 g/cm3, and 2.46 g/cm3, respectively.

The remanent magnetization at Site 1151 is very similar in behavior to that at Site 1150. The upper 78 m has an unambiguous normal polarity direction, with a steep downward inclination. The location of the Brunhes/Matuyama reversal appears to be well resolved by the abrupt downhole change to negative inclinations at ~78.2 mbsf in Hole 1151C, 80.1 mbsf in Hole 1151D, and 82-84 mbsf (between Cores 186-1151A-2R and 3R) in Hole 1151A. However, the reversals lower in the section are not easily correlated with the geomagnetic polarity time scale. Below 700 m, virtually the entire section has very stable positive inclinations that average ~60. We interpret this interval to be partially or totally overprinted by a recent normal polarity field direction. The stable declinations from these cores have proved useful for reconstructing structural orientations of the microfractures and bedding planes. We have found that the orientation of fracture planes changes downhole with dip azimuths dominantly to the west-northeast and east-southeast in the upper domain but dominantly east and west in the middle and lower domains. Below 900 mbsf, the dip angles of bedding planes are >10 and preferentially dip toward the east.

Three logging runs (one triple combo and two FMS/sonic runs down to 850 mbsf) were achieved in Hole 1151D by extending the second APC/XCB hole. The hole condition (caliper log) was much more stable at Site 1151, and logging was accomplished without difficulty.

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