Whole-Core Measurements

Multisensor Track (MST)

Natural gamma-ray (NGR) activity, gamma-ray attenuation porosity evaluator (GRAPE) density, magnetic susceptibility, and P-wave velocity were measured on whole-round samples (see "Physical Properties" in the "Explanatory Notes" chapter). All measurements were made to the base of APC coring in Holes 1096A and 1096B, at depths of 140.7 mbsf (Core 178-1096A-15H) and 166.7 mbsf (Core 178-1096B-20H), respectively. P-wave velocity measurements were not made on the XCB cores because of poor core quality, but the other properties were measured down to 588.6 mbsf (Core 178-1096C-41X).

Magnetic Susceptibility

Whole-core magnetic susceptibility was measured at 2-cm intervals (averaged over 2 s). The raw data are provided on CD-ROM and the World Wide Web (see "Related Leg Data" in the Table of Contents), and are shown in Figure F33. The spurious data associated with the ends of sections at Site 1095 were minimized at Site 1096 by not measuring the upper and lower 6 cm of each section. After low-pass filtering (Fig. F34), depth-scaled susceptibility shows a positive correlation with the GRAPE density. The average susceptibility and the susceptibility's variance also increase between ~100 and 160 mbsf (Fig. F34). This trend may be the result of a downward increase in silt content toward the base of lithostratigraphic Unit II and the high silt variability of the unit. Unit II is composed mainly of fine-grained silt and mud turbidites (see "Lithostratigraphy"). Paleomagnetic polarity reversals were used to convert the data to the age scale shown in Figure F35.

GRAPE Bulk Density

Density was measured by gamma-ray attenuation at 2-cm intervals (averaged over 2 s at each point). The raw data are provided on CD-ROM and the World Wide Web (see "Related Leg Data" in the Table of Contents), and are shown before (Fig. F33) and after (Figs. F34, F35) low-pass filtering.

GRAPE density values rise to ~2.0 g/cm3 at ~160 mbsf, then fall to ~1.5 g/cm3 at ~580 mbsf. The greater part of this density drop corresponds closely to the base of lithostratigraphic Unit II (see "Lithostratigraphy"), which coincides with the beginning of XCB coring. However, the decrease in density at the base of Unit II occurs over an interval of ~40 m, which suggests that it is not caused solely by the change in coring method. It reflects a physical properties change that was also noted in the index properties and magnetic susceptibility (see below). Superimposed on the broad trends of the filtered data are peaks that show a positive correlation with the magnetic susceptibility data and correspond to lithostratigraphic changes.

P-wave Velocities

Whole-core P-wave measurements were only recorded continuously down to 106.3 mbsf (Core 178-1096A-12H-3) in Hole 1096A and to 107.48 mbsf (Core 178-1096B-12H-6) in Hole 1096B. Below, core disturbance related to XCB coring was too great for measurements to be reliable. The anomalously high values associated with the ends of core sections at Site 1095 were removed from Site 1096 data sets by omitting data within 10 cm of the beginning and end of each section. The raw data can be found on CD-ROM and the World Wide Web (see "Related Leg Data" in the Table of Contents), and are presented in Figure F33. The main features in this data set are the increase in P-wave velocity downhole and the correlation between the peaks in the filtered P-wave velocities and sediment layers with high silt content (see "Lithostratigraphy").

Natural Gamma Radiation

Whole-core natural gamma-ray emissions (averaged over 15 s) were counted at 15-cm intervals. The change to XCB coring did not cause any change in the signal. The raw data set is provided on CD-ROM and the World Wide Web (see "Related Leg Data" in the Table of Contents), and presented in Figure F33.

The filtered gamma-ray count (Figs. F34, F35) shows an increase with depth in the first 60 mbsf, followed by a broad decrease with depth to the base of Hole 1096C at ~580 mbsf, possibly with a weak 50-m cyclicity. There is an inverse correlation between the MST measurements (GRAPE density, magnetic susceptibility, and NGR data) and the biogenic component of the sediments.

Split-Core Measurements

Index Properties

Gravimetric and volumetric determinations of index properties were made for 36 samples in Hole 1096A (Cores 178-1096A-1H through 15H), 30 samples in Hole 1096B (Cores 178-1096B-13H through 32X), and 102 samples in Hole 1096C (Cores 178-1096C-1H through 41X). One sample was taken every first, third, and fifth section per core where possible.

Samples were not taken from the reconstituted sediment surrounding the "biscuits" in the XCB cores or in regions of flow-in in APC cores. Wet mass, dry mass, and dry volume were measured, and from these measurements, percentage water weight, porosity, dry density, bulk density, and grain density were calculated (see "Physical Properties" in the "Explanatory Notes" chapter; for raw data see "Related Leg Data" in the Table of Contents).

Index properties bulk density and GRAPE density agree well in the upper 220 m. Below, they show a similar decrease with depth to the base of the hole (Fig. F36). The index properties grain density shows a similar decrease with depth from 170 mbsf, as does the bulk density. High grain density coincides with lithostratigraphic Unit II (see "Lithostratigraphy"), but grain density does not show the sharp increase with depth of the bulk density, within the upper 100 mbsf. The trend of porosity and percentage bulk water content (Fig. F37) is the inverse of that shown by the bulk density. After the initial decrease down to 100 mbsf, porosity and water content increase steadily downward from 52% to 60% and from 27% to 37%, respectively. These results were not expected, as porosity normally decreases with depth because of compaction. A possible explanation is the increase in biogenic silica content (Fig. F37A), seen previously to inhibit compaction (Bryant and Rack, 1990). The highest values of porosity and water content are found in the upper 30 mbsf, corresponding to lithostratigraphic Unit I, where there is a high biogenic content but also a low lithostatic load.

Yield Strength

The yield strength of the sediments was measured using the vane shear and pocket penetrometer equipment. Residual strengths were also obtained from the vane shear equipment. The raw data are presented in Figure F38A and provided on CD-ROM and the World Wide Web (see "Related Leg Data" in the Table of Contents). The most notable features in the data sets are the decreased strengths found with the vane shear equipment below 140 mbsf and the change in the pocket penetrometer data variability at 210 mbsf.

Down to 140 mbsf, the peaks in the vane-determined shear strengths coincide with the lowest biogenic abundances in the sediments (Fig. F38B) and with the minimum porosity (Fig. F38C) and water contents. Below this level, the relationships are not as clear, which suggests that the yield strength peak at 140 mbsf may represent the highest reliable measurement possible with the equipment. However, the general relationship of higher porosity and lower strengths remains valid below 140 mbsf.

The change in variability of the pocket penetrometer data (at ~200 mbsf) is likely to reflect a stiffening of the sediment to a level at which remolding became obvious and disturbed areas were sampled less. The vane shear equipment has a larger footprint than the pocket penetrometer and is therefore less likely to record the effect of remolded areas on the average behavior of the sediment.

Yield strength normalized by the overburden stress (see "Related Leg Data" in the Table of Contents for raw data), gives an empirical estimate of consolidation levels. Typically, values >0.5 suggest overconsolidation and <0.2 underconsolidation (Skempton, 1970). From Figure F38C, it may be seen that the material tested is underconsolidated, in agreement with the proposed sedimentation mechanisms and rates (see "Lithostratigraphy" and "Sedimentation Rates").

Discrete P-wave Velocities

Discrete P-wave velocity measurements using all three sensors (PWS1, PWS2, and PWS3) of the velocity-strength (SV) system were made throughout Site 1096. The upper 40 m of Cores 178-1096B-1H through 5H were soft enough to use the penetrative transducer pairs of PWS1 (measurement direction = longitudinal; transducer spacing = 69.5 mm) and PWS2 (measurement direction = transverse; transducer spacing = 34.8 mm). Results of the transverse and longitudinal measurements are shown in Figure F39 (average measurement separation = 1.4 m). The curves are in close agreement, but no consistent velocity anisotropy is evident.

Hamilton Frame (PWS3) measurements (average measurement separation: 2.0 m) on Cores 178-1096B-5H through 32X and 178-1096C-1H through 38X cover the depth interval 40-554 mbsf, which is too compacted to be measured using the PWS1 and PWS2 transducers (Fig. F39B). The data from all three transducer pairs are provided on CD-ROM and the World Wide Web (see "Related Leg Data" in the Table of Contents).

Thermal Conductivity

Thermal conductivity was measured once per core, on average, for all holes at Site 1096, usually in the middle of Section 3. Thermal conductivity was needed at Site 1096, in combination with downhole temperature measurements, to estimate heat flow and hence the temperature at the bottom of the hole and the depth to a theoretical methane hydrate BSR.

Above ~300 mbsf, measurements were taken by needle probe on the unsplit core, with the implicit assumption that the core was undisturbed before insertion of the needle and would remain so afterward. Most likely, this was not the case for much of the biscuited core recovered by XCB drilling below 167 mbsf in Hole 1096B and below 176 mbsf in Hole 1096C.

The other available method is appropriate for hard sediment or rock. It uses a different ("half-space") geometry, in which the needle is embedded at the flat surface of a plastic block of known thermal properties (see "Physical Properties" in the "Explanatory Notes" chapter). The block is clamped to a flat surface of the specimen, and thermal contact is assured by use of a proprietary thermal joint compound. The operator is advised to submerge block and specimen in water, to provide a more stable thermal environment. However, it was clear that sediment from below 300 mbsf, too indurated for measurement by needle insertion, was too soft for immersion in water. To take the measurement in air was to expose the specimen to larger changes in ambient temperature than the (TK04) software would accept in default mode; a valid measurement could be achieved only by doubling the acceptable limit of standard deviation of the decay curve fit. This may have biased, or increased the scatter of, measurements on deeper samples.

Measurements on cored material from all three holes at Site 1096 are combined in Figure F40. Each needle insertion produced three values, which show an internal scatter of about 5%. In addition, sediment thermal conductivity, measured by whatever means as a part of marine geothermal heat flow determinations, shows scatter of up to 10% within apparently uniform cores. The origin of this scatter is uncertain, and it has become common practice to determine an average value or (for longer sections) to fit a straight line by least squares. At Site 1096, however, even allowing for this scatter, a dependence of the thermal conductivity values on the methods of measurement and coring is apparent in Figure F40.

Thermal conductivity in fine-grained sediments is to first approximation a bulk property and therefore a linear combination of the conductivities of the grains and the interstitial water. It therefore depends upon porosity and lithology. The thermal conductivity of water is ~0.6 W/(m·K), and of most sediment-forming minerals is much higher. Thus, lithology aside, thermal conductivity should increase downhole as porosity decreases. The gradient in thermal conductivity in the uppermost 170 m in Figure F40 illustrates this, although it has been argued that APC coring compacts sediments, particularly at depth, so that the gradient might be artificially high. GRAPE and grain density measured on discrete samples both decrease downward around 170 mbsf, where there is a lithostratigraphic boundary and a change from a calcareous to a siliceous biofacies. However, the reduction in thermal conductivity below 170 mbsf is considered in large part an artifact of the change from APC to XCB. During this transition, it is possible that the needle probe has been inserted into the higher porosity drilling matrix surrounding a biscuit, or has split a biscuit during insertion, allowing the crack to fill with water or air.

Below 300 mbsf it was possible to find larger biscuits, apparently intact after coring and splitting, which could be measured by the half-space method. There is a slight concern here, however, that the search for a measurable piece might in fact result in a bias toward the more indurated components of the rock so that measurement would be biased toward higher values. Anomalously low values might result, on the other hand, if thermal contact was inadequate, or the specimen was either too small or had hidden cracks that would not have been open in situ.

For all these reasons, it was considered best to fit a simple straight line to the main body of data, neglecting values anomalously high and low and, in particular, ignoring the low values associated with needle probe measurements on unsplit XCB cores below 170 mbsf. The straight line gives a thermal conductivity equal to 1.0325 + 0.00043x W/(m·K), where x is in meters.

Downhole Temperatures and Geothermal Heat Flow

Six valid temperature measurements were obtained at Site 1096. The Adara tool was used at the mudline and after firing APC Cores 178-1096B-4H and 7H, and the Davis-Villinger tool was used after cutting Cores 178-1096B-17X, 26X, and 32X. The Adara deployment during Core 178-1096B-4H was subject to corer motion during the thermal decay period but can be used; an additional Adara deployment with Core 178-1096B-10H recovered no data because the batteries failed. Temperatures are plotted against depth in Figure F41.

The downhole increase in thermal conductivity is incompatible with a constant temperature gradient downhole, if heat flow is constant. The assumption of constant heat flow means that heat generation within the upper sediments by radioactive decay is neglected, which is reasonable: the logs show only minor radioactivity downhole (see "Downhole Measurements"). The straight-line fit to thermal conductivity measurements (Fig. F40) can be combined with downhole temperatures to determine geothermal heat flow and to extrapolate in situ temperature to the base of the hole. The result is a logarithmic curve, shown in Figure F41 as a line through the measured temperatures and extrapolated to 600 mbsf. The line is a good fit, except perhaps for the temperature from Core 178-1096B-4H, mentioned above. The extrapolated temperature at the base of Hole 1096B (607 mbsf) is 43.4ºC.

Heat flow at this site is 83 mW/m2. To make a comparison with the theoretical value of heat flow for ocean floor of this age, it is necessary to make an allowance for developments in the Magnetic Reversal Time Scale (MRTS) since the original empirical relationship was derived (Parsons and Sclater, 1977). These authors used the time scale of Heirtzler et al. (1968). The site lies on ocean floor dated by marine magnetic anomalies at ~37 Ma (Cande and Kent, 1995; "Background and Scientific Objectives"), or at ~42.5 Ma by the MRTS of Heirtzler et al. (1968). According to Parsons and Sclater (1977), the heat flow appropriate to this age (from a global average, bearing in mind oceanic lithospheric evolution) is 72 mW/m2, so the measured heat flow is ~15% higher than expected. This is considered a valid determination, however, because neither the temperatures nor the thermal conductivity measurements are in error to that extent. No allowance has been made for the effects of sedimentation (e.g., Hutchison, 1985).

The average temperature gradient downhole of about 80°/km gives a depth for the base of the methane hydrate stability zone of 300 mbsf using the ODP Pollution Prevention and Safety Panel (PPSP) hydrate stability equation (PPSP, 1992). Any BSR associated with that base would therefore lie at ~340 ms on seismic reflection profiles through the site and could not be linked to the observed BSR at 6-700 ms, which we attribute to silica diagenesis (see "Background and Scientific Objectives" and "Seismic Stratigraphy").