Whole-round core samples from Holes 1202A, 1202B, 1202C, and 1202D were run through the MST during Leg 195 if they were >40 cm long. These samples were collected from depths between 0 and 140 mbsf. Additional core recovered from depths between 140 and 410 mbsf in Hole 1202D were run through the MST during Leg 196 because time constraints did not permit measurement of this material during Leg 195.
The MST measured the volume magnetic susceptibility (MSL), the density using gamma ray attenuation (GRA), and the P-wave velocity. The P-wave velocity logger (PWL) was able to obtain P-wave velocity measurements only to a depth of 20 mbsf.
Vane shear strength measurements were made on samples from Hole 1202A, the only cores from Site 1202 split during Leg 195. Thermal conductivity measurements were made on every core run through the MST. Because of time constraints, no index properties samples were taken at Site 1202.
Variation of the volume magnetic susceptibility is often used as a proxy for climate change. It was measured at Site 1202 on cores from all four holes to obtain a continuous record of the magnetic susceptibility signal; the records from the individual holes overlap and can potentially be spliced into a single record with no coring gaps. Figure F10 shows the magnetic susceptibility of samples recovered from Holes 1202A, 1202B, 1202C, and 1202D to a depth of 140 mbsf. At a scale of several meters, the records show an excellent correlation between holes. At smaller scales (<1 m) the amount of noise in the records may preclude correlation between holes. This noise is caused by gas expansion voids and by coring deformation in the XCB parts of the section. Figure F11 compares the volume magnetic susceptibility of samples recovered from 0 to 420 mbsf in Hole 1202D measured during Leg 196 with the volume magnetic susceptibility values from 0 to 140 mbsf measured during Leg 195. This graph shows that there is excellent agreement between the measurements made during Leg 195 and those made during Leg 196. The magnetic susceptibility did not change over time, so the measurements made on the lower samples (140-420 mbsf) are reliable. In addition, the variability and range of the lower measurements (140-420 mbsf) are similar to the upper measurements (0-140 mbsf).
The largest value of magnetic susceptibility is equal to 6000 x 10-5 (SI units) at a depth of ~85 mbsf in Hole 1202D (this peak is off the scale of Fig. F11). The depths of the largest values of magnetic susceptibility are similar in the other three holes. These values may indicate a thin layer of high-susceptibility material. Another peak of lesser magnitude (nearly 400 x 10-5 SI units) is present at a depth of 293 mbsf in Hole 1202D. This peak is also off the scale of Figure F11.
The results of the GRA bulk density measurements for Holes 1202A, 1202B, 1202C, and 1202D from a depth of 0-140 mbsf are shown in Figure F12. The densities measured in the four holes are all lowest near the seafloor, between 1.6 and 1.8 g/cm3, and increase to ~2.0 g/cm3 at 10-15 mbsf.
As was observed for the MSL measurements, the density records correlate at a scale of several meters. Again, the noise introduced by gas fractures and XCB drilling disturbance could make correlation difficult at a smaller scale. As a result, the largest density values will provide the best estimates of the bulk density for these sediments. The decrease in density between 80 and 90 mbsf corresponds to an increase in magnetic susceptibility shown in Figure F10.
A comparison of the GRA bulk density measurements made during Legs 195 and 196 on samples from Hole 1202D is shown in Figure F13. This graph shows that the measurements made during Leg 196 are reduced by ~0.3-0.5 g/cm3 from the Leg 195 measurements for the same depths (0-140 mbsf). The cause of this reduction in GRA densities is almost certainly dessication or pooling of water at the liner bottom, since the core liners were systematically perforated immediately after recovery to allow gas to escape. The values of GRA density measured during Leg 196 are thus suspect and should not be used to estimate the density of the sediments.
Porosity (
)
was calculated from the GRA bulk density using Equation 1, which assumes generic
values of 2.7 g/cm3 for the grain density and 1.0 g/cm3
for the pore water density. The porosities calculated are plotted vs. depth for
all four holes in Figure F14.
Although the data show significant scatter inherited from the variability in the
density data, porosity clearly decreases overall with increasing depth, from
between 50% and 70% at the seafloor to between 40% and 60% at a depth of ~120
mbsf:
= (
grain
- bulk)/(grain
- pore
water). (1)
The results of natural gamma radiation (NGR) emissions measured during Leg 196 on core recovered from Hole 1202D are shown in Figure F15. The rate of NGR emissions is relatively high throughout the section. The variations follow the GRA density measurements more closely than the magnetic susceptibility measurements, particularly in the first 50 mbsf. There is an overall increasing trend from 30 counts per second (cps) at the seafloor to a maximum of 60 cps at 380 mbsf. The count rate increases most in the first 10 m below the seafloor to a peak of ~47 cps. Between 30 and 70 mbsf, there is a distinct drop in emissions to ~40 cps.
The PWL measured sediment velocities in the four holes to a maximum depth of 20 mbsf. Below 20 mbsf, the PWL was unable to measure velocities because the cores developed voids and small cracks from gas expansion and poor coupling between the core and core liner caused attenuation of the signal. The velocities shown in Figure F16 are typical of ocean-bottom sediments at shallow burial depths. They show a slight increase of velocity with depth from ~1.5 km/s at the seafloor to nearly 1.6 km/s at a depth of 20 mbsf. The large amount of scatter in the data is likely caused by the gas expansion voids and poor liner coupling.
The shear strength of the sediments from Hole 1202A was measured once per core in the working half of the core within 30 min of splitting. The peak shear strengths are shown in Table T3 and Figure F17. The peak shear strength increases nearly linearly with increasing depth from ~0 kPa at the seafloor to 45 kPa at 110 mbsf. The highest shear strength measured in Hole 1202A, 53 kPa at 77 mbsf, is almost double that predicted by the trend of all the other measurements and does not correspond to any obvious increases in density at the same depth (Fig. F12).
Thermal conductivity measurements (Fig. F18) show relatively little variation with depth and low scatter around a mean value of 1.04 W/(m·K). Occasional low values in the upper 50 m are probably caused by the presence of voids in the core formed by gas expansion. Thermal conductivity increases from ~1 W/(m·K) at the seafloor to a maximum of ~1.3 W/(m·K) at ~10 mbsf and then falls back to ~1 W/(m·K) by 30 mbsf and remains roughly constant to 250 mbsf. At depths >250 mbsf, the values become more variable but there are insufficient measurements to identify any trend.
In situ temperature measurements were made in Hole 1202A using the Adara temperature probe. These temperature records are shown in Figure F19. Also shown on the figure are the seafloor or mudline temperature (Tml) and the estimated sediment temperature at depth (Tsed).
The sediment temperatures at depth were determined using the curve fitting program TFIT. The estimates of sediment temperature can vary several degrees, depending on the value of the thermal conductivity of the sediment and the region of the decay curve chosen for the estimation. The estimated sediment temperatures shown in Figure F19 were determined using only the latter portion of the middle temperature series, the temperature decay curves.
The mudline temperature was determined to be 4.6°C for the two measurements. The sediment temperature at a depth of 64.1 mbsf was 7.2°C, giving a thermal gradient of 0.04°C/m. The sediment temperature at a depth of 119.5 mbsf was 9.3°C, giving the same thermal gradient. The integrated thermal resistivity, calculated as the area under the curve of depth vs. the inverse of the measured thermal conductivity, is shown in Figure F20. Temperatures (from Fig. F19) and integrated thermal resistivities corresponding to equivalent depths (from Fig. F20) are plotted in Figure F21. The slope of the best-fit line through the three points gives a heat flow for the site of 0.040 W/m2, slightly less than the approximate global average of 0.050 W/m2 (Garland, 1979). The linearity of the three points in Figure F21 indicates a constant heat flow at this site.
