PHYSICAL PROPERTIES

At Site 1252, one hole was drilled and used for complete physical property analyses. Because of pervasive gas expansion, no compressional (P)-wave velocity (V_P) and shear strength measurements were conducted. The physical properties (especially bulk density) measured at Site 1252 typically match the lithostratigraphic units.

IR imaging was carried out on all cores recovered prior to and after sectioning and curating. This site has very good core recovery and allowed detailed imaging of the entire sediment column. Only two samples thought to contain gas hydrate were recovered at Site 1252.

IR imaging was used to identify the location of hydrate in cores on the catwalk. The track-mounted IR camera was used twice, both prior to and after sectioning the core liner. The second scan could be directly correlated postcruise to the VCDs, particularly the presence of mousselike texture that is indicative of the location of hydrate.

Data from the first IR scan were used to generate a downhole temperature profile at Hole 1252A (Fig. F20A). The temperatures in the upper 130 mbsf show an overall linear trend of increasing temperature with depth common to many APC cored holes. The gradient is much less than in situ temperature gradients and apparently reflects a combination of increasing in situ temperature, cooling of the cores during retrieval, and gas expansions or exsolution.

Definite negative T anomalies first appear at a depth of 43 mbsf (Fig. F21; Table T6) and continue to a depth of 133 mbsf. A single cold anomaly is present at 183 mbsf. Given that this anomaly is present by itself and near a core top, it was probably caused either by drilling fluid incursion during core retrieval or gas expansion/gas exsolution. The depth of the base of GHSZ at Site 1252 is estimated to be at ~170 mbsf, derived by projecting a prominent BSR westward a few hundred meters to the hole location. The lowest definite thermal anomalies are present at 133 mbsf, indicating that negative Ts are present within the stability zone for gas hydrate. The lack of thermal anomalies between 133 and 170 mbsf suggests that while pressure-temperature conditions are appropriate for gas hydrate stability in this zone, the chemical composition is not (e.g., lack of methane or water to create sufficient hydrate to cause a thermal anomaly in core samples).

The downcore temperature pattern for Cores 204-1252A-13H and 14H (Fig. F20B) suggests a significant lithologic or gas content change at the bottom of Core 13H and top of Core 14H (118.9 mbsf). At that point, the downcore temperature becomes more irregular as a result of an increase in the number of voids greater than ~2 cm in width. At this depth, the accretionary complex sediments were first encountered, overlain by glauconite sand layers (see "Lithostratigraphy"). Core 204-1252A-13H exploded on the catwalk just after IR scanning was completed. Both cores contain small amounts of hydrate based on anomaly frequency, thickness, and T magnitude. The difference between the exploding core (Core 204-1252A-13H) and the highly fractured core (Core 14H) is probably a reflection of their respective mechanical properties. Core 204-1252A-14H coincides with the presence of indurated carbonate nodules and layers that may be more brittle than other parts of the core (e.g., Core 13H). Further analysis of the wireline logs and acoustic logs is expected to provide additional insight into the gas content and mechanical properties of this part of the hole (see "Downhole Logging").

Sediment density shows a different downhole trend compared to that observed at other Leg 204 Sites (Fig. F22; Table T7). The density profile shows several distinct breaks where density drops sharply to lower values. These breaks correspond in part to the boundaries of the lithostratigraphic units (see "Lithostratigraphy") and can be correlated to seismic data (Fig. F23). Five breaks were identified, located at 7, 28, 115, 180, and 210 mbsf, respectively. Each of these breaks is associated with large variations in the MS (see next section), with one exception at 180 mbsf.

The GRA density shows higher densities at the seafloor, with density values around 1.6 g/cm³ for the upper 8 m. Values drop below this depth to ~1.5 g/cm³, then increase with depth to ~28 mbsf, where a second decrease in density occurs. This second break in the density profile is less pronounced and accounts for a decrease in density of ~0.05 g/cm³. The interval between 28 and 115 mbsf is characterized by a normal density increase with depth. Density values are ~1.75 g/cm³ at 115 mbsf and decrease to 1.5 g/cm³ over an interval of 25 m before they start to increase again to a depth of 180 mbsf. Between 180 and 220 mbsf, density increases again, then remains at around 1.65 g/cm³ for the remaining depth range.

Porosity decreases relatively uniformly from values of ~70% at the seafloor to 55% at the boundary between lithostratigraphic Units II and III (114 mbsf). The top of lithostratigraphic Subunit III is characterized by very low densities and high porosities as well as slightly lower than average grain densities.

At a depth of 183 mbsf, several carbonate layers are present (see "Lithostratigraphy"). Samples taken from one of these layers (Samples 204-1252A-21X-1, 47-49 cm; 21X-1, 52-54 cm; and 21X-1, 64-66 cm) show high bulk densities of >1.8 g/cm³, with a maximum of 2.17 g/cm³ in the center of the carbonate layer.

The MS profile is marked by several distinct intervals of high values (see Fig. F22). Most of these intervals of elevated values correspond to breaks in the density profile as described above, with the boundary defined at the top of the interval that shows high MS values (not the peak itself). This is particularly evident for the top of lithostratigraphic Units II and III at 71.5 and 114 mbsf, respectively. There is no clear change in MS values associated with the boundary between lithostratigraphic Subunits IA and IB.

As a result of pervasive gas expansion, no V_P was measured either using the MST or the Hamilton Frame.

Thermal conductivity was measured regularly on the whole-round cores (see "Physical Properties" in the "Explanatory Notes" chapter). The values vary between 0.78 and 1.1 W/(m·K), with an average value of 0.915 W/(m·K) (Table T8). The measurements are affected by the pervasive gas-expansion cracks, but the locations for individual measurements were chosen to minimize distortion. There is significant scatter in the data, but thermal conductivity values follow the same downhole trend as the bulk density, indicating that thermal conductivity is correlated with moisture content of the sediments (see "Physical Properties" in the "Site 1251" chapter).

As a result of pervasive gas expansion, no shear strength measurements were made.

Physical properties measured at Site 1252 correlate well with lithostratigraphic boundaries and the results from wireline logging. The GRA density and MS profiles provide the strongest basis for the correlation. Moisture and density (MAD) properties are biased by low sampling frequency and thus provide only general trends. The distinctive sedimentological feature at Site 1252 is the abundance of carbonate layers, either as solid concretions or as disseminated layers. These carbonate layers show high bulk density, low porosity, and high grain density values.

Site 1252 has very low hydrate abundance overall based on IR imaging and the low abundance of mousselike texture as defined from VCDs. Several cold-spot anomalies were observed but make up a relatively small volume of the core and never exceed a T of -3°C.