CORE PHYSICAL PROPERTIES

Physical properties at Site 1198 were measured and evaluated on whole cores, split cores, and discrete core samples. The multisensor track (MST) was used on whole cores to perform nondestructive measurements of bulk density, magnetic susceptibility (MS), and natural gamma radiation (NGR). Color reflectance was measured on the archive halves of split cores. Compressional wave velocity was measured in the x-, y-, and z-directions on split cores and core samples. Moisture and density (MAD) analyses were performed on core samples. Thermal conductivity was evaluated on unlithified whole cores and on samples from semilithified and lithified cores.

Gamma ray attenuation (GRA) and MAD analyses provided two independent measures of bulk density at Site 1198. From 0 to 200 mbsf, which corresponds to the interval cored with the APC, GRA bulk density values are an average of 0.1 g/cm³ higher than MAD bulk density values (Fig. F21). Low core recovery prevented bulk density measurements from 200 to 350 mbsf. Below 350 mbsf, MAD bulk density exceeds the average GRA bulk density by 0.1-0.2 g/cm³ (Fig. F21). The low GRA bulk density below 350 mbsf is likely a result of undersized cores and/or sample disturbance as a result of RCB coring (see "Core Physical Properties" in the "Site 1192" chapter). Downhole bulk density trends of both data sets, however, are similar.

Bulk density within lithologic Unit I is nearly constant, averages 1.75 g/cm³, and ranges from 1.65 to 2.00 g/cm³ (Fig. F21). Throughout lithologic Units III and IV, bulk density increases from an average of 2.1 to 2.3 g/cm³. The highly altered olivine basalt basement (lithologic Unit V; see "Lithostratigraphy and Sedimentology") is characterized by the highest bulk density of 2.4 g/cm³.

Grain density decreases downhole from 2.8 to 2.7 g/cm³ within lithologic Unit I (0-200 mbsf). Lithologic Units III and IV have an average grain density of 2.7 g/cm³. Acoustic basement has the lowest grain density (2.5-2.6 g/cm³) at Site 1198 (Fig. F21).

Porosity, calculated from MAD data (Fig. F21; also see "Core Physical Properties" in the "Explanatory Notes" chapter), is nearly constant at 60% throughout lithologic Unit I. A slightly lower porosity (50%-55%) is observed from 45 to 55 mbsf and correlates with the transition between lithologic Subunits IA and IB (see "Lithostratigraphy and Sedimentology"). The general lack of compaction, shown by the nearly constant porosity, may result from high fluid pressures generated during deposition or induced by fluid migration. High porosity and potentially elevated fluid pressures may have reduced sediment stability, contributed to sediment deformation, or aided in the initiation of the mud slumps observed within lithologic Unit I (see "Lithostratigraphy and Sedimentology") and on seismic data (see "Seismic Stratigraphy").

Throughout lithologic Units III and IV, porosity decreases from 40% to 25%. The porosity () profile in these units can be related to depth (z) using an exponential function and a porosity decay parameter (k) (e.g., Athy, 1930),

(z) = _oe^-kz.

The interpreted normal compaction trend for lithologic Units III and IV (_o = 65%; k = 0.0014; Fig. F21) is similar to that observed in similar hemipelagic sediments from other sites (see "Core Physical Properties" in the "Site 1192" chapter; "Core Physical Properties" in the "Site 1193" chapter; "Core Physical Properties" in the "Site 1194" chapter; and "Core Physical Properties" in the "Site 1195" chapter). Lithologic Unit I was excluded from the porosity regression because this porosity relation assumes hydrostatic fluid pressures (Athy, 1930). Porosity in the olivine basalt basement is <10%.

Compressional wave velocity was measured at discrete intervals for Site 1198. PWS velocity is nearly constant within lithologic Unit I at ~1625 m/s (Fig. F22). This corresponds to the constant porosity of this unit (Fig. F21). Few velocity measurements were obtained from 200 to 400 mbsf (lithologic Unit II) because of poor recovery. Of the obtained data, average velocity is 2225 m/s with two high velocity measurements (4250 and 3750 m/s) (Fig. F22). The higher velocity corresponds to a horizon containing phosphatic nodules (200 mbsf; boundary of lithologic Units I and II) (see "Lithostratigraphy and Sedimentology"). From lithologic Unit III to IV velocity increases from ~2000 to ~2800 m/s. The basalt of lithologic Unit V is characterized by an abrupt velocity increase to values ranging from 3750 to 4250 m/s.

The overall velocity structure at Site 1198 is isotropic (Fig. F22) (see "Core Physical Properties" in the "Explanatory Notes" chapter). Anisotropy calculations were made in zones where transverse (x- and y-direction) and longitudinal (z-direction) velocity values were obtained.

Velocity can often be related to porosity. The porosity and velocity data from Site 1198 do not match the time-average relationship of Wyllie et al. (1956) (Fig. F23) (see "Core Physical Properties" in the "Site 1193" chapter) but can be described with a power law relation,

V_P() = a^-b,

where V_P is the compressional wave velocity, and a (12232 m/s) and b (0.49) are empirical constants determined from a least-squares regression (correlation coefficient = 0.90) (Fig. F23). One high velocity/high porosity outlier deviates significantly from the model (496 mbsf, = 52%, and V_P = 2750 m/s; Fig. F23). This high porosity-velocity point correlates with the onset of the laminated packstone/grainstone intervals at the base of lithologic Subunit IIIB (see "Lithostratigraphy and Sedimentology"). Low porosity and compositional changes control the high basement velocity.

Four APC temperature tool temperature measurements were taken in lithologic Unit I without success. The observed temperature decay curves are erratic, possibly due to heave-induced probe movement. Reliable in situ temperatures, therefore, are not available.

Thermal conductivity values are nearly constant at 1.2 W/(m·K) in lithologic Unit I and are consistent with the porosity within this interval (Figs. F21, F24). From 0 to 40 mbsf, thermal conductivity increases from 0.8 to 1.2 W/(m·K). Such a variation is not observed in any other data and may be the result of poor contact between the needle probe and the high water-content sediment. From 350 to 450 mbsf, thermal conductivity decreases slightly with depth and then increases below 450 mbsf with significant data scatter (Fig. F24).

Thermal conductivity (K_bulk) can be described with a power law relationship (e.g., Keen and Beaumont, 1990),

K_bulk() = K_wK_grain^{(1 - )},

where K_w is the thermal conductivity of the interstitial water and K_grain is the thermal conductivity of the solid grain (see "Core Physical Properties" in the "Explanatory Notes" chapter). The observed thermal conductivity follows this relationship within the range for the encountered sediments (Fig. F25), which gives confidence to the observations.

MS and NGR increase, on average, from 0 to 200 mbsf, and have significantly high values at the hardground between lithologic Units I and II (200 mbsf) (Fig. F26). The gradual increase in MS and NGR correlates with the steady downhole increase in clay content (see "Lithostratigraphy and Sedimentology" and "Geochemistry"). The wireline gamma radiation log shows a similar increase over this interval (see "Downhole Measurements"). MS shows 25-50 m scale variations (0-10 x 10^-6 SI) superimposed on the average trend. These variations may relate to variations in clay content or the presence of pyrite (see "Lithostratigraphy and Sedimentology"). NGR data are scattered throughout lithologic Unit I (Fig. F26), but small-scale (< 25 m) variations seem to exist. Similar NGR variations also can be observed in wireline log data (see "Downhole Measurements"). The MS and NGR trends inversely correlate with sediment lightness, which decreases downcore from ~70% to 50% over this interval (Fig. F26).

Lithologic Units III and IV are characterized by MS that decreases downhole from 10 x 10^-6 SI to nearly 0 SI (Fig. F26). This overall gradual decrease is mirrored by a gradual increase in lightness (45% to 55%). NGR throughout this interval is highly variable (Fig. F26). Local MS maxima may correspond to phosphate and pyrite-rich intervals at the top of lithologic Unit IV (see "Lithostratigraphy and Sedimentology"). Within the olivine basalt basement, NGR and MS values increase greatly.