PHYSICAL PROPERTIES

Physical properties at Site 1217 were measured on whole cores, split cores, and discrete samples. MST measurements (bulk density, MS, P-wave velocity, and NGR) and thermal conductivity comprised the whole-core measurements. Compressional wave velocity measurements on split cores and moisture and density (MAD) analyses on discrete core samples were made at a frequency of one per undisturbed section in Hole 1217A. Intervals that were disturbed or not recovered in Hole 1217A were sampled in Hole 1217B for velocity and MAD analyses. Light absorption spectroscopy (LAS) analyses were performed on the MAD samples as well as an additional one sample per section (located ~50 cm from the MAD sample) in Hole 1217A.

Two methods were used to evaluate the wet bulk density at Site 1217. GRA provided an estimate from whole cores. MAD samples gave a second, independent measure of wet bulk density as well as providing DBD, grain density, water content, and porosity from a discrete sample (Table T13). The MAD and GRA bulk density measures display the same trends, but the MAD densities are offset to higher values by 0.05-0.10 g/cm³ (Fig. F20). Crossplots of wet bulk density and DBD vs. interpolated GRA density (Fig. F21) show that despite the offset, the overall match between the data sets is excellent.

Wet bulk density at Site 1217 is highest between the seafloor and 22 mbsf, with values averaging 1.37 g/cm³. Below 22 mbsf, wet bulk density decreases sharply, reaching a minimum of 1.18 g/cm³ at 25.58 mbsf. The density of the nannofossil ooze in lithologic Unit I (between 27 and 32 mbsf) is distinctly higher at ~1.28 g/cm³. Below the nannofossil ooze, the increase in abundance of radiolarians results in lower wet bulk density. Density values average 1.19 g/cm³ with little variation in the sediments recovered from the lower part of Units I and II. The nannofossil chalk of Unit III was sampled at 128.83 mbsf and has a wet bulk density of 1.79 g/cm³.

Grain density (_s) averages 2.62 g/cm³ in the uppermost 5 m at Site 1217. Below 5 mbsf, it decreases and becomes more variable, coinciding with the LAS-indicated decrease in illite and increase in smectite in the sediment (Fig. F20). A grain density increase below 22 mbsf accompanies the LAS-indicated increase in calcite. Between 5 and 34 mbsf, grain density averages 2.53 g/cm³. Below 34 mbsf, grain density is lower and more variable, which coincides with an increase in radiolarian abundance. The average grain density for the lower part of Units I and II is 2.32 g/cm³. The grain density in Unit III is 2.73 g/cm³.

Porosity and water content vary inversely with wet bulk density (Fig. F20). Porosity averages 75% in the uppermost 22 m at Site 1217. Below 22 mbsf, porosity increases sharply and remains high, averaging 88%. The porosity of the Unit III nannofossil chalk is 55%.

LAS studies were conducted on cores from Hole 1217A at a frequency of two samples per section (see Vanden Berg and Jarrard, this volume, for a discussion of the LAS technique). Semiquantitative mineral concentrations were calculated from the collected spectra, assuming a four-component system: calcite, opal, smectite, and illite (Table T14). LAS analyses display the major changes in mineralogy over the different lithologic boundaries (Fig. F22). The upper 10 m of clay contain the distinct illite/smectite transition zone also seen at Sites 1215 and 1216. In the nannofossil ooze between 27 and 32 mbsf, calcite concentrations increase to ~43%, whereas clay contents decrease. The high opal sample (58%) located in this region marks a small bed of radiolarian ooze located at 28 mbsf. An increase in opal concentrations from 25% to 50% marks a change from clay-rich sediments (smectite rich) to a radiolarian ooze with clay at 52 mbsf. Over the interval from 52 to 90 mbsf, clay (smectite) contents increase downcore, whereas opal contents decrease. Lithologic Subunit IIB was not sampled because of severe core disturbance. The final LAS sample is located at 128.8 mbsf and contains 63% calcite, which is consistent with the lithologic description of a nannofossil chalk.

Compressional wave velocity was measured by the P-wave logger (PWL) on whole cores from Holes 1217A, 1217B, and 1217C and the insertion and contact probe systems on split cores from Holes 1217A and 1217B (Table T15). The match between the whole-core and split-core measurements is relatively good for the insertion probe system, but the contact probe values are ~25 m/s higher than the PWL values (Fig. F23). The general trends in the velocities are an increase from 1470 m/s near the seafloor to 1550 m/s at ~30 mbsf, a decrease to ~1510 m/s at 35 mbsf, and higher, more variable velocities below 35 mbsf. The maximum velocity measured was 1595 m/s for the nannofossil chalk of lithologic Unit III.

Velocity anisotropy was calculated from longitudinal (z-direction) and transverse (y-direction) measurements provided by the insertion probe system (Table T15) to evaluate burial-induced changes in sediment fabric. The anisotropy ranges from -0.7% to 1.8% and averages 0.9% (omitting an anomalous value of 3.0%) with no consistent trend with depth.

Thermal conductivity was measured on the third section of cores from Hole 1217A (Table T16). The thermal conductivity averages 0.75 W/(m·K). As is the case at Sites 1215 and 1216, there is a general inverse relationship between thermal conductivity and porosity at Site 1217.

NGR was measured on all whole cores at Site 1217 and displays trends similar to the other physical properties (Fig. F24). Between the seafloor and 25 mbsf, NGR values are high (~25 counts per second [cps]). Unlike Sites 1215 and 1216, the LAS-indicated illite/smectite transition in the upper 10 m is not marked by a decrease in NGR values. Below 25 mbsf, the NGR values decrease to ~1 cps and remain at this level to the bottom of Hole 1217A.

Whole-core MS measurements display greater variability than other properties and only somewhat follow the identified changes in lithology (Fig. F25). The susceptibility increases from 50 x 10^-6 SI at the seafloor to nearly 200 x 10^-6 SI at 12 mbsf. This increase coincides with a 0.2 g/cm³ increase in the GRA bulk density (Fig. F20) and the transition from illite to smectite clay. Between 12 and 25 mbsf, MS values decrease back to the range of 75 to 125 x 10^-6 SI. Significantly lower susceptibility (~35 x 10^-6 SI) marks the calcite-rich nannofossil ooze between 27 and 32 mbsf. Between 32 and 51 mbsf, MS values again increase to ~80 x 10^-6 SI as a result of an increase in clay content. Lithologic Subunit IIA, radiolarian-rich clay, is characterized by another drop in MS to values of ~50 x 10^-6 SI.