PHYSICAL PROPERTIES

Physical properties at Site 1218 were measured on whole cores, split cores, and discrete samples. MST (bulk density, MS, P-wave velocity, and natural gamma radiation) 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 Cores 199-1218A-1H through 22X and in every other section in Cores 23X through 30X. 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). Six in situ temperature measurements were obtained using the Adara tool in Holes 1218B and 1218C.

Two methods were used to evaluate the wet bulk density at Site 1218. GRA provided an estimate from whole cores. MAD samples gave a second, independent measure of wet bulk density, along with providing DBD, grain density, water content, and porosity from discrete samples (Table T19). The MAD and GRA bulk density measures display the same trends, but the extent of the agreement between data sets differs between APC cores and XCB cores. In APC cores, the wet bulk density is offset by values up to 0.10 g/cm³ higher than the GRA bulk density (Fig. F24). This offset is most consistent in lithologic Unit I (0-52.10 mbsf), and results from the difference between the assumed GRA coefficient used in data processing and the attenuation coefficient of the radiolarian-rich sediments. Variation in core diameter and homogenized intervals between drilling biscuits both contribute to the difference between the MAD and GRA densities in the XCB-drilled section. Crossplots of wet bulk density and DBD vs. interpolated GRA density (Fig. F25) show excellent correlation between the MAD and GRA data for sediments recovered with the APC and more variable and underestimated GRA density for XCB cores.

Wet bulk density is ~1.20 g/cm³ at the seafloor in Hole 1218A and increases to an average of 1.25 g/cm³ in a broad maximum between 14 and 31 mbsf. From 31 mbsf to the bottom of lithologic Unit I at 52.10 mbsf, wet bulk density decreases to 1.18 g/cm³. The boundary between the radiolarian clay of Unit I and the nannofossil ooze of Unit II is marked by a sharp increase in density to 1.59 g/cm³ at 57.95 mbsf. Wet bulk density is highly variable in the upper part of Unit II (52.1-112.50 mbsf in Hole 1218A), with values ranging from 1.19 to 1.64 g/cm³ and averaging 1.49 g/cm³. The variation in density follows the alternating lithology. Dark colored clayey radiolarian nannofossil ooze is less dense than the lighter colored nannofossil ooze. Between 112.50 and 187.45 mbsf in Unit II, the range in wet bulk density is narrower (1.48-1.72 g/cm³), and the average density is higher than sediments above (1.64 g/cm³). The pattern of lower-density dark colored sediments and higher-density light colored sediments continues in Unit II. A prominent bulk density minimum of 1.62 g/cm³ is present at 152.99 mbsf. From 196.76 mbsf to the base of Unit II at 216.90 mbsf, variability in bulk density increases, and the average density is lower (1.59 g/cm³). The greater abundance of diatoms, radiolarians, and clay in the radiolarite of Unit III (216.90-267.44 mbsf) is reflected by wet bulk density that is lower and more variable than the density in the lower part of Unit II. Average wet bulk density for Unit III is 1.42 g/cm³, and the range is from 1.28 to 1.66 g/cm³. Unit IV is marked by an overall increase in density with depth. Wet bulk density ranges from 1.48 to 1.61 g/cm³ in Subunit IVA (250.2-267.4 mbsf) and from 1.88 to 1.97 g/cm³ in Subunit IVB (267.4-274.3 mbsf).

Variation in grain density (_s) in Hole 1218A generally matches changes in lithology. Grain density averages 2.60 g/cm³ in the uppermost 5 m in Hole 1218A. Below 5 mbsf, it decreases and becomes more variable (Fig. F24), coinciding with the LAS-indicated decrease in illite (_s = 2.66 g/cm³) and increase in smectite (_s = 2.2-2.6 g/cm³) in the red clays. Along with the change in clay mineralogy, the changes in the mixture of radiolarians (opal; _s = 2.2 g/cm³), zeolites (_s = 2.2 g/cm³), and nannofossils (calcite; _s = 2.7 g/cm³) contribute to the large range in grain density (2.17-2.82 g/cm³) in Unit I. The average density for Unit I is 2.58 g/cm³. Grain density averages 2.69 g/cm³ between 52.10 and 112.50 mbsf in lithologic Unit II. Densities range from 2.59 to 2.80 g/cm³, with lower values generally associated with the darker colored, more clay-rich sediments. Between 112.50 and 187.50 mbsf, grain densities are tightly grouped about an average of 2.72 g/cm³. Within this interval, there is a slight decrease in grain density from ~2.73 g/cm³ at 150 mbsf to 2.64 g/cm³ at 187.45 mbsf, which coincides with a decrease in the CaCO₃ content (see "Geochemistry"). Below 187.50 mbsf, the variability of grain density in Unit II increases, but the average decreases to 2.67 g/cm³. Unit III is characterized by highly variable grain density as a result of increasing abundances of radiolarians and diatoms. Grain density ranges from 2.15 to 2.89 g/cm³ and averages 2.46 g/cm³ in this unit. A peak in iron and manganese concentrations within this interval (see "Geochemistry") coincides with the high grain density. In Unit IV, grain density increases more or less continuously from 2.42 g/cm³ at 255.62 mbsf to 2.78 g/cm³ at 273.75, reflecting an increase in calcite and dolomite in the sediments.

Porosity and water content vary inversely with wet bulk density (Fig. F24). Features prominent in the bulk density profile, including the sharp change at the boundary between Units I and II, the higher variability in the upper part of Unit II and Unit III, and the prominent maxima (density minima) at 153 mbsf, are also present in the porosity profile. The highest porosity (90.7%) is present in the radiolarian clay of Unit I at 38.95 mbsf. The lowest porosity (43.7%) is present at 268.08 mbsf in the dolomitic nannofossil chalk (Subunit IVB) (see "Subunit IVB"  in "Lithostratigraphy").

LAS studies were conducted on cores from Hole 1218A 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 T20). LAS analyses do not display the major lithologic boundaries as well as at previous sites (Fig. F26). The light color of the clays in Units I and III may have caused an overestimation in calcite.

Lithologic Unit I shows high clay contents and a gradual increase in calcite downcore. Also, the illite-smectite transition is clearly seen in the upper 10 m of the hole. Illite concentrations between 20 and 50 mbsf are higher than expected. These higher than expected values may reflect an increase in metal oxides that darken the color of the sediment. The carbonate-rich lithologic Unit II shows an expected increase in calcite values to ~90% as well as a corresponding drop in clay content. The clay that is present is mainly smectite. The top of Unit III marks the E/O boundary with a decrease in calcite and an increase in clay. Nannofossils are still abundant in Unit III resulting in the high calcite values in this region. Unit IV is another nannofossil chalk and again contains higher calcite values (~60%).

Compressional wave velocity was measured by the P-wave logger (PWL) on APC whole cores from Holes 1218A, 1218B, and 1218C and by the insertion and contact probe systems on split cores from Hole 1218A (Table T21). For XCB cores, cube samples were cut with the dual-bladed rock saw, allowing determination of velocities in the y- and z-directions with the contact probe system. The match between the whole-core and split-core measurements is relatively good for the insertion and the contact probe systems, with only a few anomalous points (Fig. F27).

Downhole trends in velocity do not simply follow changes in lithology or bulk properties (Fig. F27). Velocity (transverse) increases with depth in Unit I from ~1500 m/s near the seafloor to 1555 m/s at 40.46 mbsf. The sharp increase in density and decrease in porosity that occurs at the boundary between Units I and II is less prominent in the velocity profile. From 51.46 mbsf, near the base of Unit I, to 54.99 mbsf, near the top of Unit II, velocity decreases from 1538 to 1512 m/s. Overall, there is a gradual increase in velocity with depth in Unit II. Small variations in velocity most likely reflect alternations in lithology. The prominent bulk density minimum and porosity maximum in Unit II at 155 mbsf is not evident in the velocity profile, and a broad velocity maximum between 170 and 180 mbsf is not reflected in the density and porosity profiles. Unit III is marked by an anomalously high velocity (1602 m/s) near the top of the unit at 218.06 mbsf. Excluding this value, the trend in Unit III is an increase in velocity from 1554 m/s at 222.94 mbsf to 1598 m/s at 248.25 mbsf. An exceptionally high velocity for Site 1218 sediments (1716 m/s) was determined for the dolomitic nannofossil chalk in Subunit IVB at 268.09 mbsf.

The lack of consistent downhole velocity trends in Hole 1218A is partly explained by the crossplot of velocity and wet bulk density (Fig. F28). The nannofossil ooze of Unit II is characterized by a general increase in velocity with increasing density. A similar trend is apparent for the nannofossil chalk of Unit IV, although the velocities are higher at the same density, most likely reflecting a difference in the sediment bulk modulus. The radiolarian clays of Unit I and the radiolarite of Unit III differ from the calcareous sediment in their relationships with bulk density. In Units I and III, there is either no relation or a weak increase in velocity with decreasing density. This pattern possibly results from the stiff sediment fabric created by the shape of radiolarians and their interlocking spines. This stiffness produces a higher shear modulus and velocities higher than expected for the high porosity of the sediment. The difference in the trends of velocity with bulk density for the calcareous and siliceous sediments explains the lack of a prominent change in velocity at the boundary between radiolarian clay of Unit I and the nannofossil ooze of Unit II.

Velocity anisotropy was calculated from longitudinal (z-direction) and transverse (y-direction) measurements provided by the insertion probe system and the cut samples measured with the contact probe system (Table T21) to evaluate burial-induced changes in sediment fabric. The anisotropy ranges from -1.0% to 1.7% and averages 1.0% for the insertion probe system (upper 34 m of Hole 1218A). The anisotropy determined with the contact probe for sediments from 196.76 to 273.76 mbsf ranges from -0.3% to 2.6% and is marked by a clear increase in anisotropy of the sediments of Subunit IVB. The average anisotropy for the sediments below 196 mbsf is 1.0% for Unit II, 1.4% for Unit III, 0.4% for Subunit IVA, and 2% for Subunit IVB.

Thermal conductivity was measured on the third section of Cores 199-1218A-1H through 19H and 199-1218B-1H through 19X (Table T22). The thermal conductivity shows a strong dependence on lithology (Fig. F29) and porosity (Fig. F30). The radiolarian clays of Unit I display a nearly constant conductivity, which averages 0.72 W/(m·K). Thermal conductivity increases sharply at the top of Unit II to 0.94 W/(m·K). In Unit II, conductivity increases with depth to 1.22 W/(m·K) at 173.46 mbsf in Hole 1218A, with a pattern that roughly mimics that of porosity. The inverse relationship between thermal conductivity and porosity is well defined at Site 1218 (Fig. F31), with a correlation coefficient of 0.97.

In situ temperature measurements were taken using the Adara tool with four cores in Hole 1218A and three cores in Hole 1218B. The tool did not stabilize in the borehole with Core 199-1218A-4H, and the temperature could not be calculated. Borehole temperatures range from 6.22°C at 60.90 mbsf to 9.95°C at 121.50 mbsf, with an average seafloor temperature of 1.78°C (Table T23; Fig. F31).

Heat flow at Site 1218 was determined according to the procedure of Pribnow et al. (2000). The laboratory-determined thermal conductivity was used to estimate in situ thermal conductivity (see "Heat Flow Calculation" in "Physical Properties" in the "Explanatory Notes" chapter), and a linear fit through these values was used to calculate the thermal resistance (Fig. F31). Because no temperature measurements were obtained from lithologic Unit I, only the conductivities from Unit II were used for the heat flow determination. Thermal resistance was estimated for the depths of the temperature measurements, and the heat flow was obtained from the inverse of the linear fit for the crossplot of temperature and thermal resistance (Fig. F31). The heat flow estimate at Site 1218 is 67 mW/m². This value is similar to a heat flow of 75 mW/m² at the nearest point (9°3.0´N, 133°40.0´W) in the global heat flow data set (Pollack et al., 1993).

Natural gamma radiation was measured on all whole cores at Site 1218 (Fig. F32). The highest NGR values are present between the seafloor and 50 mbsf, where they average 8.6 counts per second (cps) in the clay-rich lithologic Unit I. Below 50 mbsf, values drop to <1 cps in the carbonate-rich Unit II and remain at this level to a depth of 216 mbsf. A slight increase in NGR values, to an average of 0.43 cps and a maximum of 9.72 cps, marks lithologic Unit III because of an increase in clay content.

Whole-core MS measurements correlate well with the major differences in lithology and to changes in bulk physical properties (Fig. F33). MS values in Unit I are relatively high, averaging 34 x 10^-6 SI. The expected direct relationship between bulk density and MS is not well developed in this unit. The MS record contains significant variation that is not present in the more uniform GRA bulk density profile (Fig. F24), although there is a match of the general trends. A significant decrease in susceptibility marks the top of the carbonate-rich Unit II at 52 mbsf. MS values are low in Unit II, averaging 10 x 10^-6 SI, but the variation that is present corresponds to alternations between nannofossil ooze (lower MS) and clayey nannofossil ooze (higher MS). An increase in susceptibility marks the boundary between Units II and III as the clay content increases and culminates with a maximum of 80 x 10^-6 SI at 225 mbsf. The average susceptibility in lithologic Unit III is 31 x 10^-6 SI. A decrease to MS values of ~10 x 10^-6 SI corresponds to increased carbonate concentrations at the top of Unit IV. Susceptibility increases within Unit IV to 35 x 10^-6 SI at the base of the unit.