Measurements of whole sections of core taken at Site 1138 included magnetic susceptibility, gamma ray attenuation porosity evaluator (GRAPE) bulk density, and natural gamma radiation (NGR). We also determined the following physical properties data: (1) index properties data (bulk density, grain density, porosity, and water content) in discrete samples, (2) wet bulk density in discrete samples, (3) compressional wave velocities (Vp) through the working half of the core and in discrete samples, (4) magnitude of velocity anisotropy, and (5) thermal conductivity for sediments and basement rocks.
We obtained index properties data (bulk density, grain density, porosity, and water content) using gravimetric methods on discrete samples from Site 1138 (Table T17). Overall downhole trends in index properties show great variability at Site 1138 (Fig. F79 and enlarged subset in Fig. F80). From the seafloor to ~112 mbsf in Unit I (foraminifer-bearing diatom clay and ooze with scattered ice-rafted pebbles) (see "Lithostratigraphy"), wet bulk density is low (<1.5 g/cm3), porosity is high (>70%), and average grain density is 2.4 g/cm3.
Between ~112 and ~155 mbsf, within Unit II, index properties change significantly. Grain density increases from 2.3 to 2.8 g/cm3, porosity decreases from ~80% to ~50%, and bulk density ranges from 1.2 to 1.8 g/cm3. These changes reflect lithologic changes from foraminifer-bearing nannofossil clay to white foraminifer-bearing nannofossil ooze. The carbonate content of these sediments also changes abruptly, from ~30% at a depth of ~112 mbsf to over 85% near a depth of ~155 mbsf (see "Lithostratigraphy"). Within the first two units (Units I and II), the porosity-depth trend changes at the boundary of Subunit IIA and IIB. Unit I and Subunit IIA sediments consist of gray foraminifer-bearing diatom clay and light gray foraminifer-bearing nannofossil clay respectively, and here the porosity changes more with depth than in the foraminifer-bearing nannofossil ooze of Subunit IIA.
From just below a depth of ~150 mbsf, below the contact between Subunits IIA and IIB, to a depth of ~310 mbsf, bulk density, grain density, and porosity change little, except for some variation in grain density in the upper 30 m of Subunit IIB.
Between ~320 and ~640 mbsf, bulk density decreases from 1.66 g/cm3 at ~320 mbsf to 2.00 g/cm3 at ~640 mbsf, and porosity decreases from an average value of 62% at the top to 37% at the bottom. Grain density maintains a nearly constant value of ~2.6 g/cm3. In this zone, the dominant lithologies are foraminifer-bearing and nannofossil chalks with intervals of nannofossil claystone, and carbonate contents are uniformly high.
In the lower part of Unit IV through Unit VI, index properties values are scattered and relatively sparse. Nevertheless, the slope of all index properties data changes noticeably at the boundary between Units V and VI (~670 mbsf), corresponding to a change in lithology from glauconitic calcareous sandstone to silty claystone.
At the boundary between Unit VI and basement (~698 mbsf), index properties change abruptly (Figs. F79, F80). Between ~718.4 and 733.3 mbsf, bulk density for basement Units 1 and 2 (flow-banded dacite, and pumice breccia and clay, respectively) ranges from 1.71 to 2.00 g/cm3, grain density changes between 2.44 to 2.75 g/cm3, and porosity varies from 44% to 55%. All index properties change markedly again near 747 mbsf of the transition to the basaltic basement units. Bulk density and grain density increase downhole with mean values of 2.56 and 2.96 g/cm3, respectively, and porosity decreases to a mean value of 21% (Fig. F80).
Bulk densities were also estimated from whole-core GRAPE measurements taken in sections recovered from Hole 1138A (Fig. F81A), which provide a semicontinuous record. The GRAPE data show fairly constant bulk densities with depth to ~320 mbsf. Deeper data exhibit trends similar to those seen in the index properties (see previous paragraphs). Maximum GRAPE densities (right side of the bulk density profile) (Fig. F81A) correlate best with bulk densities obtained from discrete samples (solid line in Fig. F81A). Discrete sample data for the basement units consistently show higher values than the GRAPE density data. As noted previously in the other Leg 183 site chapters, the larger scatter in the GRAPE bulk density data for the basement units results from the fractured nature and narrow diameters of the cores, which do not fill the core liner (see "Physical Properties" in the "Explanatory Notes" chapter).
NGR measurements in the sedimentary sections of Hole 1138A show positive peaks of >10 counts per s (cps) at depth ranges centered around ~100, ~660, and ~700 mbsf (Fig. F81). These intervals correspond to the foraminifer-bearing diatom clay in Unit I, the glauconite-bearing calcareous claystone and sandstone in Units IV and V, and the dark brown siltstone in Unit VI, respectively. In the rest of the Site 1138 section, NGR values vary little, except for a relatively significant increase (>5 cps) at ~490 mbsf, probably reflecting the K/T boundary (Core 183-1138A-52R). In basement Units 1 and 2 (718.4-833.3 mbsf), NGR values fluctuate between 8 and 32 cps with a mean value of 17 cps. NGR values for other basement units average 7 cps (Fig. F81C).
We determined magnetic susceptibility on all cores from Site 1138, with whole-core sections measured at 4-cm intervals by the Bartington meter, and split-half sections at 2-cm intervals by the point-susceptibility meter. The characteristic susceptibility peaks and troughs correlate with flow boundaries. Detailed results are discussed in "Paleomagnetism" in conjunction with the NRM pass-through and discrete sample measurements.
Variations in compressional wave velocity downhole commonly correlate with changes in lithology. At Site 1138 we calculated compressional wave velocity from discrete samples in split-core sections or cut samples (Fig. F79D). Measurements were generally made into the core (x direction), although we also measured some discrete samples in the other two directions to investigate velocity anisotropy (Table T18).
The compressional wave velocity data for sedimentary Units I and II show very little scatter ranging from 1514 to 1865 m/s (Table T18; Fig. F79D). Velocities in Unit III increase with depth, ranging between 1700 and 3190 m/s. A slight change in the velocity trend just above the boundary of Subunit IIIB may correspond to a diagenetic change in the sediment (see "Lithostratigraphy"). Velocities for Units IV through VI are more scattered with a mean around 2500 m/s.
Velocities for the basement units vary more (Fig. F80), from 1884 to >6000 m/s, with a mean value of 4014 m/s. The highest velocities of >6000 m/s correspond to lava flows forming basement Units 13 through 19 with significant changes in flow morphology (see "Physical Volcanology"). In particular, velocities in basement Unit 13 increase dramatically downhole, from ~3000 m/s at the top to >6000 m/s at the bottom of the unit. This increase in velocity appears to correspond to the vesicular basalt grading into massive basalt. Velocities >6000 m/s could be a consequence of measurements on cut samples with a relatively small length that often gives larger uncertainties, combined with the calibration method (see "Physical Properties" in the "Explanatory Notes" chapter).
Velocity anisotropy at Site 1138 is generally low (<15%). However, velocity anisotropy is found in the chalks of Subunit IIIB and Unit IV. As shown in Figure F79D, measurements made in the x and y directions (parallel to layers) yielded relative larger velocities than measurements done in the z direction (perpendicular to the layers). This velocity anisotropy in sediments with laminations may arise because the densest layer and less stress release provides a better route for wave propagation, whereas the perpendicular wave must propagate through all layers.
We determined thermal conductivity on selected lithified sediments and basaltic rock samples from Site 1138 (Fig. F82, Table T19). In the sedimentary section, thermal conductivity values for Unit I average 0.7 W/(m·K). Between ~115 and ~300 mbsf (Unit II and part of Unit III), thermal conductivity averages 1.1 W/(m·K). The volcanic basement units exhibit higher thermal conductivity values ranging between 0.6 and 2.2 W/(m·K), with a mean value of 1.3 W/(m·K) (Table T19).
The physical properties data obtained at Site 1138 vary greatly downhole. Offsets and changes in slope can be correlated with distinct changes in lithology. Changes from siliceous to calcareous-dominated lithologies are evident in fluctuations in index properties and sonic velocities. The variations in index properties and velocities within the basement units reflect the effect of alteration and brecciation on the physical properties of basalt at this site.