PHYSICAL PROPERTIES

Physical properties at Site 1220 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 1220A and in Hole 1220B, Cores 199-1220B-9H through 20X. 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). Three in situ temperature measurements were obtained using the Adara tool in Hole 1220B.

Two methods were used to evaluate the wet bulk density at Site 1220. GRA provided an estimate from whole cores. MAD samples are a second, independent measure of wet bulk density, along with providing DBD, grain density, water content, and porosity from discrete samples (Table T16). The MAD wet bulk densities are ~0.05 g/cm³ greater than the GRA bulk density in the clay of lithologic Unit I (0-19.1 mbsf) (Fig. F23). In the radiolarian oozes of Units II (19.1-38.0 mbsf) and IV (70.6-182.5 mbsf), the discrete sample densities are consistently 0.10 g/cm³ greater than the GRA density. The two density measures are more nearly equal in the nannofossil ooze of Unit III (38.0-70.6 mbsf) and the nannofossil chalk of Unit V (187.9-200.0 mbsf). The latter unit was drilled with the XCB, but the gaps between the sediment and core liner were minimal and the match between the data sets is good.

The P/E boundary section within Unit V (Section 199-1220B-20X-2) (see "P/E Sediments" in "Lithostratigraphy") was well imaged by the GRA detector (Fig. F24). The boundary is marked by a change in density from 1.32 g/cm³ in the dark colored, clay-rich sediment above the boundary to 1.77 g/cm³ in the calcareous chalk below the boundary. The increase in metal oxides directly above the boundary is not great enough to substantially increase the bulk density (see "P/E Boundary" in "Geochemistry").

In three short intervals (92.9-94.0, 95.6-99.9, and 173.6-176.4 mbsf), GRA densities are displaced to values 0.10 to 0.15 g/cm³ greater than adjacent GRA densities. The offset is an electronic artifact. 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 XCB.

In Unit I, the clay increases in bulk density with depth as a result of compaction, from 1.25 g/cm³ near the surface to 1.35 g/cm³ at 16.25 mbsf. From 16.25 mbsf to the base of Unit I, density decreases as a result of the transition to the lower-density radiolarian oozes of Unit II. The average wet bulk density of Unit II is 1.19 g/cm³. The variable bulk density in Unit II (from 1.16 to 1.31 g/cm³) reflects the interbedding of clay and nannofossil ooze with radiolarian ooze. The wet bulk density of Unit III is higher, averaging 1.42 g/cm³, as a result of the predominance of nannofossil ooze in the unit. Radiolarian ooze and clay are interbedded with the nannofossil ooze in Unit III, resulting in a significant range in density, from 1.19 to 1.64 g/cm³. Unit IV is characterized by uniform wet bulk density. Excluding a stiff clay layer with a density of 1.34 g/cm³ at 150.25 mbsf, density ranges only from 1.17 to 1.25 g/cm³ for Unit IV. The expected increase in density with increasing overburden is minimal for the radiolarian ooze in Unit IV, most likely the result of an open fabric formed by interlocking radiolarians that resists particle rearrangement and collapse. For the sediments in Unit IV recovered with the APC (70.6-152.0 mbsf), wet bulk density increases at a rate of 0.027 g/cm³ per 100-m depth. Wet bulk density increases sharply to 1.40 g/cm³ at the boundary between Units IV and V. Within Unit V, density increases with depth, with a maximum of 1.71 g/cm³ in the nannofossil chalk of Core 199-1220B-19X at 192.94 mbsf. Core 199-1220B-20X was not sampled, but a maximum bulk density of ~2.04 g/cm³ was measured by the GRA device in Section 199-1220B-20X-2 at 198.55 mbsf.

Variation in grain density (_s) also corresponds well to the changes in lithology at Site 1220 (Fig. F23). The clays of Unit I display grain densities that range from 2.11 to 2.75 g/cm³. The range is typical of other pelagic clays at Leg 199 sites and is attributed to the range of densities in the mixture of smectite (_s = 2.2-2.6 g/cm³), zeolite (_s = 2.1-2.6 g/cm³), and calcite (_s = 2.7 g/cm³). Grain density is lower in Unit II, with most values clustered about 2.42 g/cm³, although the range in densities is from 2.30 to 2.78 g/cm³. Between the top of Unit III (38 mbsf) and 57 mbsf, grain density of the nannofossil ooze is ~2.70 g/cm³. Below 57 mbsf, grain density decreases steadily to the top of Unit IV. Grain density for the radiolarian ooze of Unit IV, excluding the stiff clay at 150.25 mbsf, averages 2.20 g/cm³ and ranges from 2.03 to 2.50 g/cm³. The nannofossil chalk of Unit V is characterized by grain density that increases from 2.50 g/cm³ at its top to 2.71 g/cm³ at 192.94 mbsf.

Porosity and water content vary inversely with wet bulk density (Fig. F23). In Unit I, porosity decreases with depth as a result of clay compaction, from 85% near the seafloor to 80% at the base of the unit. The radiolarian-rich sediments of Units II and IV are marked by high porosities, which average 88% and 85%, respectively. A slight decrease in porosity with depth occurs in Unit IV. A wide range in porosity, from 64% to 87%, characterizes the alternating nannofossil ooze and radiolarian ooze in Unit III. The porosity of the bulk of the nannofossil ooze in this unit varies between 70% and 75%. Porosity is significantly lower in Unit V, ranging from 75% at the top of the nannofossil chalk to 60% at 192.94 mbsf.

MAD analyses were performed on three chert samples, two from Unit IV (Samples 199-1220B-15X-CC, 0-2 cm, and 17X-CC, 4-6 cm) and one from Unit V (Sample 199-1220B-19X-1, 0-2 cm) (Table T16). The average wet bulk density, grain density, and porosity for the Unit IV cherts are 2.11 g/cm³, 2.33 g/cm³, and 16.9%, respectively. The chert from Unit V is distinctly harder. Wet bulk density, grain density, and porosity for this rock are 2.52 g/cm³, 2.61 g/cm³, and 5.7%, respectively, indicating more complete conversion to quartz.

LAS studies were conducted on sediments from Hole 1220A and Cores 199-1220B-9H through 19X at a frequency of two samples per undisturbed 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 T17). The results of the LAS analyses correlate well with the major lithologic boundaries (Fig. F26).

Lithologic Unit I is composed of smectite-rich (average = 84%) clay, with minor amounts of opal and calcite. The illite-smectite transition is not present in this section because of poor recovery of the water/sediment interface. Except for a few anomalous high values, illite concentrations for the most part are negligible at Site 1220, similar to all other sites. The radiolarian ooze of lithologic Unit II is marked by the expected decrease in clay (to an average of 40%) and increase in opal (from an average of 8% in Unit I to an average of 26% in Unit II). The high calcite samples (~70%) at 24 mbsf represent an interval of nannofossil ooze (see "Unit II" in "Lithostratigraphy").

Lithologic Unit III is composed of calcite-rich nannofossil ooze. Calcite concentrations increase from an average of 7.0% in Unit I and 29% in Unit II to an average of 73% in Unit III. Conversely, smectite concentrations drop to an average of 13% in Unit III and opal concentrations to an average of 11%. The radiolarian ooze of lithologic Unit IV contains an average of 48% opal, 37% smectite, and 12% calcite. Clay contents also increase downhole from concentrations of ~25% near the top of Unit IV to ~40% near the bottom of this unit. This downhole increase in LAS-calculated clay concentration matches the description for clay content of the Unit IV lithology (see "Unit IV" in "Lithostratigraphy"). The stiff clay interval at 150.25 mbsf is distinctly different than adjacent sediments and is characterized by a smectite concentration of 76% and a negligible opal content. An increase in calcite (and subsequent decrease in opal) at 193 mbsf reflects the presence of nannofossil chalk in Unit V.

Compressional wave velocity was measured by the P-wave logger (PWL) on all whole cores from Hole 1220A and Cores 199-1220B-1H through 16X. The insertion and contact probe systems were used to measure velocities on split cores from Hole 1220A and Cores 199-1220B-9H through 19X (Table T18). For Units I and III, the agreement between the PWL and split core velocities is good. The data sets diverge in the radiolarian ooze of Units II and IV. The split-core transverse velocities are ~15 m/s greater than PWL velocities in Unit II and 25 m/s greater than PWL velocities in Unit IV (Fig. F27). In Unit II, the PWL and split-core velocities follow the same trends; however, a consistent pattern of variation is lacking for the PWL and contact probe velocities in Unit IV.

In Unit I, velocity increases from 1500 m/s near the seafloor to 1550 m/s at 19 mbsf (Fig. F27). Velocities in Unit II average 1545 m/s. Lower velocities between 24 and 26 mbsf coincide with more dense lighter-colored sediment, which represent an increase in calcareous constituents in the radiolarian ooze. Unit III displays more variability in velocity than the other lithologic units, with split-core velocities ranging from 1509 to 1558 m/s. The general pattern is that more dense calcareous intervals are characterized by lower velocities than the less dense siliceous intervals. Velocities increase markedly at the top of Unit IV. The transverse velocity determined by the contact probe averages 1565 m/s for the radiolarian oozes of this unit; whereas, the PWL velocities are ~1540 m/s in the unit. The coarse grain size of the radiolarian ooze and possible dewatering between PWL and split-core measurements may result in higher contact probe velocities. Velocities in the nannofossil ooze of Unit V are lower than velocities in the overlying radiolarian ooze. The average velocity for Unit V is 1548 m/s, with a maximum velocity of 1570 m/s at 192.95 mbsf. Velocities measured for the three chert pieces that were sampled range from 3180 m/s for the cherts from Unit IV to 4893 m/s for the chert from Unit IV (Table T18).

The crossplot of velocity and wet bulk density (Fig. F28) shows lithology-dependent differences in the relationship between velocity and the bulk-sediment properties. The clays of Unit I, the nannofossil oozes of Unit III, and the nannofossil chalks of Unit V are characterized by a general increase in velocity with increasing density. The radiolarian oozes of Units II and Unit IV display either no relationship between density and velocity or a weak increase in velocity with decreasing density. The lack of a relationship between density and velocity results from the stiff sediment framework 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.

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 T18) in order to examine the burial transformation of sediment fabric. The clays of Unit I are essentially isotropic, with an average anisotropy of 1.0%. Velocity anisotropy was determined for the two chert pieces from Unit IV, which contain different colored laminae. Anisotropies of 5.2% and -1.6% were determined for Samples 199-1220B-15X-CC, 0-2 cm, and 17X-CC, 4-6 cm, respectively, suggesting residual presence of bedding in the chert.

Thermal conductivity was measured on the third section of all cores from Hole 1220A and Cores 199-1220B-1H through 18X (Table T19). The conductivity is low in the clays of Unit I and the radiolarian oozes of Unit II (Fig. F29). Excluding an anomalous value of 0.95 W/(m·K) at 22.71 mbsf, the average thermal conductivity for Units I and II is 0.75 W/(m·K), which is typical for similar sediments at other Leg 199 sites. Conductivity is higher and more variable in the nannofossil ooze of Unit III. From the top of Unit III to 56.76 mbsf, it increases with depth, reaching a maximum of 1.14 W/(m·K). Below 56.76 mbsf, conductivity decreases with depth to 0.71 W/(m·K) at the top of Unit IV. Thermal conductivity is nearly constant in the Unit IV radiolarian ooze and clay, averaging 0.74 W/(m·K). The one conductivity measurement in the nannofossil chalk of Unit V has a value of 0.94 W/(m·K). The pattern of thermal conductivity dependence on porosity is similar to that at Sites 1218 and 1219 (Fig. F30). Conductivity increases with decreasing porosity for clay and nannofossil ooze sample, as a result of the decrease in the interstitial spacing. Thermal conductivity in the radiolarian oozes does not display a relationship with porosity because of the uniformly high porosity, ~85%, and the poor heat conduction in the biogenic silica that comprises the radiolarians.

In situ temperature measurements were taken using the Adara tool for three cores in Hole 1220B. Borehole temperatures range from 4.50°C at 49.50 mbsf to 7.15°C at 102.50 mbsf, with an average seafloor temperature of 1.45°C (Table T20; Fig. F31).

Heat flow at Site 1220 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). The thermal resistance was calculated assuming constant in situ conductivities in four layers, 0.72 W/(m·K) for Units I and II, 0.96 W/(m·K) for Unit III, 0.71 W/(m·K) for Unit IV, and 0.93 W/(m·K) for Unit V (Fig. F31). 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 cross plot of temperature and thermal resistance (Fig. F31). The heat flow estimate for Site 1220 is 46 mW/m², which is significantly lower than the 66 mW/m² determined for only slightly younger crust at Site 1219. The value calculated for Site 1220 is consistent with the 44 mW/m² determined at the closest point (11°3.0´N, 142°28.0´W) from the global heat flow data set (Pollack et al., 1993).

Natural gamma radiation was measured on all whole cores in Hole 1220A (Fig. F32). The highest natural gamma radiation values are present in the clay-rich lithologic Unit I and average 13.1 counts per second (cps). NGR maxima in Unit I at 8.3 mbsf (15.0 cps) and between 13.7 and 16.3 mbsf (18.0 cps) correlate with similar increases in GRA bulk density (Fig. F23). The higher values between 13.7 and 16.3 mbsf also correlate with a substantial increase in MS (Fig. F33) and high concentrations of phillipsite (K-Ca zeolite) (see "Unit I" in "Lithostratigraphy"). Below Unit I, NGR values decrease to near background levels. Values recorded in Units II, III, and IV average 2.0, 1.2, and 1.0 cps, respectively.

Whole-core MS measurements were made on all cores from Site 1220. MS in the clay-rich lithologic Unit I increases downhole from ~30 x 10^-6 SI to values as high as 145 x 10^-6 SI at 16 mbsf (Fig. F33). This susceptibility maximum correlates with a small increase in GRA bulk density, an increase in NGR, and increased concentrations of Al and Fe (see "Solid-Phase Geochemistry" in "Geochemistry").

Below 19.5 mbsf, susceptibility does not reflect major changes in lithology as well as at other Leg 199 sites. MS in the radiolarian-rich Unit II averages 25 x 10^-6 SI, with the lowest values corresponding to a layer of nannofossil ooze at ~23 mbsf. The calcite-rich nannofossil ooze of Unit III displays a small decrease in susceptibility to an average of 15 x 10^-6 SI. The upper part of Unit III (38-58 mbsf) contains several peaks in susceptibility that correlate with higher concentrations of clay in this interval (see "Unit III" in "Lithostratigraphy"). Between 58 and 69 mbsf, the sediment contains a higher concentration of nannofossils and displays more uniformly low susceptibility (~10 x 10^-6 SI).

The radiolarian-rich lithologic Unit IV has slightly higher MS values than the surrounding lithology (average = 17 x 10^-6 SI). Low susceptibility (~10 x 10^-6 SI) at 104 mbsf correlates with the lowest grain density values in the radiolarian ooze (Fig. F23) and a local maximum in LAS opal concentration (Fig. F26). Below 145 mbsf, the susceptibility data are limited by poor core recovery; however, the P/E boundary section, Section 199-1220B-20X-2, was well documented by the MS detector (Fig. F24). The transition from nannofossil chalk to the metal oxide-rich clay is marked by an increase from ~25 to 113 x 10^-6 SI at 199.38 mbsf. This difference is comparable to the change at the boundary between the clay of Unit I and the radiolarian ooze of Unit II but occurs over a much narrower interval. Below 199.38 mbsf, susceptibility decreases to a minimum of 45 x 10^-6 SI, coinciding with the reddish layer. The small spike to a low MS value at 199.53 mbsf is suspected to be an anomalous point. A second, broad peak of 88 x 10^-6 SI at 199.70 mbsf lies just above the P/E boundary (Fig. F24). The changes in susceptibility correlate well with zones of metal enrichment in the multicolored clay (see "P/E Boundary" in "Geochemistry"). Below the boundary, the susceptibility of the nannofossil chalk returns to 25 x 10^-6 SI.