At Site 1226 we collected a full range of physical property data from the sediment/water interface to immediately above basalt basement. The data are described below and compared with those from Site 846. At Site 846, APC coring ended at Core 138-836B-22H (206.6 mbsf). In Hole 1226B, APC coring extended to a depth of 271.9 mbsf (Core 201-1226B-29H) and in Hole 1226E to a depth of 326.0 mbsf (Core 201-1226E-20H). This allowed us to extend the depth of reliable physical property data by ~120 m. In addition, we collected IR emission and electrical resistance data that were not collected at Site 846.
Whole-round cores were first scanned for IR emission on the catwalk prior to sectioning. Immediately after sectioning, exposed section ends adjacent to the interstitial water and microbiology sections were IR scanned for temperature determination. The cores were then equilibrated to the laboratory temperature, and each section was run on the multisensor track (MST). The measurements made on the MST were magnetic susceptibility (spacing = 5 cm, data acquisition scheme [DAQ] = 2 x 1 s), gamma ray attenuation (GRA) bulk density (spacing = 10 cm, count time = 5 s), P-wave velocity (spacing = 10 cm, DAQ = 10), and natural gamma radiation (NGR) (spacing = 30 cm, count time = 15 s). Thermal conductivity measurements were made on the third section of each whole-round core in Hole 1226B. MST data were collected on cores from Holes 1226A, 1226B, 1226C, 1226D, and 1226E. IR emission and thermal conductivity measurements were made only on Hole 1226B cores. Some sections were removed from the catwalk for microbiology and interstitial water sampling. Physical properties were measured on these sections only if intact parts remained following the sampling. The likelihood of this declined after we switched from APC to XCB coring.
After splitting we took discrete undisturbed samples from each core for measuring moisture and density (MAD) (dry volume and wet and dry mass). We also measured compressional wave velocities using the digital sonic velocimeter and resistance using a third-party device.
Our sampling strategy was designed to address specific leg and related research objectives while maintaining core flow through the laboratory. Hole 1226A was scanned on the MST but not sampled for moisture and density. A full suite of physical property measurements in whole- and split-core mode were conducted on Hole 1226B cores, which extended from the sediment/water interface to the basalt basement. Hole 1226C comprised a single mudline core that was run at higher resolution and precision on the MST but without discrete MAD and P-wave data. Holes 1226D and 1226E were logged at the same rates as 1226B (see above); Hole 1226D was a disrupted mudline core, and Hole 1226E focused on areas of interest identified from Hole 1226B data. Voids and cracking were logged for the MST where separation was visible, and data were not recorded over these intervals.
MAD data, P-wave measurements from the velocimeter, and resistance data were collected only from Hole 1226B because of the high degree of correlation between the Site 846 and Hole 1226B MST data. Two MAD samples were taken per section for the top 60 m (Sections 201-1226A-1H-1 through 6H-7) and, where possible, for the lowermost 90 m (Sections 36X-1 through 47X-2). In between, MAD properties were measured at one per section. In sections where methane samples were collected, the MAD samples were co-located with the methane samples. Spot sampling of MAD was also carried out in Hole 1226E in order to confirm Hole 1226B measurements.
MAD measurements were uploaded to the ODP (Janus) database and used to calculate water content, bulk density, grain density, porosity, void ratio, and dry bulk density. PWS3 data were collected on all sections. Resistance measurements were used to compute formation factors for Hole 1226B and were compared with porosity data.
Instrumentation and measurement principles are discussed in "Physical Properties" in the "Explanatory Notes" chapter.
Each core from Hole 1226B was scanned from top to bottom on the catwalk with an IR camera, except where there were problems booting the camera and/or computer. As mentioned in "Infrared Scanner" in "Physical Properties" in the "Site 1225" chapter, extensive postcruise processing is needed to create a downcore scale. Therefore, correlations with other physical properties are not possible at this preliminary stage. However, in order to gain a greater understanding of the factors involved with taking accurate temperature measurements on the cores, we examined temperature profiles generated along different longitudinal sections of core to determine if reflection significantly alters the surface temperature. We briefly examined the differences between IR scanning during the day and night and between APC and XCB cores. Emissivity of a representative sample of mud from this site was established in order to obtain accurate temperature measurements of the section ends when they are cut on the catwalk.
The well-focused IR scan of Core 201-1226B-13H was selected to generate multiple core liner temperature profiles. The profiles differ by location of the profile line on the core liner (Fig. F11). The temperature range covered by the profiles was 18.2° to 20.7°C. The variation between profile measurements was 0.63°C, on average, with a maximum variation of 1.4°C (Fig. F11B).
A preliminary comparison of night and day scans was done by visual interpretation of the core scans using the IR software. The only noticeable difference was in the visibility of the scale. It appears that at night it is more visible on the image than during the day.
The disturbance associated with XCB coring, as well as areas where there was poor contact between the sediment and the core liner, were not identifiable with the current infrared scanning configuration. However, a large void (~30 cm) at the bottom of Core 201-1226B-43X was identifiable in the core liner images by its higher temperature (Fig. F12).
Emissivity (e) was determined using a residue from Sample 201-1225C-30H-6, 74.5-76.5 cm. A piece of electrical tape with a known emissivity (e = 0.95) was placed on the surface of the sediment. The temperature at the outer surface of the tape read by the IR camera was 27.4°C, and a thermistor inserted in the sediment directly beneath the tape confirmed that 27.4°C was the temperature of the sediment. The surface temperature of exposed sediment (i.e., not covered with tape) immediately adjacent was measured by the camera as 26.4°C (Fig. F13A). The emissivity of the camera was then adjusted so that the exposed sediment gave a temperature reading of 27.4°C (Fig. F13B). This resulted in an empirical value for the sediment emissivity of 0.84. This emissivity was subsequently used to recalibrate the temperature measurements of the section ends measured on the catwalk.
Low-field volume magnetic susceptibility was measured on the MST using the Bartington loop sensor as described in "Magnetic Susceptibility" in "MST Measurements" in "Physical Properties" in the "Explanatory Notes" chapter. The data correlate well with the Site 846 data across the entire downhole profile (Fig. F14).
Average magnetic susceptibility is very low from 0 to 265 mbsf, varying between slightly negative and ~2 x 10-5 SI units, essentially indicating the absence of ferrimagnetic minerals. There is a very slight shift to more negative values at 52 mbsf, which correlates with the lithologic boundary between lithostratigraphic Subunits IA and IB. Between 50 and 120 mbsf, a meter-scale cyclicity in the magnetic susceptibility and reflectance data are inversely correlated (Fig. F15). Using an average sedimentation rate of 40 k.y./m (Site 846; Shipboard Scientific Party, 1992), preliminary spectral analysis suggests these are Milankovitch precessional cycles. The origin of the signal is intriguing because negative susceptibilities correlate with high values in the ratio of red-green to blue electromagnetic (EMR) spectral reflectivity, which is the opposite of the expected response. More commonly, a darker color, reflecting higher organic content, correlates with low or absent magnetite. We suspect the inverse correlation may be a function of the water content of the sediment, as water has a slightly negative magnetic susceptibility (-0.72 x 10-6 cgs units) (Carmichael, 1982, p. 270). Although the chromaticity value results from a more complex computation, it probably also reflects the wavelength-specific interaction of EMR and water.
Leg 138 scientists started XCB coring at 207 mbsf, whereas we extended APC coring to 326 mbsf. Between these depths, the two data sets remain very well correlated, including the slight increase at 264 mbsf. This suggests that the coring method does not adversely affect the susceptibility measurements at meter-scale resolution. Particularly, the peak of nearly 50 x 10-5 SI units centered on depth of 243.5 mbsf is real.
The average value of magnetic susceptibility steps up to ~3 x 10-5 SI units at ~264 mbsf, which is approximately coincident with the Subunit IC/ID lithostratigraphic boundary at 272 mbsf. This is coincident with a decrease in reflectance (see "Color Reflectance Spectrophotometry" in "Lithostratigraphy"). Below 264 mbsf (the top of Subunit ID), magnetic susceptibility increases to a peak of ~10 x 10-5 SI units at a depth of 290 mbsf. It then declines to a low for Subunit ID at a depth of ~300 mbsf. Between 300 and 315 mbsf, it increases sharply to a peak of ~25 x 10-5 SI units and falls back to 6-8 x 10-5 SI units at ~325 mbsf. From this depth to the base of Unit I (400.12 mbsf) the susceptibility steadily increases to an average value of ~25 x 10-5 SI units.
Magnetic susceptibility continues to increase across Unit II to the base of the hole, where it reaches a downhole maximum of ~45 x 10-5 SI units.
At Site 1226, we collected 29 discrete samples for paleomagnetic measurements from Hole 1226E, which was not continuously cored. The sampling frequency was one sample from each core for Cores 201-1226E-9H through 14H (70.9-269.0 mbsf) and two samples from each core below this interval to the bottom of the hole (Cores 201-1226E-15H through 25X; 269.0-418.4 mbsf). Alternating-field (AF) demagnetization of the natural remanent magnetization (NRM) was conducted up to 40 mT in 10- or 5-mT steps. Anhysteretic remanent magnetization (ARM) was measured to 40 mT in 10-mT steps with a 29-µT direct current-biasing field. AF demagnetization of the ARM was conducted to 40 mT in 10-mT steps on ~24 samples. Additional samples will be processed by stepwise thermal demagnetization on shore.
Magnetic intensity and susceptibility increase in lithostratigraphic Subunit ID below 300 mbsf (see "Subunit ID" in "Unit I" in "Description of Lithostratigraphic Units" in "Lithostratigraphy") (Fig. F16). White clay- and radiolarian-bearing diatom-rich nannofossil ooze (Sample 201-1226E-18H-6, 55-57 cm) and pale yellow diatom-bearing nannofossil ooze (Sample 20H-5, 54.5-56.5 cm) in the upper and middle parts of lithostratigraphic Subunit ID show three magnetic components, including a downward drilling-induced overprint and two horizontal components (Fig. F17). In the lower part of lithostratigraphic Subunit ID, a pale yellowish white diatom-bearing nannofossil chalk with pale gray laminae (Sample 201-1226E-22X-5, 54.5-56.5 cm) has a stable downward inclination (Fig. F18).
Light reddish brown to brown foraminifer- and diatom-bearing clay-rich nannofossil chalk containing hematite in the lower part of lithostratigraphic Unit II directly overlies the oceanic basement. A dominant light brown layer (Sample 201-1226E-25X-3, 112-114 cm) and a dark brown layer (Sample 25X-CC, 13-15 cm) at the bottom of Hole 1226E show stable magnetic directions after removing the drilling overprint by 15-mT AF demagnetization (Figs. F19, F20).
Density data were measured on the MST by the GRA densitometer (spacing = 10 cm, count time = 5 s) and were calculated from the moisture and density data. Comparison of GRA data from Holes 1226B and 1226E relative to Hole 846B data reveals a consistent high-resolution density profile (Fig. F21). Figure F22 displays a 5-m moving average of the GRA density estimates from Holes 1226B and 846B, showing that the two surveys are statistically identical except between (1) ~210 and 270 mbsf, corresponding to the extended interval of advanced piston coring during Leg 201, and (2) ~375 and 410 mbsf, where the Hole 846B data erroneously predicted a density decrease in the increasingly consolidated sediments (see "Lithostratigraphy").
GRA density increases from 1.20 to 1.52 g/cm3 from 0 to 15 mbsf and then declines back to a low of ~1.20 g/cm3 at 52 mbsf. Below 52 mbsf, GRA density first increases to ~1.52 g/cm3 at a depth ~120 mbsf and remains at 1.52 g/cm3 until ~250 mbsf. At 250 mbsf, it starts to decrease to a low of 1.22 g/cm3 at 300 mbsf. From 300 mbsf to the bottom of the hole, it increases to the downhole maximum of 1.74 g/cm3. This complex profile was interpreted to be controlled by sediment accumulation rate and mix of nannofossil-diatom components (Shipboard Scientific Party, 1992) (see "Lithostratigraphy"). Regular or cyclic downhole variability in GRA density data has a spatial frequency on a scale of ~10 m.
Moisture and density measurements for Hole 1226B (Fig. F23) compare favorably to both Leg 201 GRA-derived and Leg 138, Hole 846B GRA-derived values (Fig. F22). The bulk density and porosity profiles are clearly controlled by grain density fluctuation tied to the diatom/nannofossil ratio. Sharp grain density decreases at the tops of Subunits IB and ID are precise indicators of diatom enrichment and are accompanied by sharp porosity increases resulting from the large open frustules. Subunit IC is composed of nannofossil ooze with grain densities generally between 2.58 and 2.68 g/cm3; porosity within this interval declines with depth consistent with one-dimensional consolidation models (Athy, 1930). Below the diatom-enriched interval at the top of Subunit ID (272-320 mbsf), increasing clay and siliciclastic content (see "Lithostratigraphy") increase grain densities from ~2.62 to 2.82 g/cm3 at the top of Unit II (400 mbsf). The rapid porosity decrease from 320 mbsf to the top of Unit II probably reflects both a trend of increasing consolidation and clay infilling of the chalk pore structure.
P-wave data from the MST P-wave logger (PWL) were recorded (spacing = 10 cm, DAQ = 10) for all APC cores from Holes 1226A through 1226E. The PWS3 velocimeter was used to measure P-wave velocities transverse to the core axis on all split cores from Hole 1226B, with measurements taken at two per section for Cores 201-1226B-1H through 6H, one per section for Cores 7H through 35X, and at least one per section for Cores 201-1226B-36X through 47X. Additional PWS3 measurements were made on split cores from Hole 1226E (Cores 201-1226E-18H through 20H and 22X and 25X), with sample cubes extracted from intact core sediments tested to provide information on drilling disturbance effects in XCB cores.
PWL measurements range from 1480 to 1520 m/s for both Holes 1226B and 1226E for the 0- to 272-mbsf interval (Subunits IA-IC), with depth-equivalent PWS measurements ~40 m/s faster (Fig. F24). PWS3 velocity fluctuations over 10-40 m show an inverse correlation with wet bulk density. PWS3 measurements on XCB split cores (272-418 mbsf) range from 1520 to 1740 m/s, with a mean gradient of ~1.3 m/s/m. Split-core P-wave velocity from 272 to 345 mbsf also appears to be controlled by wet bulk density in the sediment's unloaded shipboard state. Velocities of the increasingly consolidated ooze-chalk sediments from 345 to 408 mbsf increase rapidly from 1540 to 1660 m/s.
Large differences between wireline log velocities determined in the downhole environment and measurements made in the shipboard laboratory are evident below 120 mbsf (Fig. F24). The difference between the logging and PWS3 velocities from 120 to 255 mbsf reflects the effective stress sensitivity of the unconsolidated skeletal frame of the pelagic ooze sediments. In the interval between 255 and 345 mbsf, the PWS3 and wireline logging velocities are inversely related. PWS3 velocity variability can be explained by variation in the grain density component of bulk density where the sediments are not under consolidation stress. At their in situ consolidation state, sediment P-wave transmission velocities are controlled by their sensitivity to the porosity component of bulk density; shipboard and downhole porosity measurements both show a large porosity increase (70%-88%) from 255 to 305 mbsf, followed by a porosity decrease (88%-67%) from 305 to 345 mbsf (Fig. F23) (see "Downhole Measurements" in the "Site 846" chapter of the Leg 138 Initial Reports volume) (Shipboard Scientific Party, 1992). From 345 to 375 mbsf, the PWS3 and logging velocities follow an identical velocity gradient (3.33 m/s/m) and then fluctuate (±50 m/s) around a mean value of 1670 m/s for PWS3 data and 1820 m/s for the downhole logging results.
We attempted to determine drilling-related velocity artifacts associated with PWS3 measurements in XCB cores by measuring (1) intact core biscuits with minimal slurry rind at the core margin; (2) Hole 1226E piston cores over 271.9-326 mbsf, which was cored by XCB in Hole 1226B; and (3) plug samples from Hole 1226E split cores from which slurry rind was removed. We used simple ray traveltimes through a slurry rind-core biscuit model, assuming that the slurry rind has a velocity between 1500 and 1530 m/s and using the PWS3 velocities as composite measurements, to estimate the true core sediment velocity for various rind thicknesses. The results are shown as the velocity range between the solid lines for selected depth intervals in Figure F24. From this analysis, drilling disturbance appears to cause a 50- to 100-m/s decrease in recorded PWS3 velocities.
Natural gamma radiation was measured on the MST for Holes 1226B (spacing = 30 cm, count time = 15 s) and 1226E (spacing = 15 or 30 cm, count time = 15 s) and by wireline logs. Logging and MST NGR measurements exhibit a strong correlation from ~71 to 320 mbsf (Fig. F25A) (71 mbsf corresponds to the base of the drill pipe above which the logging NGR signal is attenuated). The MST peak at ~53 mbsf corresponds to the top of Subunit IB, which is composed of alternating layers of nannofossil-rich diatom ooze and diatom or nannofossil ooze. This interval has been previously shown to be exceptionally high in organic carbon (Shipboard Scientific Party, 1992) and enriched in uranium (Fig. F25B), reflecting highly reducing conditions at shallow burial depths. An anomalous drop in logging NGR measurements at ~318 mbsf, not reflected in the MST data or uranium concentrations, may be related to a sharp transition from indurated diatom ooze to nannofossil ooze.
Thermal conductivity measurements were made on Hole 1226B sediments at a rate of one per core (usually the third section, 75 cm, if this was available). Values range between 0.69 and 1.09 W/(m·K) (average = 0.85 W/[m·K]). Average normalized thermal conductivity and bulk density show a high correlation (Fig. F26), indicating that the thermal conductivity is a direct function of water content of the sediments (Bullard, 1963), consistent with Leg 138, Site 846 measurements. Thermal conductivity measurement quality was degraded between 280 and 320 mbsf and 365 and 380 mbsf, due to core disturbance and unfilled core liners.
Formation factor was determined for Hole 1226B as described in "Physical Properties" in the "Explanatory Notes" chapter, with two measurements taken per section for Cores 201-1226B-1H through 6H, one per section for Cores 7H through 35X, and at least one per section for Cores 36X through 47X. Where possible, intact core biscuits were tested to minimize drilling disturbance effects in XCB cores (Cores 201-1226B-30X and below).
Lithostratigraphy and porosity are clearly reflected in the formation factor profile (Fig. F27). Down to ~270 mbsf (Subunits IA-IC), formation factors fluctuate between 1.8 and 3.0, with transitions at (1) ~50 mbsf (Subunit IA/IB boundary), coincident with a sharp porosity increase; (2) ~119 mbsf (Subunit IB/IC boundary), coincident with a steep porosity decrease; and (3) ~265 mbsf (Subunit IC/ID boundary), coincident with the initial increase in porosity associated with the porosity high between 275 and 320 mbsf. Data quality in the interval from 275 to 300 mbsf is mixed, as drilling-related artifacts bias the resistivity measurements downward. Outlined data points in Figure F27 were measured on core biscuits, with the electrodes inserted into drilled pilot holes for samples below 360 mbsf. The large change in formation factor (from 3 to 4.5) over the 300- to 375-mbsf interval is associated with both a large porosity decrease (from 87% to 55%) and increasing induration of the sediments (see "Lithostratigraphy"). Electrical conductivity anisotropy ranges from 2% to 14% (average = 7%) in Subunits IA-IC. Anisotropy associated with the highlighted measurements from 300 to 375 mbsf ranges from 4% to 10%.
We can identify five physical property zones at Site 1226, only two of which correspond closely to lithostratigraphic subdivisions (Subunit IA and Unit II; see "Description of Lithostratigraphic Units" in "Lithostratigraphy"). Lithostratigraphic Subunits IA, IB, and IC are characterized by an absence of ferrimagnetic minerals. Although the signal is very low in Subunit IB (50-120 mbsf), it seems to be cyclic and may represent a primary depositional record. There is a subtle decline in the magnetic signal at the top of lithostratigraphic Subunit IB, but the boundary is much more strongly recognized in the lower bulk density and higher porosity and NGR, which clearly define the diatom-dominated ooze between 50 and 70 mbsf.
A sharp increase in the density is present at ~120 mbsf, which corresponds to the boundary between lithostratigraphic Subunits IB and IC, but as Figure F22 indicates, this is probably just part of a general downhole increase. Magnetic susceptibility, P-wave velocity, and grain density show little variation across this lithostratigraphic boundary, and porosity continues to decline continuously from the top of Subunit IB to the base of Subunit IC. We therefore cannot confidently recognize the Subunit IB/IC boundary using the physical property data.
A number of significant changes in the downhole trends of all physical properties occur across the interval from 264 to 272 mbsf, at or near the base of Subunit IC. Magnetic susceptibility becomes measurably positive at ~264 mbsf and then steps up again at 272 mbsf. The most significant change in density, from ~1.45 to 1.32 g/cm3, takes place at 264 mbsf, coincident with a drop in grain density and P-wave velocity and an increase in porosity. Density increases again for a short interval at ~270 mbsf, and magnetic susceptibility increases appreciably at this level. Overall, this interval corresponds to the increase in linear sedimentation rate from ~15 m/m.y. below 264 mbsf to >30 m/m.y. above this level (see "Lithostratigraphy").
Across the interval from 264 to 320 mbsf, all physical properties significantly deviate from general downhole consolidation trends and we recognize this as a distinct zone with respect to physical properties. It corresponds to an interval of mainly diatom ooze. Across this interval, bulk densities decline to an average of ~1.30 g/cm3 or less. Porosity and in situ P-wave velocity (the latter from Hole 846B; Shipboard Scientific Party, 1992) both reflect the increase in water content of these sediments, similar to the situation we described at Site 1225, Subunit IC (see "Summary and Discussion" in "Physical Properties" in the "Site 1225" chapter). Between 300 and 320 mbsf, a subzone characterized by higher magnetic susceptibility is present near the base of the diatom-rich interval.
From ~300 mbsf, bulk density and P-wave velocity increase and porosity declines on a downhole consolidation trend to the base of the hole. Average grain density increases from 2.65 to 2.85 g/cm3 down to 400 mbsf and then increases to 2.95 g/cm3 in Unit II. The increase in average grain density over this interval is probably in part a result of an increase in iron-bearing minerals, indicated by the reddish color of Unit II and the increase in magnetic susceptibility from 320 mbsf to the base of the hole, but with a sharp increase at 400 mbsf coincident with the lithostratigraphic boundary.