In this section we describe the downhole distribution of physical property data collected from Site 1225 and compare these data with those from Site 851. Where data were collected at higher spatial resolution than at Site 851, we explain the scientific reasons and objectives for this sampling strategy and interpret the results. New data that were not collected at Site 851 include IR emission and electrical resistance.
A variety of physical property measurements were routinely collected on whole-round sections at Site 1225 to provide downhole data for correlation purposes and for integration with chemical and biological samples. A scan of IR emission along the entire core surface was recorded for most cores from Hole 1225A prior to sectioning and sampling on the catwalk. After sectioning, multisensor track ([MST] magnetic susceptibility, GRA density, P-wave velocity, and natural gamma radiation [NGR], for all holes) and thermal conductivity measurements (for Hole 1225A only) were made on whole-round sections, with the exception of those sections removed on the catwalk for microbiological sampling. Several IR images of exposed section ends were taken to monitor inner core temperature and warming during catwalk sampling. From split cores, we took discrete undisturbed samples for measuring moisture and density (MAD) properties (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. For Holes 1225A and 1225C, measurement spacings, count times, and data acquisition schemes (DAQs) used for the MST were as follows: magnetic susceptibility meter (spacing = 5 cm, DAQ = 2 x 1s), GRA densitometer (spacing = 10 cm, count time = 5 s), P-wave logger (PWL) (spacing = 10 cm, DAQ = 10), and NGR (spacing = 30 cm, count time = 15 s). Hole 1225B comprised a single core that was run at higher resolution and precision on the MST: magnetic susceptibility meter (spacing = 2 cm, DAQ = 1 x 10 s), GRA densitometer (spacing = 2 cm, count time = 10 s), PWL (spacing = 10 cm and 2 cm, DAQ = 10), and NGR (spacing = 15 cm, count time = 30 s). Voids and cracking were logged for the MST where separation was visible, and data were not recorded over these intervals.
MAD, P-wave velocity from the velocimeter, and resistance data were only collected from Hole 1225A because of the high degree of correlation between the Site 851 and Hole 1225A MST data. Samples were taken at four per section for the top 10 m (Sections 201-1225A-1H-1 through 2H-6) and the lowermost 20 m (Sections 33H-6 through 35X-4). In between, properties were measured at one per section. MAD measurements were uploaded to the ODP (Janus) database and were used to calculate water content, bulk density, grain density, porosity, void ratio, and dry bulk density. P-wave velocities were measured in two directions (PWS1 and PWS2) over the interval 0-3.54 mbsf (Section 201-1225A-1H-3), and thereafter, only PWS3 data were able to be collected. Resistance and porosity measurements were used to compute formation factors for Hole 1225A.
Most of the physical property data show breaks or variation in downhole trends that coincide with the lithostratigraphic boundaries defined by the sedimentology (see "Lithostratigraphy"). We have indicated these on the accompanying diagrams and explain their physical origin in the summary.
Instrumentation and measurement principles are discussed in "Physical Properties" in the "Explanatory Notes" chapter.
Except where there were problems booting the camera and/or computer, each core from Hole 1225A was scanned from top to bottom on the catwalk with an IR camera. The camera was mounted on a trolley and pushed by hand along the core, which resulted in inconsistent scan times ranging between 30 s and 1 min and scan video files ranging from 150 to 330 images. Because of these inconsistencies, extensive postprocessing is required to create a composite downhole plot of depth against core liner temperature. This precluded correlations with other properties. However, a preliminary examination of the data set was conducted.
Selected files were analyzed to produce downhole plots of core liner temperature vs. scan time (i.e., vs. relative depth) (Fig. F13). The plots illustrate that core liner temperature varies by up to 2°C along each core, but there is no consistent pattern to this variability from core to core. We speculate that this absence of a pattern between cores, some scanned only an hour apart, might be attributed to two factors. First, the camera has a significant sensitivity to reflection and ambient light. Although we employed a shield to minimize this effect, the shield may not be adequate. Inconsistencies in ambient light arise from the time of scanning (e.g., day vs. night) and possibly from a slight rocking in the motion of the trolley. Second, the cores are handled by many people prior to scanning on the drill floor and on the catwalk, and it is possible that the duration of their contact with the core results in the minimal temperature variation measured along the length of the core liner.
A comparison of scan profiles of cores from similar depths in Holes 1225A and 1225C (Cores 201-1225A-12H and 201-1225C-12H) was attempted, but no correlation was found, despite similar scan times (Fig. F14).
Single IR images were recorded from a selection of section ends before capping, in order to obtain information about the radial temperature distribution at the time of microbiological sampling (see Fig. F15 as an example). We conducted two experiments on the catwalk during which time-lapse images of a warming transverse (base of Section 201-1225A-11H-6) and longitudinal section (Section 26H-6) were acquired every 15 s over a continuous period of 18 min. The ambient air temperature was 28°C. For the transverse section, the initial temperature profile core center temperature was 9.6°C and liner temperature was 18.4°C at time zero. At the end of the experiment, temperatures were 18.5° and 21.0°C, respectively (Fig. F16). We conducted a similar longer-term experiment in the shipboard laboratory to examine the warming of the interior of the cores. Every 15 min, 10 cm was cut from Section 201-1225C-20H-7. For these video sequences, the temperatures seen on the IR images were calibrated against core temperature measured by thermistor. Thermal conductivity was also recorded, and a sample was removed for emissivity analysis of the sediment.
From these preliminary tests we can conclude that the cores from this site have surface temperatures that vary on the order of 2°C along the core when they arrive on the catwalk. The temperature at the center of each core was on the order of 10°C lower than the surface temperature. The ends exposed after sectioning take at least 20 min to equilibrate to ambient temperature. Overall, the setup used for data collection at Site 1225 does not lead to consistent and repeatable temperature measurements of the cores in the range of conditions encountered.
Low-field volume magnetic susceptibility was measured on the MST using the Bartington loop sensor described in "Magnetic Susceptibility" in "MST Measurements" in "Physical Properties" in the "Explanatory Notes" chapter. The data correlate well with the Site 851 data across the entire downhole profile (Fig. F17). Average magnetic susceptibility varies between 3 x 10-5 and 8 x 10-5 SI units from 0 to 69 mbsf.
From 69 to 72 mbsf, the magnetic susceptibility declines steadily, consistent with the record from Site 851. From 72 to 190 mbsf, magnetic susceptibility remains level at ~0 x 10-5 SI units. Between 190 and 230 mbsf, magnetic susceptibility increases to ~10 x 10-5 SI units. From there it declines steadily to 300 mbsf, where it again reaches 0 x 10-5 SI units, except for a peak at 256 mbsf. Between 300 mbsf and the base of the sedimentary section, magnetic susceptibility climbs back to an average of ~2 x 10-5 to 3 x 10-5 SI units.
Our data for Hole 1225A show high magnetic susceptibilities (up to 1292 x 10-5 SI units), primarily in sections 1 and 2 of every core, and sporadically below this in most cores below Core 201-1225A-3H. Chips of rusty metal, 1-2 mm in diameter and present variably in layers up to 10 cm thick or as individual grains, are visible through the core liner. We believe these chips came from the drill pipe, concentrated at the base of the hole during each core trip, and then were sampled at the top of the succeeding core. The problem persisted to the base of Hole 1225A but did not affect Holes 1225B or 1225C.
Cores from Hole 1225A were run on the cryogenic magnetometer to correlate with data collected at Site 851 and to investigate whether the paleomagnetic record is preserved in the lower part of the hole where magnetic susceptibility is high. Paleomagnetic data were not collected at Site 851 below 150 mbsf. Study of the Site 1225 cores only included those taken from zones of high magnetic susceptibility (Cores 201-1225A-1H through 11H [0-90 mbsf] and Cores 21H through 34H [190-300 mbsf]). The first two cores were measured for natural remanent magnetization (NRM) and were demagnetized to peak alternating fields (AFs) of 5, 10, 15, and 20 mT. The remaining cores were measured for NRM and cleaned at 15 mT. In order to correct for orientation, the magnetic readings from the Tensor tool and the local geomagnetic deviation (8.9°) were added to the declination measured by the cryogenic magnetometer.
The results correlated well to the detailed magnetic work conducted at Site 851. There is more noise at the top of the cores, likely due to the iron filings from the drilling, mentioned above. In the interest of time, our paleomagnetic interpretation was limited to the identification of normal and reversed epochs in the uppermost 90 m (Fig. F18). The reversal record for the lowermost section (190-300 mbsf) is less clear (Fig. F18). However, it does appear to contain a reversal record. Therefore, we conclude that the signal has not been completely degraded by time. This could suggest stability in the iron geochemistry of this zone over millions of years.
At Site 1225, we collected 32 discrete samples for paleomagnetic measurements from the working halves of the core. The sampling frequency was one sample from each core from Hole 1225C (Cores 201-1225C-1H through 27H; 0-254.5 mbsf). AF demagnetization of the NRM was conducted up to 40 mT in 10- or 5-mT steps on ~29 samples. 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. The rest of the samples will be thermally demagnetized on shore.
The intensity after 20-mT AF demagnetization in Hole 1225C samples agrees with the intensity after 15-mT AF demagnetization measured by the cryogenic magnetometer in Hole 1225A samples (Fig. F19). Lithostratigraphic Subunits IA and IC (see "Lithostratigraphy") exhibit higher magnetic intensity. The uppermost brown sand-sized foraminifer-, diatom-, and radiolarian-rich nannofossil ooze containing iron and manganese oxides (Sample 201-1225C-1H-2, 93.5-95.5 cm) strongly shows the effect of downward drilling-induced overprint (Fig. F20). A pale brown layer (Sample 201-1225C-1H-3, 109-111 cm) below the brown layer shows a more stable magnetization (Fig. F21). The white foraminifer-, diatom-, and radiolarian-rich nannofossil ooze with purple pyrite laminae layers (Sample 201-1225C-5H-1, 138-140 cm) in lithostratigraphic Subunit IA shows a magnetic direction that was not demagnetized by AF demagnetization up to 40 mT (Fig. F22).
Pale yellow and yellowish white diatom-bearing nannofossil ooze layers (Samples 201-1225C-23H-5, 69-71 cm, and 26H-3, 87-89 cm) in lithostratigraphic Subunit IC show higher intensity than those in Subunit IA. AF demagnetization as high as 40 mT could not demagnetize these samples, which is probably due to postdepositional chemical remanent magnetization (for an example see Fig. F23). Thermal demagnetization is needed to determine the magnetic carrier and original directions.
Density data were measured on the MST by the GRA densitometer and calculated from the MAD data. GRA data from Holes 1225A and 1225C were compared with the Hole 851B data (Fig. F24). Data from Holes 1225A and 1225C were collected with a 5-s count time at 10-cm spacing. GRA density increases from 1.40 to 1.62 g/cm3 over the interval 0-190 mbsf and then declines back to 1.4 g/cm3 by 250 mbsf. Below 250 mbsf, GRA density first increases to 1.64 g/cm3 at a depth of ~260 mbsf and then falls to a low of 1.2 g/cm3 at 270 mbsf. At 294 mbsf, GRA density peaks at a downhole maximum of 1.74 g/cm3 before declining to ~1.54 g/cm3 at the base of the Hole 1225A.
The scatter for the 2- and 5-s count times (Holes 851B and 1225A, respectively) was compared using a 5-m moving average, and it was determined that the two are not statistically significantly different (Fig. F25). The downhole profiles correlate well and can be used for meter-scale correlation with confidence. Regular or cyclic downhole variability in GRA density data with a spatial frequency on a scale of ~10 m is greater than for any other physical property data collected at comparable spatial resolution at Site 1225.
Moisture and density measurements for Hole 1225A (Fig. F26) compare favorably to both Leg 201 GRA-derived and Leg 138, Hole 851B GRA- and mass/volume-derived values (Fig. F25). Grain density measurements (Fig. F26B) exhibit much greater scatter than the results of Hole 851B, which we attribute to initial pycnometer (volume) errors at this site. Combining Holes 851B and 1225A grain density data sets results in the following average grain densities:
The subunit-specific bulk density-porosity pattern is thus dominated by water content variability and not by changes in grain densities. Of particular interest are the four ~40-m porosity cycles in Subunit IB, in which porosities fluctuate by 10%-12%. Postcruise analysis must be performed to determine the lithologic, diagenetic, and consolidation attributes responsible for this pattern.
P-wave data from the MST PWL were recorded from all cores, with the exception of the XCB core at the base of Hole 1225A (Core 201-1225A-35X). P-wave velocities were also measured using the P-wave velocimeter insertion probe system along the core axis (z-axis) and across the core axis (y-axis) (PWS1 and PWS2, respectively). The PWS3 contact probe system was used to measure P-wave velocities through the cut half of the core (x-axis). Measurements were taken either in the z- and y-axis directions (Section 201-1225A-1H-3 and shallower) or in the x-axis direction. Very rarely were all three directions able to be recorded at a single location. Expansion cracks were variably present in the cores, probably resulting in some attenuation of the acoustic signal, but only in a few places was wetting of the core required to allow transmission of the acoustic pulse. The velocity profiles recorded by the PWL on the MST read consistently slower than those from the velocimeter (Fig. F27). From 0 to 182 mbsf, the PWS3 was the faster by ~20 m/s. Between 182 and 250 mbsf, the two are in reasonable agreement, but below this interval, the PWS3 value is again generally faster by ~15 m/s. Both downhole velocity profiles illustrate an increase from seafloor velocities to peaks at ~20 mbsf, and then they decline to lows at ~65 mbsf. Below this level velocities gradually increase, reaching ~1550 m/s just below 150 mbsf (based on the velocimeter data). Velocities then decline to downhole lows of ~1495 m/s at ~220 mbsf. Below 220 mbsf, velocities increase irregularly to reach a downhole high of ~1580 m/s. Although the general shape of the velocity profiles is similar to other physical property parameters, there is no direct correlation between peaks and troughs at depth.
Natural gamma radiation was measured on the MST track for Holes 1225A and 1225C (spacing = 30 cm, count time = 15 s) and was also measured by wireline logs. The NGR profile shows a minimal downhole trend, decreasing from ~13-14 counts per second (cps) total emission near the top of both holes to ~12-13 cps at the base of both holes (Fig. F28). There are narrow peaks of 17-19 cps at depths of 84, 110, 265, and 302 mbsf, and above 3 mbsf, the count is consistently >20 cps. There is good correlation between the GRA density, the NGR, and the wireline log data. The GRA data tend to be noisier than the wireline data, with some cyclicity present in the latter that are not obvious in the former.
Thermal conductivity measurements were made in Hole 1225A at a rate of one per core (usually section 3 at 75 cm, if this was available). Values range between 0.77 and 1.11 W/(m·K) (average = 0.89 W/[m·K]). A slight but steady downhole increase is the only perceptible trend in these data (Fig. F29A).
A plot of average normalized thermal conductivity vs. bulk density (Fig. F29B) shows almost exact correlation, indicating that the thermal conductivity is a direct function of water content of the sediments. The only obvious deviation from this relationship is at a depth of ~170 mbsf, where the thermal conductivity is lower than expected for the measured bulk density.
Several methods were used to analyze the temperature data measured by the downhole probe and to reconcile these with the thermal conductivity data. A linear fit of the temperature profile has a correlation coefficient of R2 = 0.97, indicating a deviation from the linear model that cannot be explained by measurement error. To examine deviations caused by variations in the thermal conductivity, a temperature gradient for each core was computed by dividing the core length by the measured thermal conductivity and multiplying by the average heat flow. These gradients were combined to form the composite temperature profile shown in Figure F29C. The composite result shows that variations in the measured sediment thermal conductivities cannot explain the temperature deviations from a linear profile. The temperature data are best fit by a profile that curves downward. In some cases, curved temperature profiles can be caused by downward increases in thermal conductivity. However, the thermal conductivities measured at Site 1225 show large variability and only a slight downward increase of 0.3 mW/(m·K)/km (Fig. F29A). Fitting the temperature profile curvature with a steady-state conductive model requires a rate of thermal conductivity increase eight times greater than that of the measured values. Curved profiles can also be caused by upward flow of fluids transporting heat from deeper in the section. Using a model of steady-state fluid flow with advective heat transport requires an upward flow velocity of ~2 cm/yr to match the curved profile. This flow rate is an order of magnitude greater than that obtained by fitting the Site 1225 chloride interstitial water concentrations (see "Interstitial Water" in "Biogeochemistry"). An alternative model that would generate the observed profile is a transient lowering of the basement temperature. The timing required for a basement temperature transient was not analyzed.
Formation factor was determined from resistance measurements taken using the four-needle electrode array. Resistance measurements were normalized against the resistance of surface seawater, which was measured periodically as the sediment measurements were made. The data show an overall downhole increase (Fig. F30), which is consistent with a decline in porosity with depth. The only zone that deviates from this trend lies between 265 and 287 mbsf, where the formation factor is consistently lower than the linear downhole trend of the data.
A plot of anisotropy of conductivity (Fig. F30) shows a wide scatter across Subunits IA and IE, indicating that laminations in the sediment are restricting interstitial water flow to dominantly horizontal pathways. In contrast, Subunits IB, IC, and ID are more homogeneous in their conductivity structure, suggesting equal vertical and horizontal flow.
The physical properties measured on cores at Site 1225 correlate well with those obtained during Leg 138 at Site 851. In this section, we summarize the correlation between different physical properties and interpret the physical origin of the lithostratigraphic boundaries and subunits defined in "Lithostratigraphy".
Subunit IA (0-70 mbsf) has low but consistent magnetic susceptibility and consistently high GRA density.
The boundary between Subunits IA and IB corresponds to the loss of magnetic susceptibility. The boundary is not discernible on the MST NGR record and lies within the attenuation zone of the drill pipe for the wireline NGR data. This boundary is also the point at which sonic velocity starts to increase from a reasonably stationary downhole trend in Subunit IA, but these data are very noisy and could not be used to reliably pick the transition. Similarly, there is no clear break in either of the density data sets that corresponds to the boundary. The thermal conductivity record has a much lower spatial resolution than the other records and cannot be used in defining the transition.
Subunit IB (70-200 mbsf) is characterized by little or no ferrimagnetic mineral content. The unit is also marked by an increase in infaunal activity, evidenced by the intensity of bioturbation, together with an intensification of color banding (see "Description of Lithostratigraphic Units" in "Lithostratigraphy"). These observations suggest that the loss of the magnetic susceptibility signal may have been caused by biogeochemical dissolution of magnetic minerals rather than by dilution through increased sedimentation.
There is no marked change in the density of the sediments between Subunits IA and IB, and this agrees with the sedimentary observations. Velocity and density increase, reaching maximum values adjacent to the bottom of the subunit at ~160 mbsf. The density data are, in general, less noisy than the P-wave velocity data. Up to four cycles with a regular spatial frequency of ~40 m are present in the former. As expected, porosity steadily declines with depth but only until the base of Subunit IB. Thermal conductivity generally increases from the surface to the base of Subunit IB.
The boundary between Subunits IB and IC (200 mbsf) is most clearly marked by the return of a magnetic susceptibility signature. P-wave velocity peaks at ~150 mbsf in the midpart of Subunit IB and then declines to a low that corresponds to the transition between Subunits IB and IC. This boundary also corresponds to an abrupt decrease in bulk density, which is the start of a continuous decline in this parameter until near the base of Subunit IC.
Subunit IC (200-270 mbsf) has a significant magnetic susceptibility signature. It is also characterized by a porosity increase and density decrease. Thermal conductivity steadily declines until the base of Subunit IC, where the minimum downhole value of 0.769 W/(m·K) was recorded. Subunit IC is characterized by trends in physical properties that deviate from a simple variance with depth relationship. Together, the physical properties indicate a higher than expected water content within Subunit IC. There is no obvious compositional variation in the sediment to explain this. Sediment descriptions indicate that the only differences between Subunits IB and IC are a progressive increase in the diatom content with depth in Subunit IC, together with an absence of the bioturbation that typified Subunit IB. The comparatively low average grain density for Subunit IC can be explained by the higher proportion of opal-A (SiO2·nH2O; grain density = 1.890 g/m3) compared with calcite (grain density = 2.710 g/m3) in this subunit.
Compositional data indicate that the Subunit IC/ID boundary (270 mbsf) is the horizon at which diagenetic restructuring of biogenic silica starts (see "Description of Lithostratigraphic Units" in "Lithostratigraphy"). Magnetic susceptibility is low to absent across Subunit ID (270-300 mbsf), and the reason for this is not clear. The sediments are not as intensely bioturbated as those in Subunit IB, but the green and orange/purple banding is present and the sedimentation and organic carbon accumulation rates are relatively low, as they were during the succeeding deposition of Subunit IC, suggesting dissolution is again more likely than dilution as the mechanism. Other physical parameters such as bulk density, porosity, and P-wave velocity return to the trends across this layer expected with burial.
Subunit IE (300-305 mbsf) consists of nannofossil ooze/chalk across which a relatively low magnetic susceptibility signal is present. P-wave velocities from the velocimeter are highly variable but generally increase with depth. Both GRA and bulk density are lower in this subunit than in the overlying sediment, but the data are highly variable, as are the porosity data.