At Site 1230, one deep hole (Hole 1230A; 278.3 mbsf penetration), one intermediate-depth hole (Hole 1230B; 105.0 mbsf penetration) and three shallow holes (Holes 1230C, 1230D, and 1230E; <15.0 mbsf penetration) were cored. Physical property measurements conducted on cores from all holes ranged from standard-resolution multisensor track (MST), IR camera imaging, and discrete sample property measurements on Hole 1230A cores, to IR camera and MST runs on Hole 1230B cores, to only MST runs on cores from the subsequent holes (Holes 1230C-1230E), with high-resolution acquisition on Hole 1230D. Continuous IR imaging was conducted on the catwalk prior to microbiological or interstitial water whole-round core sampling. MST data were obtained from all intact whole-round cores that were not used for microbiological and geochemical sampling. These MST data provide a good near-continuous record of physical property variation from the seafloor to 135 mbsf. For sediments recovered from below 135 mbsf, APC injection disturbance, degassing, and depressurization expansion artifacts almost completely destroyed the in situ sediment fabric. As a result, accurate electrical resistivity (formation factor) measurements were not possible and P-wave velocities were only obtained for discrete samples from the upper ~155 mbsf and from 230 to 245 mbsf. The P-wave logger (PWL) on the MST track was turned off after running the first core (~10 m) in Hole 1230A because the data were inconsistent for the reasons mentioned above.
Each section of whole-round core analyzed for physical properties was degassed on the catwalk (up to 2 hr, if necessary for safety, due to high hydrogen sulfide levels), equilibrated to laboratory temperature (2-4 hr), and then run on the MST. The standard-resolution measurements were magnetic susceptibility (spacing = 5 cm, data acquisition scheme [DAQ] = 2 x 1 s), gamma ray attenuation (GRA) density (spacing = 10 cm, count time = 5 s), and NGR (spacing = 30 cm, count time = 15 s). Thermal conductivity measurements were made on the third section of each whole-round core in Hole 1230A, where possible.
Moisture and density (MAD) properties and P-wave velocity from the digital velocimeter (PWS3) were collected at a frequency of one per section, where possible, and at higher resolution in sections with many voids or lithologic transitions. MAD samples were co-located with the methane headspace extractions, where possible, to facilitate the volumetric analysis of methane concentrations.
Instrumentation, measurement principles, and data transformations are further discussed in "Physical Properties" in the "Explanatory Notes" chapter.
Physical property data at Site 1230 can conveniently be described in terms of the lithostratigraphic subdivisions. We are able to describe the interval of low core recovery (135-225 mbsf) because of continuous triple combo and FMS-sonic tool wireline logs across this zone. All physical property data suggest a sharp boundary at 216 mbsf, the depth assigned to the Unit I/II contact (see "Description of Lithostratigraphic Units" in "Lithostratigraphy"). Above this level, variations in physical property data are small and can be interpreted in terms of sediment compositional variation, burial and compaction, and the presence of gas hydrates between 80 and 160 mbsf. GRA-based bulk density measurements are extremely noisy, reflecting depressurization and gas expansion. This process also results in artificially low density for the sediments, which masks the expected compaction signature throughout Unit I. MAD bulk density data are much less noisy, show the expected downhole compaction profile, and match the wireline density measurements except for a slight low bias of 0.06 g/cm3 in the lower half of Unit I. Whole-core magnetic susceptibility and NGR emission measurements are lower than in situ values because of volumetric expansion during recovery. However, both data sets retain sufficient downhole variability to discriminate subunits based on these properties. We note that over the common intervals the wireline logs track the complementary physical property data extremely well, with the exception of the neutron porosity tool, which has a 12%-15% high bias relative to the corresponding MAD-derived porosity.
The trend of the bulk resistivity data best defines the Unit I/II boundary with stationary trends above and below a 1.6-
m increase at 216 mbsf. Below 216 mbsf (Unit II), MAD density is higher than that in Unit I but the GRA density data are inconsistent because of degassing effects. On average, grain density, magnetic susceptibility, and P-wave velocity are all slightly higher in Unit II than in Unit I. The downhole records of each physical property are described and interpreted in detail below. The correlation between gas hydrates and physical properties is discussed in "Gas Hydrate".
Each core from Holes 1230A and 1230B was scanned with an IR camera immediately upon its arrival on the catwalk (see "Infrared Thermal Imaging" in "Physical Properties" in the "Explanatory Notes" chapter), except when the cores were very short or when hydrogen sulfide was a safety hazard. A key improvement at Site 1230 was the use of the external liquid crystal display screen, which provided a live view of the core scan, allowing immediate identification of voids and cold spots. Cold spots were of particular interest at Site 1230, as they were likely indicators of the presence of hydrates. Partial automation of image analysis allowed depth-matched downhole plots to be generated within a day. Three method development goals were therefore achieved at this site: (1) development of procedures and criteria for identification of hydrates using the IR camera, (2) correlation of camera temperatures with other physical property measurements, and (3) comparison of temperature distributions between holes (1230A and 1230B).
Hydrates were visually identified in Cores 201-1230A-18H and 19H. At this time, we discovered that the core liner temperatures at hydrate locations could be as high as ~16°C rather than the 0°-10°C previously assumed. Subsequently, using the 16°C criterion, the camera identified a cold area of interest in Core 201-1230A-26H. This area was sectioned and immediately split, revealing very cold, bubbling sediment, which was interpreted as containing disseminated hydrate. A composite image of this core is illustrated in Figure F13. The catwalk scan minimum temperature of the intact cores was 15.7°C (Fig. F13A), whereas the split core minimum temperature was -1.7°C (Fig. F13B). Temperatures as low as -3.2°C were imaged when the core was disrupted during sampling (Fig. F13C).
Downhole profiles were compiled using the method described in "Infrared Thermal Imaging" in "Physical Properties" in the "Explanatory Notes" chapter. The downhole plot of temperature recorded by the IR camera for Hole 1230A has a distinctive feature at ~70 mbsf (Fig. F14). At that depth, variability of core-scan temperatures increased, indicating increasingly common voids and cold spots.
Downhole plots of core liner temperature for Holes 1230A and 1230B are compared in Figure F15. The profiles of average scan temperature correlate well above 70 mbsf, in the hydrate-free sediment. In the sediment below 70 mbsf, the complex temperature structure was not reproducible between holes.
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. Data were collected on whole-round core sections from Holes 1230A, 1230B, 1230C, and 1229E at standard resolution (spacing = 5 cm, DAQ = 2 x 1.0 s). Hole 1230D was run at high resolution (spacing = 1 cm, count time = 10 s). Positive magnetic susceptibility is present in all cores (Fig. F16). A duplicate continuous record from Holes 1230A and 1230B is present from 0 to 90 mbsf. Below this level, the record is mostly continuous in Hole 1230A to a depth of 135 mbsf. Magnetic susceptibility was not recorded at Site 685.
Subunit IA (0-64 mbsf), a diatomaceous and nannofossil ooze, is characterized by increasing magnetic susceptibility from ~6 x 10-5 SI units at the seafloor to ~13 x 10-5 SI units at 64 mbsf. Regular oscillations of ~8 x 10-5 SI units over a 2- to 3-m scale are present. The susceptibility low of 5 x 10-5 SI units at 58 mbsf breaks the general downhole increase in this subunit.
In Subunit IB (a clay-rich diatomaceous ooze), susceptibility declines to ~6 x 10-5 SI units to 82 mbsf. Between 82 and 118 mbsf, the magnetic susceptibility data fluctuate between 5 x 10-5 and 18 x 10-5 SI units, with a peak of 28 x 10-5 SI units at 93 mbsf. There is no apparent lithologic explanation for the oscillations over this interval, although we note gas hydrates were visually observed in the stratigraphic section at ~82 mbsf. Whether this correlation is an artifact of degassing or has some significance linked to hydrate formation remains to be investigated. At the base of Subunit IB (118 mbsf), the susceptibility is ~15 x 10-5 SI units.
Across Subunit IC (118-148 mbsf), magnetic susceptibility falls to ~8 x 10-5 SI units at 121 mbsf. It subsequently peaks at ~16 x 10-5 SI units at 130 mbsf. Below this level the record is incomplete, but susceptibility appears to remain at 14 x 10-5 to 16 x 10-5 SI units to the top of Subunit ID.
Subunit ID (148-218 mbsf) had poor core recovery, so its magnetic susceptibility trends are unclear. Magnetic susceptibility on recovered segments ranges from ~5 x 10-5 to 25 x 10-5 SI units.
The magnetic susceptibility record of Unit II (216-277 mbsf) is similarly incomplete. From the few intact sections available, magnetic susceptibility ranged from 5 x 10-5 to 25 x 10-5 SI units, similar to sediments in Unit I.
At Site 1230 we collected 25 discrete samples for paleomagnetic measurements. The sampling frequency was one sample from each core from Hole 1230A (Cores 201-1230A-1H through 35X; 0-254.5 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.
Lithostratigraphic Subunits IC through ID (see "Description of Lithostratigraphic Units" in "Lithostratigraphy") exhibit higher magnetic intensity of NRM after 20-mT AF demagnetization (Fig. F17). Demagnetization of the dark brown pyrite-bearing diatom-rich silty clay in lithostratigraphic Subunit IC reveals magnetic directions that are unstable (Fig. F18). In lithostratigraphic Subunit ID, magnetic intensity correlated with magnetic susceptibility despite poor core recovery (Fig. F17). Lithostratigraphic Subunit ID, which consists of dark brown clay-rich diatom ooze alternating with lighter olive diatom ooze, shows stable magnetization. In general, the former appears to be less fractured and brittle than the latter. The less brittle clay-rich, well-consolidated layer (Sample 201-1230A-19H-2, 10-12 cm) has a stable direction (Fig. F19). The fractured diatom-rich fissile layer (Sample 201-1230A-22H-1, 60-62 cm) shows a downward drilling-induced overprint (Fig. F20). Further demagnetization is needed to determine original directions.
Bulk density data were measured on the MST by the GRA densitometer (spacing = 10 cm, count time = 5 s, for Holes 1230A and 1230B) and were calculated from mass/volume measurements of discrete samples from Hole 1230A. Additional MAD data, including porosity and grain density, were also calculated from these mass/volume measurements. Wireline bulk density data are available for the interval from 80 to 270 mbsf. The wireline log measurements are generally 0.02-0.08 g/cm3 higher than the discrete sample results across the entire common interval for Hole 1230A (Fig. F21). A similar suite of density data are available for Site 685 (Shipboard Scientific Party, 1988).
MST bulk density data for both Holes 1230A and 1230B are noisy over the interval of high core recovery (0-135 mbsf). These data range from 0 g/cm3 (presumably gas voids) to 1.55 g/cm3 with the majority between 1.3 and 1.5 g/cm3 (Fig. F21A). The poor quality of the GRA measurements is evident from values <1.2 g/cm3, extreme variability over <1 m, and the -0.15-g/cm3 bias relative to MAD values. We attribute these problems to the effects of depressured core gas expansion, although care was taken to not collect MST data over voids visible through the core liners. A 5-m moving average of the data from Hole 1230A shows variation over the range 1.3-1.45 g/cm3 on a scale of 10 to 15 m (with the average varying little across Subunits IA, IB, IC, and most of ID) (Fig. F21B). GRA density data from Site 685 (not shown) reflect the consequences of recording at a 6-mm spacing without regard to the location of voids. They are extremely noisy, variable, and 15%-30% lower than comparable Hole 1230A measurements.
MAD data define downhole trends more clearly than the MST records and are consistent with wireline log results (Fig. F21B). Discrete sample bulk density increases slightly with depth from a near-seafloor value of ~1.35 g/cm3 to ~1.55 g/cm3 at the base of Unit I (Fig. F22A). An initial steep increase in bulk density (from 1.35 to 1.45 g/cm3) corresponds to a rapid decrease in porosity (from 78% to 70%) across the first 20 mbsf (Fig. F22C). From 20 mbsf to the base of Subunit IA (64 mbsf), bulk density remains relatively constant at ~1.45 g/cm3, whereas porosity continues to decline slightly to 68%. Grain density is widely variable across the upper 20 m of Hole 1230A and then remains at ~2.44 g/cm3 to the base of Subunit IA (Fig. F22B).
In Subunit IB, MAD-derived bulk density displays three 10-m cycles. Each of these cycles is characterized by a gradual upward decrease from ~1.5 to 1.4 g/cm3 followed by a sharp return to ~1.5 g/cm3 at the top of the cycle. Both the grain density and porosity plots show the same pattern, indicating that the bulk response is dominantly a function of sediment particle type. The wireline bulk density log (Fig. F21B) records the lower two of these cycles and suggests that each cycle consists of thinly interstratified low- and high-porosity bands.
Subunit IC is characterized by a slight downhole decline in bulk density (from ~1.59 to 1.55 g/cm3 in the wireline record) (Fig. F21B). Grain density and porosity data suggest this interval is dissected by a depositional discontinuity at 128 mbsf. The resolution of the density data is not sufficient to allow us to unequivocally interpret this feature. Resistivity data (Fig. F37) suggest that the discontinuity may be linked to a hydrate-bearing zone between 126 and 130 mbsf.
Subunit ID is marked by low core recovery. The MAD-derived bulk density profile follows a smoothed trend of the wireline bulk density log (Fig. F21B). Average bulk density is relatively constant at ~1.46 g/cm3 from 148 to 180 mbsf, increases to 1.61 g/cm3 at 199 mbsf, and then drops to 1.46 g/cm3 at the base of Unit I.
In Unit II, density and porosity measures are widely variable. Average porosity is ~55% and ranges between 40% and 65%. Bulk density ranges between 1.45 and 2.0 g/cm3. Grain density exhibits similar variation between 2.2 and 2.7 g/cm3. Given the close match between the MAD and wireline records where there is sufficient recovery, the data in Unit II indicate interbedded or alternating lithologies with variable physical properties. This is consistent with the lithologic descriptions of ooze with varying clay content, tectonic fabric development, and intermittent carbonate cementation zones (see "Description of Lithostratigraphic Units" in "Lithostratigraphy").
The neutron porosity wireline data depict a profile that mimics the MAD-derived porosity measurements, although the wireline data are extremely variable (±15%) and ~15% higher than the MAD-based values (Fig. F26). The cause of this discrepancy is enigmatic. Despite the discrepancy, the wireline data do show, consistent with the MAD measurements, a slightly higher porosity over Subunit ID than would be otherwise expected by simple burial and compaction of a relatively homogeneous sedimentary section.
P-wave data from the MST PWL were recorded at a 10-cm spacing for the first core from Hole 1230A and at 2-cm spacing for Holes 1230D and 1230E. Discrete samples were removed from split cores at a rate of about two per core, depending on lithologic variation and core condition, cut into 1.8- to 3.0-cm3 cubes, and resaturated with seawater for up to 2 days to overcome the loss of interstitial water arising from degassing of the cores. The cubes were oriented so that P-wave velocity could be measured using the PWS3 velocimeter in the x-, y-, and z-directions (two horizontal and one vertical plane, respectively, defined with the vertical direction along core axis).
The PWS3 velocimeter data (Fig. F23) show a trend in Unit I consistent with downhole burial compaction observed in the bulk density profile. P-wave velocities range from 1500 m/s at the seafloor to ~1600 m/s at the deepest reliable measurement at 170 mbsf. PWL and PWS3 measurements both record the same rapid compaction across the uppermost 10-20 mbsf, as observed in the density data. The PWS3 data yield average velocities ~80-110 m/s slower than those recorded by the sonic wireline log over the common intervals. We attribute this difference to the disruption of sediment fabric caused by gas expansion and the effective pressure difference between in situ and shipboard states. Despite this disruption, the PWS3 data still clearly retain a relict compaction signature.
At the Unit I/II boundary, wireline P-wave velocity increases from ~1700 to 1820 m/s over an interval of ~2 m, followed by a sharp peak at 222 mbsf of ~2440 m/s. A carbonate-cemented breccia was recovered from within this 222- to 225-mbsf interval. Below 222 mbsf, the P-wave velocity wireline log and the sparse discrete PWS3 data vary over a range of 1560-2150 m/s across the 30-m interval of Unit II. We attribute this variation, similar to that seen in other physical properties, to a heterogeneous distribution of tectonic fabric and degree of cementation.
NGR was measured on the MST for all Site 1230 holes (spacing = 30 cm, count time = 15 s), except for Hole 1230D, which was run at a higher spatial resolution (spacing = 15 cm, count time = 30 s). In addition, natural gamma radiation was recorded with the NGR sonde on the triple combo tool string during the wireline logging run. MST data sets for Holes 1230A and 1230B are shown with the wireline results in Figure F24. The wireline response is suppressed above ~80 mbsf because of attenuation by the drill pipe.
Natural gamma radiation decreases slightly across the upper part of Subunit IA from ~40 counts per second (cps) at the seafloor to ~35 cps at 50 mbsf. Just below 50 mbsf, radiation emissions fall to ~25 cps. This discontinuity does not correlate with features in any other physical property record. A slight offset in NGR at 64 mbsf corresponds to the Subunit IA/IB boundary. The MST NGR record remains relatively constant at ~30 cps until a depth of 82 mbsf, coincident with the depth at which the unattenuated wireline record starts.
Both the MST NGR and the wireline NGR log record low values (<20 cps and ~20 American Petroleum Institute NGR units [gAPI], respectively) at 82 mbsf and then rapidly increasing values to a depth of 96 mbsf. This NGR cycle correlates with a similar rapid downward increase in wireline bulk density (Fig. F21B). The shape of the high-resolution NGR wireline log, in particular, indicates this to be a trend of the type normally associated with upward-coarsening grain size or decreasing clay content. The same trend is recorded by the MST NGR data. From 96 to 118 mbsf, two cycles with similarly upward-decreasing NGR signatures are present. Their responses are not as apparent as in the uppermost cycle, with the lowest values at ~ 40 gAPI units and the high at ~50 gAPI units. Each of these lower two cycles is ~10 m thick and corresponds to similar patterns in the bulk density and resistivity logs. The base of Subunit IB is located at a gamma ray emission peak of 65 gAPI units. A match between the MST and the higher-resolution wireline logs is not clear over this interval.
Across Subunit IC, both the gamma ray wireline log and the MST NGR data are reasonably constant. Values range between 60 and 50 gAPI units, with a slight decline from the top to the base. The noisy but uniform pattern suggests either a thinly bedded or bioturbated, interstratified clay and ooze sequence.
Across the low-recovery interval (Subunit ID), MST NGR data are too sparse to be useful. The wireline log indicates a trend similar to that in Subunit IC for the upper 5 m of Subunit ID. Between 155 and 160 mbsf, emission rates of ~70 gAPI units cap a thick upward-decreasing NGR cycle that extends from 160 to ~202 mbsf. Over this interval, the record increases from 40 to 93 gAPI units. From the base of this cycle to the Unit I/II boundary at 218 mbsf, average NGR declines only slightly from ~78 to 68 gAPI units, with high variability. The actual Unit I/II break in the NGR record is located at 220 mbsf, 2 m below the main break in all other physical property records.
The record from Unit II is characterized by two cycles of upwardly increasing gamma radioactivity. The partial MST NGR record over this interval confirms this pattern. The boundary between the two cycles is at 230 mbsf. The NGR data are not as noisy as the other physical properties over Unit II, and the change at 230 mbsf in the NGR data is not clear in any other property. These cycles probably represent zones of lithologically homogeneous sediment separated by tectonically altered, variably cemented intervals (Shipboard Scientific Party, 1988) (see "Lithostratigraphy").
Thermal conductivity measurements were made on Hole 1230A sediments at a rate of one per core (usually the third section, at 75 cm, if this was available). Values range between 0.68 and 1.07 W/(m·K) (average = 0.83 W/[m·K]) (Fig. F25A). Average normalized thermal conductivity and bulk density profiles are correlated (Fig. F25B), indicating that the thermal conductivity is generally controlled by water content of the sediments. The consistency and correlation with water content of these measurements is distinctly different from the results reported for Site 685 (Shipboard Scientific Party, 1988; their figure 69). The Site 685 needle probe measurements range over 0.74-0.98 W/(m·K) from 0 to 180 mbsf. At any particular depth, the variation within a 3-m interval is of this same order (0.20 W/[m·K]), clearly indicating ubiquitous measurement error given the homogeneity of sediment type and low porosity variations over the same interval (within 2%). The empirical estimate of thermal conductivity from wireline logs for Site 685 is also suspect, as the method predicts decreasing values across the interface at 216 mbsf between the slope apron and the accretionary wedge.
We attempted to collect electrical resistance data for calculation of formation factor on cores from Hole 1230A, but degassing of the cores on the catwalk resulted in loss of interstitial water and severe disruption of sediment fabric. Resaturation of cores using surface seawater proved unsuccessful, with measurements of formation factors >3 and some readings up to 8. For comparison, equivalent depth measurements made on diatomaceous ooze at Sites 1227, 1228, and 1229 ranged from 1.7-2.3. Periodic attempts, especially in sediments that appeared consolidated, met with similar results. No formation factor results are reported for Site 1230.
The shallow resistivity log was used as a proxy for comparison (Fig. F29D). These data cover the interval from 81 to 275 mbsf (lower 38 m of Subunit IB, Subunits IC and ID, and Unit II) and provide a detailed electrical resistivity record over the gas hydrate zone. As is standard, these logs have not been temperature corrected, but the thermal effect for the 5°C range over the logged interval is calculated at ~0.2 m.
Resistivity increases suddenly at the Unit I/II boundary as a result of the abrupt porosity decrease (65%-44%). A sequence of sharp resistivity modulations is present in Unit II (at 222-225, 231-234, 240-246, and 246-248 mbsf, respectively), with values ranging from 1.0 to 3.5 m. These intervals correlate with positive abrupt bulk density (Fig. F21B) and sonic P-wave velocity (Fig. F23) anomalies that, combined with lithostratigraphic indicators, suggest zones of intense cementation and/or tectonic fabric.
Physical property data were derived from MST and discrete sample measurements and two wireline tool (triple combination and FMS-sonic) logging runs. Cores recovered above ~160 mbsf were characterized by high in situ gas content. Rapid depressurization on the catwalk caused considerable disturbance to sediment fabric. This disturbance inhibited P-wave velocity measurements, eliminated formation factor determination, and biased MAD-based bulk density by -0.02 to -0.08 g/cm3 relative to in situ wireline estimates. Wireline- and laboratory-based physical property profiles, with the exception of the porosity data, are nonetheless in agreement where core recovery allowed for continuous shipboard measurements. The porosity discrepancy between the discrete property and logging data (~15%) has not been satisfactorily resolved, but several possible explanations exist. For instance, incomplete drying of closed diatom frustules could result in a low discrete porosity estimate relative to the wireline log. Equally, degassing of sediments could widen the gap between porosity measurements made in the laboratory and those made in the borehole because of the different properties used by each to calculate porosity. In overpressured sediments exposed in the borehole, free gas escapes from the formation to be substituted by water, whereas in the extracted sediments the same process in air results in dewatering of the sediments. Wireline porosity values >100%, however, suggest a systematic bias in the neutron sonde processing.
The stratigraphic section is composed of two units divided by a sharp physical property boundary at 216 mbsf. The principal consolidation process in Unit I is consistent with burial compaction as shown by the bulk density/porosity and P-wave velocity profiles. Wireline resistivity and P-wave velocity anomalies, coupled with catwalk IR thermal imaging analysis and direct observations, support prediction of several in situ gas hydrate zones in Subunits IB and IC (see "Gas Hydrate"). Unit II physical properties reflect distinct consolidation-related modifications, most clearly shown as carbonate cementation and strain fabric development. Such lithologic modification is characteristic of convergent margin wedge sediments.
