At Site 1227 we collected a full range of physical property data from the sediment/water interface to the limit of coring at 151.1 mbsf. The data are described below and compared with those from Site 684 (Shipboard Scientific Party, 1988). Higher-resolution sampling than that used at Site 684 was feasible with current laboratory instruments.
Whole-round cores were degassed for up to 2 hr on the catwalk when necessary for safety, were equilibrated to laboratory temperature (2-4 hr), and then each available section was run on the multisensor track (MST). The standard MST measurements were magnetic susceptibility (spacing = 5 cm, data acquisition scheme [DAQ] = 2 x 1 s), GRA bulk density (spacing = 10 cm, count time = 5 s), P-wave velocity (spacing = 10 cm, DAQ = 10), 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 1227A, where possible. Physical properties were measured on the microbiology sections only if intact parts remained following the sampling. This greatly limited the continuity and, hence, spatial resolution of the physical property record below ~30 mbsf. Hole 1227B was logged at higher resolution and precision on the MST, whereas Holes 1227C, 1227D, and 1227E were logged at the same rates as Hole 1227A.
Moisture and density (MAD) properties, P-wave velocity from the digital velocimeter, and resistance data (translated to formation factor, as detailed in "Formation Factor" in"Physical Properties" in the "Explanatory Notes" chapter) were collected regularly only from Hole 1227A because of MAD processing time constraints. MAD samples were taken at a frequency of one sample per section and at higher resolution in sections with many voids or lithologic transition areas. Where possible, MAD samples were co-located with the methane headspace extractions to facilitate the volumetric analysis of methane concentrations. Spot sampling for MAD was also carried out in Hole 1227D in order to confirm measurements from Hole 1227A. Even though core recovery decreased significantly below ~52 mbsf, the collected data are sufficient to allow characterization of the physical parameters of each lithostratigraphic unit and to be confident of the correspondence of our data to those from Site 684.
Instrumentation, measurement principles, and data transformations are further discussed in "Physical Properties" in the "Explanatory Notes" chapter.
The infrared scanner was not employed at this site, due to expected rapid recovery and high concentrations of hydrogen sulfide that created safety concerns.
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. Leg 112 magnetic susceptibility data are not available in digital format. However, a visual comparison between our data and the postcruise records collected for Sites 684 (Fig. F12A) (Merrill et al., 1990) shows a reasonable match across the uppermost 70 mbsf, where the records are most continuous.
Lithostratigraphic Units I, II, and III comprise a series of laminated to partly bioturbated biogenic oozes, interstratified with unconsolidated mixed detrital and biogenic silts and sands (see "Description of Lithostratigraphic Units" in "Lithostratigraphy"). The oozes are typified by relatively low magnetic susceptibilities in the range of 1 x 10-5 to 5 x 10-5 SI units. The coarser-grained layers are associated with magnetic susceptibility peaks that range from 25 x 10-5 to 50 x 10-5 SI units. The most prominent of these layers are located between 5 and 7 mbsf (Unit I), 9 and 12 mbsf (Unit I/II boundary), 14 and 18 mbsf (Unit II), and 41 and 53 mbsf (Unit III) (Fig. F12B).
These peaks in the susceptibility signal, coincident with grain size and NGR increases, are probably the result of a change in the origin of the sediment. Using a permanent (Nd) magnet, extracts from dried samples and slurries were obtained from the layers at 5-7, 9-12, and 41-53 mbsf for optical and XRD analysis. Preliminary studies suggest the source of the magnetism is primary magnetite. Rounding and size of the extracted crystals suggest they have been transported rather than formed in situ by diagenetic or microbial processes.
The magnetic susceptibility record in the lower part of Hole 1227A is incomplete. Most of the intact parts of cores are composed of recovery-induced gas-fractured oozes that have a very low magnetic susceptibility signature. Magnetic susceptibility spikes appear at the top of each core below ~60 mbsf (Section 201-1227A-7H-3), the depth at which we encountered a high-susceptibility sand and calcite-cemented unit. This material apparently fell down the hole after each successive core and was therefore present at the top of each core as a drilling artifact. The possible exception is the peak at ~110 mbsf, which appears to be associated with an in situ coarser-grained layer.
At Site 1227 we collected 18 discrete samples for paleomagnetic measurements. The sampling frequency was two samples from each core in Cores 201-1227A-1H through 3H (0.0-24.6 mbsf) and one sample from each core below this interval to the bottom of the hole (Cores 201-1227A-4H through 17H; 24.6-141.6 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.
Diatomaceous and siliciclastic sediments at Site 1227 show low magnetic intensity and susceptibility (Fig. F13) compared with the other sites occupied during Leg 201. The uppermost dark brown clay-rich diatom ooze sample (Sample 201-1227-1H-2, 14-16 cm) shows a less stable magnetic direction (Fig. F14). Although samples from lithostratigraphic Unit II (see "Description of Lithostratigraphic Units" in "Lithostratigraphy") exhibit higher susceptibility, we were not able to isolate a clear magnetic signal.
Density data were measured on the MST by the GRA densitometer (spacing = 10 cm, count time = 5 s) and calculated from split-core mass/volume (MAD) measurements. The GRA data from Site 1227 show much less scatter than the Site 684 data (Shipboard Scientific Party, 1988) (Fig. F15) and are consistently at the high end of the values for Site 684, though the trends in the two data sets are nearly identical. Figure F16 displays a 5-m moving average of the GRA density estimates from Holes 1227A and 684C, showing that the two surveys have good correlation until ~30 mbsf. Below this depth, the continuity of both records decreases because of poor core recovery.
GRA density is generally between 1.2 and 1.4 g/cm3 throughout the profile. As with magnetic susceptibility, there are several intervals where density peaks. The density increases to ~2.0 g/cm3 at 5-7, 9-11, and 42-53 mbsf (Fig. F15B). These peaks correlate with a coarse-grained foraminifer ooze located at the base of Unit I, a foraminifer-enriched silt at the Unit I/II boundary, and a glauconitic silt comprising Unit III. There also appears to be an increase in bulk density from 140 mbsf to the bottom of the hole, but the record is noisy and the trend may be an artifact of coring.
Between 50 and 125 mbsf, the MAD-calculated density increases slightly from ~1.2 to ~1.4 g/cm3 (Figs. F16, F17A). This compaction-related trend is not reflected in the GRA data, which show a bulk density decrease that is attributed to unfilled core liner effects and decompression/degassing disturbance.
Porosity data derived from the MAD measurements (Fig. F17C) indicate an initial porosity for fine-grained sediment of ~85% at the seafloor. This declines to ~70% for the deepest measurement at ~140 mbsf. The coarser-grained units described above have distinctly lower porosities, consistent with the more equant grain shapes and framework support of the fabric. The uppermost silts in Unit I have porosities of ~60%, whereas the thicker silt layer in Unit III has a porosity of <50%.
Grain density data (Fig. F17B) support the mineral identifications discussed in "Lithostratigraphy". Grain densities of ~2.8 g/cm3 are consistent for coarse-grained silts, compared with values of 2.2 to 2.4 g/cm3 for the finer-grained sediments that dominate Units II and IV (Carmichael, 1982).
P-wave data from the MST P-wave logger (PWL) were recorded (spacing = 10 cm, DAQ = 10) for all APC cores from Holes 1227A to 1227E and at 2-cm spacing for Hole 1227B. The PWS3 velocimeter was also used to measure P-wave velocities on split cores from Hole 1227A, with measurements taken at a minimum of one per section (depending on lithologic boundaries) for Sections 201-1227A-1H-1 through 5H-6. Below Section 201-1227A-5H-6 (43.1 mbsf), reliable PWL and PWS3 measurements were unobtainable because of decompression and drainage effects, which created partially saturated cracklike voids of ~2-10 mm length oriented perpendicular to the core axis.
Between 0 and 55 mbsf, PWL measurements range from 1485 to 1695 m/s, whereas PWS3 velocities are bounded within 1540-1700 m/s (Fig. F18). The PWS measurements were generally 40-50 m/s faster (similar to measurement differences at Site 1226). The breakdown of PWS velocities based on lithostratigraphic units is as follows: Unit I (diatom ooze) ranges from 1550 to 1570 m/s; Unit II (diatom-bearing clay- and pyrite-rich silt) ranges from 1550 to 1580 m/s; and Unit III (glauconite-bearing pyrite-rich silt) ranges from 1535 to 1560 m/s. Based on several measurement sets of 10-cm-spaced velocity profiles over ~1.5 m, the 20- to 30-m/s velocity variation within the respective units represents natural meter-scale variability and is not indicative of a particular sedimentary sequence.
Three high-velocity intervals (beginning at 6.2, ~14.0, and ~42.0 mbsf, respectively) (Fig. F18) correlate with coarse-grained foraminifer ooze (Unit I), foraminifer-enriched silt (Unit II), and glauconitic silt (Unit III). The velocities in these intervals are at least 1700 m/s, which we consider to be an in situ minimum because of the unconsolidated nature of these sediments in split cores.
NGR was measured on the MST for all Site 1227 holes (spacing = 30 cm, count time = 15 s). Hole 1227B was run at a higher spatial resolution (spacing = 15 cm, count time = 15 s). As with other physical properties at this site, the NGR data (Fig. F19) track the compositional anomalies in the layers located between 5 and 7 mbsf (Unit I), 9 and 12 mbsf (Unit I/II boundary), 14 and 18 mbsf (Unit II), and 41 and 53 mbsf (Unit III). The higher emission signal in these layers is probably linked to the presence of feldspars and glauconite (see "Description of Lithostratigraphic Units" in "Lithostratigraphy").
Thermal conductivity measurements were made on Hole 1227A sediments at a rate of one per core (usually the third section, at 75 cm, if this was available). Values range between 0.70 and 0.94 W/(m·K) (average = 0.76 W/[m·K]). The maximum thermal conductivity is at 47.0 mbsf. This corresponds to the glauconitic silt in Unit III, the interval of lowest downhole porosity. Average normalized thermal conductivity and bulk density show a high correlation (Fig. F20), indicating that the thermal conductivity is an inverse function of water content of the sediments. Thermal conductivity measurement quality decreased for Core 201-1227A-13H and below, due to unfilled core liners and drilling disturbance.
Formation factor (longitudinal and transverse) was determined for Hole 1227A sediments as described in "Formation Factor" in"Physical Properties" in the "Explanatory Notes" chapter, with a minimum of one sample per section for Cores 201-1227A-1H through 17H. Measurements in all cores below Core 201-1227A-12H (beginning at ~110 mbsf) were made after saturating pilot holes in the split core face with seawater before inserting the four-pin probe, to account for decompression voids and water content decrease resulting from extended air exposure after splitting (hydrogen sulfide degassing precaution). Data reported herein correspond to high-quality measurements taken in minimally disturbed APC intervals.
Longitudinal (parallel to core axis) formation factors range from 1.5 to 2.9 except in the coarser-grained intervals at 6.2 and 41.0 mbsf, where measurements make steplike increases to >4.5 (Fig. F21). The low values below ~116 mbsf are probably decompression artifacts, as the sediment framework shows consistent expansion features that would tend to make electrical conductivity pathways less tortuous than in situ conditions. Electrical conductivity anisotropy typically ranges from 3% to 15% (average = 7%). Overall, the formation factor measurements track the changing lithostratigraphy, delineating the sharp grain-size and component-mix changes in Units I and III.
The physical properties of the units at Site 1227 are due to the dramatic compositional variation between the coarser and finer sediments and the variable impact of burial on the different sediments. The coarser layers have diagenetic pyrite and are in a foraminifer matrix. In Unit I, the coarser layers comprise a mixture of clastic grains, primarily quartz, and hornblende. Unit III has redeposited shallow-water glauconitic grains. High magnetic susceptibility, high bulk density, high P-wave velocity, high formation factor, low porosity, and high natural gamma radiation characterize these layers. The coincidence of high bulk density and low porosity can be explained by the composition of the material and the compactional history of the sand and silt compared with the ooze. The NGR response most likely arises from decay of potassium in the feldspars and glauconite.
The finer sediments are primarily diatomaceous oozes. The ooze has characteristic low NGR values and shows slightly increasing density and decreasing porosity downhole. These are the expected trends as a result of compaction.