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doi:10.2204/iodp.proc.317.103.2011

Physical properties

The shipboard physical properties program at Site U1351 included nondestructive measurements of gamma ray attenuation (GRA) densitometer bulk density, magnetic susceptibility (loop sensor; MSL), natural gamma radiation (NGR), and P-wave velocity (measured with the P-wave logger [PWL]) on all whole-round core sections. Additionally, magnetic susceptibility (point sensor; MSP), discrete P-wave velocity (measured with P-wave bayonets [PWB] and the P-wave caliper [PWC]), spectrophotometry and colorimetry, moisture and density (MAD), and sediment strength were determined on working section halves from Holes U1351A and U1351B. Unless otherwise noted, all depths in this section are reported in m CSF-A.

All measurements were made on all APC cores from Holes U1351A and U1351B to 28.0 m (Section 317-U1351A-6H-2) and 512.4 m (Section 317-U1351B-59H-3), respectively. Continuous APC coring in Hole U1351B ended with Core 317-U1351B-13H (99.7 m). Below this depth, coring was accomplished predominantly with the XCB system, and only occasional APC cores were taken. Because of drilling disturbance associated with XCB coring and the slightly smaller diameter of the core relative to that of the core liner, whole-round magnetic susceptibility and P-wave measurements were degraded in quality for XCB cores. In general, physical property measurements of sections from Holes U1351A and U1351B correlate well. Whole-round and section-half core logging measurements below ~290 m in Hole U1351B were affected by poor core recovery.

Gamma ray attenuation bulk density

GRA bulk density was measured at 5 cm intervals (measurement time = 5 s). Raw data ranged from 0 to 2.3 g/cm³ (Fig. F32). After Gaussian low-pass filtering (50 passes), this range narrowed to 1.6–2.2 g/cm³ (Fig. F33). Cyclic variations in GRA density may reflect varying concentrations of sands and/or clays related to glacial–interglacial changes.

A comparison of GRA densitometer data with MAD data from samples in the uppermost 28 m of Holes U1351A and U1351B yields nearly identical results (Fig. F34). One spurious MAD result was found in Hole U1351B, but otherwise the four data sets are remarkably consistent. Note that other spurious GRA densitometer results obtained at core section ends were removed from this plot. The remaining low values may be the result of incomplete filling of the core liner. Deeper in the hole (below ~400 m), GRA and MAD data diverge, with GRA data following the trend of MAD samples but yielding slightly lower values (Fig. F35A).

A comparison of bulk density measured with different methods and expanded to include downhole logging results (also using a GRA densitometer) from Hole U1351B is shown in Figure F35A. Significant parts of Hole U1351B were wider than 20 inches, suggesting less than ideal results for density and porosity downhole tools. Nevertheless, GRA bulk density from cores is consistently lower than density from downhole logging. Figure F36 illustrates two hole sections with fairly complete core recovery, one where the hole exceeded the caliper measurements (>19.5 inches) and one where the hole diameter was <19.5 inches.

In the narrow-diameter hole interval (~140–210 m), downhole logging and MAD data are nearly identical (Fig. F36A). The MAD outlier at 175 m is from a small concretion found in the core and represents a higher bulk density than the overall sediment at that depth. GRA density values are somewhat lower than both MAD and downhole logging data values. Variations related to coring disturbance are evidenced by the repeating pattern of GRA densitometer measurements, with relatively higher densities near the core base and lower values toward the top. The higher values are generally consistent with downhole logs and MAD measurements, whereas the lower values are not.

In the larger diameter hole interval (210–270 m), downhole logging again yielded the highest densities and the GRA densitometer gave the lowest estimates (Fig. F36B). Variations in bulk density are indicated by both MAD and GRA estimates from cores. However, GRA results again tended to be affected by the coring process in that densities were low at the top of each core and increased with depth. This is consistent with core compaction, which occurs as the core liners are progressively filled during the coring process. This trend was not seen in MAD data, nor was it observed in the downhole log.

MAD data are minimally affected by coring disturbance and are therefore generally more reliable than GRA density data from cores. MAD data usually have bulk density values very close to those of downhole density data (Fig. F36A). The discrepancy between values from MAD analyses and those from downhole logging (Fig. F36B) therefore points to the possibility that downhole density logs were affected by the large diameter of the hole.

Magnetic susceptibility

Magnetic susceptibility (MSL) was measured at 5 cm intervals (measurement time = 2 s). The raw data ranged from 3.5 to 134.5 instrument units in Hole U1351A and from 0 to 872.8 instrument units in Hole U1351B (Fig. F32). Note that extremely high or low values, interpreted as noise, are not shown. Extremely high values at the tops of core liners may be linked to caved shell-hash material. Contamination by caved sediments is also associated with high magnetic remanence values (see "Paleomagnetism"). Comparable low magnetic susceptibility was also obtained at Site 1119 (Shipboard Scientific Party, 1999) and potentially associated with voids. To help illustrate key trends, data were cleaned using a Gaussian low-pass filter (50 passes; Fig. F33).

Magnetic susceptibility (MSP) was measured at 5 cm intervals (measuring time = 1 s) (Fig. F32). Measurements were made on all section halves unless drilling or surface disruption precluded the collection of meaningful results. MSP data at Site U1351 correlate well with MSL data from whole-round core sections; this is most readily apparent in the uppermost 180 m of Hole U1351B (Figs. F37, F38). Again, extreme low and high values are not plotted. Filtered data (using the same 50-pass Gaussian low-pass filter applied to MSL data) are plotted for Hole U1351B in Figures F33 and F38.

MSP and MSL measurements are clearly cyclical in the uppermost ~180 m of Hole U1351B, particularly in the uppermost 80 m where core recovery was most complete (Figs. F37, F38). This cyclicity is similar to that noted in magnetic susceptibility and NGR data from Site 1119 (Shipboard Scientific Party, 1999). At Site 1119, NGR cycles were interpreted as glacial–interglacial cycles (Carter and Gammon, 2004). In Hole U1351B, magnetic susceptibility shows a degree of correlation with NGR and GRA bulk density, most noticeably in the raw (unfiltered) data of the uppermost 180 m (Fig. F37).

Natural gamma radiation

NGR was measured with the Natural Gamma Radiation Logger (NGRL) on all core sections at 10 cm intervals. Measured values ranged from 5 to 75 counts per second (cps), with higher values associated with muddy lithologies and lower values associated with sands. A marked cyclicity is apparent in the uppermost 75 m, where four (not necessarily contiguous) cycles occur. At greater depths, poor recovery precluded cycle recognition (Figs. F32, F33).

Where they overlap in coverage, NGRL measurements correspond moderately well with those from the downhole triple combo gamma ray tool, as do measurements made through the drill pipe down to 90 m WSF. The downhole record displays gamma cyclicity down to ~220 m WSF, which is therefore the probable base of the interval of marked sand/mud cyclicity that characterizes the upper part of Hole U1351B (see "Downhole logging").

The uppermost 20 m of the NGRL record displays a cyclical pattern similar to that of marine isotope Stages (MIS) 1–6, as documented previously at nearby Site 1119 (Shipboard Scientific Party, 1999) (Fig. F39); however, below this depth, the NGR pattern becomes irregular and is difficult to match with earlier parts of the oxygen isotope curve. Carter and Gammon (2004) have shown the inverted NGR record to be a close proxy for MIS in offshore eastern South Island to at least 2.4 Ma. Although the record from Hole U1351B may seem to display an out-of-pattern sharp gamma low near the inferred time of peak MIS 6 cooling, Carter et al. (2004) interpreted a similar peak at Site 1119 as representing the passage of the Subtropical Front over the site.

Assigning ages of 127 ka to the MIS 5e peak and 12 ka to the inferred latest Pleistocene point (where NGR suddenly decreases from ~58 to 20 cps) allows a provisional timescale to be derived for the uppermost 25 m of Hole U1351B (Fig. F39). In turn, this age model suggests that an almost 3 m thick section of Holocene age may occur at the top of the core. If these correlations are correct, the MIS 1–5 glacial–interglacial cycle is 16 m thick in Hole U1351B compared with 46 m thick at Site 1119, indicating that over this part of the record Hole U1351B has a sedimentation rate about one-third that of Site 1119.

Below 22 m (i.e., below MIS 6), the lack of age control and the almost certain occurrence of other significant unconformities make it impractical to use the NGR record to contribute to age control.

P-wave velocities

P-wave velocity measurements were recorded nearly continuously on sections from Holes U1351A and U1351B at 5 cm intervals using the PWL on the Whole-Round Multisensor Logger. P-wave velocities were also measured on section halves using the PWC and PWB (Fig. F40). In a Cartesian coordinate system, the PWC measures P-wave velocity vertically to the sectional plane of the working half (x-axis), whereas the PWB measures the cross section (y-axis) and long axis (z-axis) of the core.

Unfortunately, all three measuring tools yielded poor results below ~22.5 m in both holes because of signal attenuation and sediment cracking caused by high gas content. Below 512 m, sediment disturbance related to XCB coring was too great for any signal to be generated. PWL, PWC, and PWB data from the uppermost 25 m in Holes U1351A and U1351B are presented in Figure F40. Poor results for z-axis P-wave velocity are not shown.

PWL data from both holes and discrete P-wave velocity from Hole U1351B are positively correlated. Within the uppermost ~22.5 m, the values range from 1411 to 1751 m/s. In the uppermost ~2.0 m, P-wave velocity decreases slightly downhole (from 1580 to 1530 m/s). Below this depth, P-wave velocity increases gradually downhole (from 1530 to 1650 m/s), matching the increase in bulk density indicated by MAD results. Discrete P-wave (x- and y-axis) velocity data from Hole U1351A are more scattered but show the same trend.

Spectrophotometry and colorimetry

Spectrophotometric measurements and associated colorimetric calculations were made on section halves at 5 cm intervals at the same positions as MSP measurements. Color data were recorded as L*, a*, and b* variations. L* values in particular show clear long-term trends in Hole U1351B (Fig. F41). Similar but less pronounced trends are also observable in b* data. Variations in a* are less pronounced than those in b* or L*. Variations in b* correlate with similar variations in magnetic susceptibility, particularly in the uppermost ~80 m of the hole (Fig. F42), which suggests that the lithologic/mineralogic controls on b* may be the same or at least similar to those that control magnetic susceptibility. A noticeable departure from this covariant relationship occurs between ~28 and ~35 m, an interval corresponding broadly to a sand-rich interval (Fig. F42). Within this interval, L*, a*, and b* values fluctuate more markedly and do not correlate well with magnetic susceptibility.

Moisture and density

MAD measurements were made on approximately one sample per section throughout Holes U1351A and U1351B (Fig. F43). Extra measurements were taken for occasional variations in facies. Measurements were coordinated with smear slide samples. Two cemented nodules at 175 and 976 m were sampled and measured, with the former having the lowest porosity measured in the core (15.2%) and the latter having a relatively low porosity (28.7%).

High variability typical of these properties was observed in the shallowest cores, followed by some reduction in variation with depth. The sampling method was tested in one case because of a fortuitous error. Samples were collected using a syringe as a miniature piston corer to minimize the compaction of samples during acquisition. Because of a sampling error, two adjacent samples were taken in Hole U1351B. One was compacted by the syringe by ~20%, whereas the other was obtained in the usual manner. The differences in calculated bulk density, grain density, porosity, and void ratio between the two samples vary between 0.9% and 1.9%, confirming that the MAD method is not sensitive to the sampling procedure and is insensitive to fabric disturbance as long as no water is lost in the process. However, a total analysis error of at least 1%–2% should be assumed.

Curve fitting revealed a subtle burial effect in Hole U1351B (Fig. F43). Statistically, bulk density had the poorest relationship with depth and increased at a rate of <0.1 g/cm³/km. The decrease in porosity with depth fits an exponential compaction curve:

where

• ρ_b = bulk density in g/cm³,
• ρ_g = grain density,
• ϕ = porosity in percent,
• v = void ratio, and
• z = depth in kilometers.

The overall uniform nature of the sediments at Site U1351 is ideal for using downhole logging methods to determine porosity. However, the diameter of Hole U1351B was often too large for reliable porosity results from downhole logging (Fig. F35B, F35C). Despite this limitation, the general log porosity trend compares well with that seen in MAD data. Low porosity values calculated from downhole log density (Fig. F35B) are consistent with MAD porosities and higher GRA densities (Fig. F35A). This is shown clearly in the expanded view of the 140–260 m interval (Fig. F44). Interestingly, where the hole was wide, neutron porosity downhole log values are entirely consistent with MAD porosities, whereas neutron porosities are somewhat higher where the hole was relatively narrow. The highest porosity values seen in the logs (below 600 m; Fig. F35B) are most likely artifacts of hole-width issues related to sands in the formation that were partially washed out by the coring process. However, many of the more moderate high-porosity intervals do not correspond to recovered cores and, as such, may correspond to high porosity in unconsolidated sands and/or gravels.

Sediment strength

Sediment strength measurements were conducted on working section halves from Holes U1351A and U1351B using automated vane shear (AVS) and fall cone penetrometer (FCP) testing systems (Fig. F45). A comparison of both measurement methods is shown in the cross-plot in Figure F45E.

Shear strength indicates that sediments range from very soft (0–20 kN/m²) to very stiff (150–300 kN/m²). Vane shear and fall cone shear strength correlate well in very soft and soft sediments, but AVS values are about three times lower in firm to very stiff sediments (standard deviation = 20.8 kN/m²) than FCP values (standard deviation = 68.9 kN/m²). These findings suggest that the applicability of vane shear in firm to very stiff sediments is limited and the vane shear test underestimates the strength of stiffer sediments. Overall, vane shear and fall cone strength data from Holes U1351A and U1351B correlate positively (Fig. F45A–F45D). Between 0 and ~250 m, shear strength increases slightly, indicating a change from very soft to firm sediments. Intervals with very poor core recovery (~250–730 m and ~810 m to hole bottom) have distinctly lower strength values, which may be associated with the disadvantageous sediment properties that led to the poor core recovery. Between ~730 and ~810 m, sediment shear strength increases from firm to stiff. In the uppermost 50 m, shear strength correlates well with cycles seen in magnetic susceptibility, GRA density, and NGR, supporting the interpretation that these fluctuations represent prominent changes in the sedimentary regime reflecting glacial and interglacial climate.

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