Previous   |    Next

doi:10.2204/iodp.proc.339.103.2013

Physical properties

The shipboard physical properties program at Site U1385 included high-resolution nondestructive measurements of gamma ray attenuation (GRA) bulk density, magnetic susceptibility (loop sensor), P-wave velocity, and natural gamma radiation (NGR) on whole-round core sections. Thermal conductivity was obtained on Section 3 of each core in Hole U1385A. Discrete measurements of P-wave velocities were determined on section halves (one per section in Hole U1385D and Cores 339-U1385E-1H through 5H). Moisture and density (MAD) samples were measured once in every section as 10 cm³ discrete samples. Color reflectance spectrometry and split-core point-logger magnetic susceptibility measurements were obtained for every section in each hole in 2 cm steps.

Whole-Round Multisensor Logger and Special Task Multisensor Logger measurements

GRA bulk density and magnetic susceptibility were measured on all core sections at Site U1385 at 2.5 cm intervals using the Special Task Multisensor Logger (STMSL) and, after allowing the cores to acclimate for 3 h, on the WRMSL (Fig. F20).

Gamma ray attenuation bulk density

Variations in GRA density may reflect variations in lithology, consolidation, cementation, and porosity. Measured GRA densities steadily increase in the upper 30–40 mbsf (Fig. F20), probably reflecting sediment compaction that decreases water content and porosity. However, GRA densities are relatively stable below 50 mbsf, parallel to the reduction in magnetic susceptibility values; thus, GRA densities also seem to reflect a long-term change in lithologic composition. Grain densities vary mostly between 2.7 and 2.8 g/cm³ with no particular depth trend, although small-scale excursions are often mirrored in the GRA densities. GRA densities measured on the WRMSL are offset from those measured on the STMSL, with the former being greater by 0.04–0.08 g/cm³. Instrument calibration, drift, and core expansion may account for the offset during analysis. MAD bulk densities are ~0.04 g/cm³ lower than STMSL GRA densities in the upper 80 mbsf in Hole U1385A (Fig. F20), but MAD bulk densities approach GRA densities below this depth.

Magnetic susceptibility

The most notable aspect of all physical property records at Site U1385 is a gradual reduction of magnetic susceptibility values beginning at ~20 mbsf (Fig. F21). In the upper few meters, susceptibility values range from 20 × 10^–5 to 40 × 10^–5 SI, increase to a maximum of 40 × 10^–5 to 50 × 10^–5 SI with peaks up to 70 × 10^–5 SI at ~20 mbsf, and decline downhole to an absolute low of 5–10 × 10^–5 SI between 50 and 60 mbsf. Further downhole, susceptibilities recover but remain relatively steady at a low level between 5 × 10^–5 to 15 × 10^–5 SI. In addition to these general trends, magnetic susceptibility maintains a distinct high-amplitude variability until 50 mbsf and becomes comparatively low and less variable further downhole. This major change between 20 and 50 mbsf seems to correspond to either a general change in lithology or a diagenetic overprint, which is mirrored in low NGR counts (Fig. F22) and high L* values (see “Natural gamma radiation”). A likely factor for the main decrease of magnetic susceptibility is the reduction of fine-grained magnetite to iron sulfides in the sulfate reduction zone. This is also supported by the low anhysteretic remanent magnetization values (see “Paleomagnetism”). Despite this diagenetic overprint, magnetic susceptibility, NGR, and sediment reflectance property a* are positively correlated between ~40 and 90 mbsf, indicating that a strong paleoenvironmental signal is retained in the magnetic susceptibility.

P-wave velocity

P-wave velocity was measured in all core sections at Site U1385 at 2.5 cm intervals on the WRMSL. Reasonable P-wave velocity measurements could only be retrieved for the upper ~70 mbsf on the WRMSL because of poor coupling between the liner and sediment in cores severely affected by core expansion (Fig. F20). P-wave velocity values closely follow GRA densities, with a steady increase from ~1490 m/s at the uppermost part of the holes to 1530 m/s at 15 mbsf to more variable data at the base of the holes (1550–1580 m/s). This relation, as well as the good match between P-wave velocities obtained by the WRMSL and on section halves (as long as signal quality permitted automatic processing), supports the reliability of the data. Problems arose below ~60 mbsf, where an increasing abundance of degassing cracks prevented good coupling between the core liner wall and sediment, deteriorating the acoustic signal of the WRMSL. Split-core P-wave velocity data were obtained further downhole to the base of Hole U1385D, where velocities vary around 1600 m/s with a maximum of 1650 m/s in the automatically processed data (Fig. F20). Caution must be taken in interpreting the manually picked data, in which greater subjectivity and poorer signal quality lead to higher scatter in the data. The method of manually picking data tends to overestimate velocities.

Natural gamma radiation

Measurements of NGR were made on all core sections, mostly at 20 cm spacing, and run on an integration time of 7 min per section (420 s per measurement). Cores 339-U1385B-1H to 3H and all cores in Hole U1385D were scanned at 10 cm resolution. Measured values range from 20 to 50 cps (Fig. F22).

Overall, NGR values increase with depth, but two different intervals can be distinguished. The first interval corresponds to the upper 50 mbsf, with values ranging from 25 to 45 cps. In the second interval, from 60 mbsf to the bottom of the hole, values range between 30 and 50 cps. In particular, the interval between 50 and 60 mbsf is characterized by a distinct decrease of NGR parallel to low magnetic susceptibility and high lightness (L*) values (Figs. F21, F22). Since L* is often coupled to carbonate content, dilution of potassium-bearing detrital material (clay) by higher carbonate contents might explain the low NGR and magnetic susceptibility signal.

NGR variations at Site U1385 exhibit regular cyclicity that becomes more prominent below 90 mbsf. High-frequency variations show a close correlation to GRA densities, likely reflecting the varying amounts of carbonate versus clay components. A positive correlation between NGR and magnetic susceptibility excursions becomes apparent below 40 mbsf. This correlation is not evident above this depth, hinting at a change in the factors influencing the sedimentary composition.

Moisture and density

Measurements of density, porosity, and grain density were undertaken on 59 samples from Hole U1385A. One sample was taken from every two sections at ~60 cm. Care was taken to avoid locations of obvious drilling disturbance. These samples were measured for wet mass and dry density to calculate bulk density and grain density.

The bulk densities from discrete samples are plotted in Figures F20 and F23 and show a data range of 1.55–1.91 g/cm³. In the upper 60 mbsf of the hole, GRA densities and bulk densities of samples show certain offsets in which discrete samples are consistently lower than those measured on the WRMSL. Nevertheless, results from both techniques are in good agreement below 60 mbsf. This depth coincides with the sulfate–methane transition (SMT) zone at ~50–60 mbsf determined by pore water analysis. Because of the increasing amount of methane in the pore space, sediment expansion occurs below 60 mbsf, promoting the formation of cracks and voids. We interpret the disappearance of the offset between discrete sample and GRA densities below 60 mbsf as a consequence of gas expansion in the methanogenic zone. The degassing cracks cause underestimation of the GRA density values, causing them to have similar values to those obtained in discrete samples. This indicates that MAD bulk density measurements systematically underestimate the true density value. The reason for this systematic underestimation is not sufficiently resolved. Notably, only the dry volume of the sample was determined, not the wet bulk volume. Hence, the difference between the two density estimates might be related to the precipitation of salt crystals in the pore space during sample drying at 105°C, which would affect the determination of dry volumes by pycnometer and all further calculations (see “Physical properties” in the “Methods” chapter [Expedition 339 Scientists, 2013]).

Porosities range from 69% to 49% and generally decrease with depth, similar to moisture content, which is ~46% in the uppermost sections and decreases to ~27% at the base (Fig. F24). Bulk and dry densities exhibit increasing values with depth (Fig. F23). These trends primarily reflect compaction and dewatering of the sediment, which in the case of bulk density overprints the lithologic variations. Grain density values range from ~2.7 to ~2.8 g/cm³ downhole without a clear trend (Fig. F24). Grain densities are slightly higher than quartz density (2.65 g/cm³) and probably reflect the varying degree of calcareous material in the sediment.

Thermal conductivity

Thermal conductivity was measured once per core using the full-space probe, usually in Section 3 near the middle of the section. Thermal conductivity in fine-grained sediments is, as a first approximation, a linear combination of the conductivities of the grains and the interstitial water and therefore depends upon porosity, water content, and lithology. Overall, thermal conductivity values range from 1.1 to 1.45 W/(m·K), without a clear increase with depth and poor correlation with moisture content.

Top of page   |    Previous   |    Next