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doi:10.2204/iodp.proc.339.107.2013

Physical properties

Physical properties at Site U1389 were determined in Holes U1389A–U1389E. High-resolution scanning on whole-round segments was performed with the WRMSL at 2.5 cm intervals. Below Core 339-U1389E-38X, the interval was set to 1 cm. NGR was determined at 20 cm intervals above and 10 cm intervals below Section 339-U1389A-25X-6, 20 cm intervals in Hole U1389B, 20 cm intervals above and 10 cm intervals below Core 339-U1389C-34X, and 10 cm intervals in Holes U1389D and U1389E. The Special Task Multisensor Logger (STMSL) was only used for Holes U1389B, U1389C, and U1389D for stratigraphic correlation purposes at a scanning interval of 2.5 cm. Thermal conductivity probes were applied on Section 339-U1389A-1H-3 to Core 339-U1389A-23X. P-wave velocity was measured on split-core segments (working half) from APC Cores 339-U1389A-1H through 11H and on discrete samples of sufficiently indurated sediment pieces from Hole U1389E. Moisture and density (MAD) measurements were determined on discrete samples from every other section of Holes U1389A and U1389E. Color reflectance analysis and split-core point-magnetic measurements were collected at 5 cm intervals.

Based on the physical property data, two main intervals were identified at Site U1389 (Figs. F25, F26, F27). The upper interval (physical properties Unit I) is from 0 to 722.7 mbsf (base of Core 339-U1389E-42R). Although distinct shifts, especially in magnetic susceptibility, are present throughout this interval, a consistent relation holds between lithology and physical properties. Beds of coarser grain size correspond to higher gamma ray attenuation (GRA) density, magnetic susceptibility, and a* reflectance values (i.e., more reddish color, probably indicative of more Fe oxides in the sediment) than for the otherwise muddy sediment. Physical properties Unit I was divided into five subunits.

Physical properties Subunit IA covers the upper 0–178 mbsf and is characterized by relatively high and variable magnetic susceptibilities and GRA densities as well as widely fluctuating a* values. A distinct interval, physical properties Subunit IB, can be discerned from 178 to 217 mbsf (Cores 339-U1389A-21X through 25X) and displays high NGR values and low a* values (i.e., more greenish colors), as well as magnetic susceptibilities and GRA densities with fluctuations similar to the subunit above but with considerably reduced scatter of the data (Figs. F25, F26). Lithologically, this interval is dominated by mud with only a few coarse-grained beds (see “Lithostratigraphy”). Physical properties Subunit IC ranges from 217 to 321 mbsf and has more scatter in the GRA density and magnetic susceptibility data than the data from physical properties Subunit IB. Neither of the discussed physical property parameters shows distinct long-term fluctuations comparable to those in physical properties Subunit IA. Exceptions are a*, with a prominent minimum between 270 and 300 mbsf, and pronounced oscillations of L* between 280 and 321 mbsf. The base of physical properties Subunit IC coincides with the base of lithologic Subunit IA (see “Lithostratigraphy”).

Physical properties Subunit ID is identified from 321 to 406 mbsf, similar in depth to lithologic Subunit IB. This subunit’s upper boundary coincides with a major unconformity in the seismic record and is generally marked by an increase in magnetic susceptibility, although poor core recovery makes detailed analyses problematic. A distinct characteristic of physical properties Subunit ID, in contrast to other subunits determined within physical properties Unit I, is that some of its muddy intervals have high magnetic susceptibility values. Downhole logging shows a distinct increase in variability in the standard (total) gamma ray (HSGR) data (see “Downhole measurements”) that is not apparent in the NGR data. This increased variability is interpreted to represent an increase in the abundance of sandy beds. Dominance of coarse-grained beds is not supported by the lithologic evidence; lithologic Subunit IB is characterized by the reduced presence of silty or sandy mud beds. The poor core recovery, however, suggests that sandy intervals were lost during core retrieval, biasing the record.

Physical properties Subunit IE, from 406 to 720 mbsf, encompasses lithologic Subunits IC and ID and represents a return to physical property characteristics similar to those of physical properties Subunit IC, particularly low magnetic susceptibility. Core recovery is slightly better than in physical properties Subunit ID, but gaps in the record and drilling disturbance result in considerable data scatter.

Below 720 mbsf to the base of Hole U1389E, physical properties Unit II is identified. The upper boundary of this interval is slightly deeper than the upper boundary of lithologic Subunit ID (see “Lithostratigraphy”). Although no particularly obvious change in the characteristics of individual physical property records is apparent, the relationship between the different parameters changes remarkably. The previously consistent positive correlation between GRA density, magnetic susceptibility, and a* becomes more complex, in that GRA densities and magnetic susceptibility are partly positively but also negatively correlated. The very low magnetic susceptibility might obscure the signal here. The boundary at ~720 mbsf roughly coincides with inferred lithologic changes at ~710 mbsf as well as the defined Pliocene/Pleistocene boundary determined from biostratigraphy (~700 mbsf) and paleomagnetic (696 mbsf) studies (see “Biostratigraphy” and “Paleomagnetism”), supporting the assumption that changes in the depositional environment occurred here. However, the offset between the discussed physical property changes and stratigraphic boundaries determined by other methods exists and needs further study. The downhole increase in the ratio of downslope-transported material to contourite deposits might serve as an explanation for the more complex relations between the physical properties as compared to the upper part of Site U1389 (especially within lithologic Subunit IA). Here, contourites are believed to be exclusively responsible for the formation of coarse-grained layers, explaining the more uniform relation between lithology and physical properties in physical properties Unit I.

Whole-Round Multisensor Logger and Special Task Multisensor Logger measurements

Analyzing cores on the STMSL was skipped for Holes U1389A and U1389E because no immediate acquisition of data for stratigraphic correlation was necessary but was resumed for Holes U1389B–U1389D. Temperature equilibration before measurements with the WRMSL was at least 3 h.

Gamma ray attenuation bulk density

GRA density at Site U1389 is highly variable, between 1.6 and 2.4 g/cm³ (Fig. F25). The considerable scatter, in particular in cores retrieved by rotary drilling, might be related to core disturbance and the presence of small cracks and voids, particularly in intervals with high sand content. Also, the natural variations in grain size and sediment texture are clearly reflected in the GRA density record; increases in grain size commonly correspond to increases in GRA density. This matches the observation that sandy beds are generally poorly sorted (see “Lithostratigraphy”), leaving less open pore space than well-sorted sand and therefore increasing bulk density. In terms of long-term variations in GRA density, physical properties Subunit IA is characterized by four major cycles of a sawtooth-like pattern (a rapid downhole increase followed by a slower decrease) with superimposed small-scale variability. Physical properties Subunit IB shows four distinct peaks in GRA density without following a major trend. Physical properties Subunits IC–IE and Unit II are affected by increasing scatter, especially below 750 mbsf in intervals of poor core recovery, masking any potential long-term variability.

Magnetic susceptibility

As observed at the previous Expedition 339 sites, magnetic susceptibility has the highest average values (as high as 40 × 10^–5 SI) in the upper part of Hole U1389A (Fig. F25), decreasing downhole to 12 × 10^–5 to 15 × 10^–5 SI in physical properties Subunit IC. A second maximum can be distinguished in the lower part of physical properties Subunit ID between ~321 and 406 mbsf, with peaks reaching again as high as 40 × 10^–5 SI. In this part of the record, relatively high magnetic susceptibility is also found in the more muddy parts of the sediment. Magnetic susceptibility decreases to values commonly around 10 × 10^–5 SI below 640 mbsf (Fig. F25). As discussed above, a remarkable coherence between susceptibility and GRA density is observed for the coarse-grained intervals in physical properties Unit I, suggesting a relatively high content of magnetite and other magnetic minerals in the fine fraction of the sand layers. In this sense, smear slide analyses indicate the presence of opaque minerals in these layers (see “Lithostratigraphy”). As discussed for the previous sites, diagenesis might have degraded the magnetic signal by reducing fine-grained Fe oxides to Fe sulfides.

P-wave velocity

The WRMSL was used to gather sonic velocities for all holes at Site U1389, and an attempt was made to determine P-wave velocities on split cores in each section of Holes U1389A and U1389E (Fig. F25). Because of poor sediment-to-liner coupling, reasonable results from the WRMSL could only be obtained for the upper ~22 m of cores retrieved with the APC. The P-wave velocity profile can be extended to 90 mbsf by using the P-wave determinations on split cores in Hole U1389A. Although the sediment surface appeared to be smooth and should have provided an adequate coupling to the transducers, no clear acoustic signal could be attained at greater depth. The formation of small cracks in the relatively stiff and brittle sediment might have negatively affected signal propagation through the sediment. In Hole U1389E, measurements on adequately indurated pieces of sediment without core liner were carried out, which provided, in some cases, a signal sufficiently large for manual signal processing.

P-wave velocities follow GRA densities in the upper 22 mbsf, with values close to 1600 m/s in the uppermost meter, a downhole decrease to 1500 m/s to 4 mbsf, and a return to values close to 1600 m/s at 13 mbsf. Below this depth, values scatter between 1450 and 1680 m/s in accordance with the fluctuations in the GRA densities (Fig. F25). Because 1450 m/s is lower than the sonic speed in water, such low values are most likely an underestimation of the true speed caused by cracks and voids in the sediment. Sonic velocities obtained for the same intervals by WRMSL and on split cores (either by manual or automatic processing) agree very well. P-wave velocities determined on discrete samples had maximum values of 1730 m/s (Core 339-U1389E-38R) but were also as low as 1450 m/s (Core 339-U1389A-11R). Here, the same problem occurs as for measuring split cores with liner: the pressure needed to provide good contact between the transducers and the sediment specimen creates small cracks that increase the traveltime of the acoustic signal through the sediment piece.

Natural gamma radiation

NGR scanning was performed on all cores from Site U1389. NGR counts fluctuate mostly between 20 and 45 cps and exhibit cyclic patterns for the cores retrieved with the APC and XCB (Hole U1389A) (Fig. F26). These patterns are also consistent with the logging data (see “Downhole measurements”). In cores retrieved with the RCB (Hole U1389E), the signal is relatively noisy and large-scale cycles are hard to identify, which is also caused by gaps in core recovery. The relation of variations in the NGR record to other physical property data is not consistent. On a broad scale, it appears that a* and NGR are anticorrelated. This is especially valid for the intervals of persistently low a*, such as 65–110, 180–205, and 265–295 mbsf, where somewhat elevated NGR values are apparent without matching a* excursions in detail (Fig. F26). Low a* is often, but not always, related to fine-grained sediment. It might be inferred that these intervals also contain more organic matter, usually associated with elevated uranium concentrations and/or higher amounts of potassium-bearing feldspars and mica, which would both enhance NGR counts. Because the sandy intervals are rather impure with a wide range of components, a more complex NGR signal could be expected.

Moisture and density

Determination of moisture content and density on discrete sediment samples was performed in every section of Hole U1389A (Fig. F27A) and downhole in Core 339-U1389E-2R (Fig. F27B). Generally, GRA and MAD densities are consistent (Fig. F25) when measured on APC and XCB cores. GRA densities measured on RCB cores are usually lower than the corresponding MAD values, resulting from the lower volume of the RCB cores that the GRA densiometer integrates over.

Moisture and porosity both decrease downhole, declining rapidly in the upper few meters from maximum values of ~40% for moisture content and 70% for porosity to values of 17%–25% (moisture content) and 41%–56% (porosity) at ~25 mbsf (Fig. F27A). Downhole to ~720 mbsf, a gradual decrease in moisture contents of 17%–22% and porosities of 35%–45% can be observed. Within this general decrease, a small shift to higher moisture contents and porosities is present at ~295 mbsf, including increased scatter in the data until ~700 mbsf. The boundary between physical properties Subunit IE and Unit II at ~720 mbsf apparently goes hand-in-hand with a halt in sediment compaction because no further decreases in porosity and moisture content are observed below this depth. It is also notable that both parameters show minimums at this depth (705–745 mbsf) with little scatter in the records. Below 830 mbsf, the variability in moisture and porosity increases considerably (Fig. F27B). The reason for the low scatter close to the interval boundary is not obvious from the lithologic observations. Possibly, diagenetic overprint plays a role here.

Notable, with respect to the MAD calculations, is the distinct change in interstitial water chlorinity, with highest values of 800–850 mM at depths between ~350 and 550 mbsf and a return to low chlorinities (<400 mM) below ~720 mbsf (see “Geochemistry”). The anomalously high chlorinities, compared to seawater, occur in an interval with a relatively high scatter in grain density. Higher porosities (increased permeability) might explain advection of briny fluids, but no increase in porosity is observed in this interval. Porosities may have been underestimated because more salt precipitated during drying of the respective sediment samples, clogging pore space. However, a correction for the increased salt contents has not been performed, as they are assumed to be insignificant for the observed salinity variations (Blum, 1997). Additional uncertainties arise from calculations of the derived parameters that are done automatically by the software assuming a concentration of dissolved ions in pore water typical for seawater (Blum, 1997). Grain densities <2.65 g/cm³ (i.e., the grain density of quartz) in the interval where high chlorinities are observed might be due to the excess precipitation of halite (grain density = ~2.20 g/cm³) and other salts from interstitial water during sample processing. Otherwise, grain densities vary between 2.65 and 2.85 g/cm³. Lower variability is observed in physical properties Subunit IB and the upper part of Subunit IC (downhole to ~250 mbsf). The relatively high grain densities are indicative of a significant amount of components other than quartz, such as carbonates and heavy minerals, in agreement with the lithologic description (see “Lithostratigraphy”). No obvious relation between grain densities in coarse- versus fine-grained sediments could be detected, pointing at a similar mineralogical composition independent of grain size.

Thermal conductivity

Thermal conductivity was measured once per core using the full-space probe, usually in Section 3 near the middle of sections of all cores at Site U1389A (see “Downhole measurements”). Because cores retrieved by XCB drilling are severely disturbed and affected by biscuiting, only thermal conductivity measurements taken from APC cores (339-U1389A-1H through 11H) are considered here. Thermal conductivity varies between 1.0 and 2.1 W/(m·K), which is in the range of what was observed at the other sites. No clear trend is apparent in the data. A relationship between the thermal conductivity and the moisture content and porosity is also not evident, although pore water content should have an effect on thermal conductivity.

Summary of main results

The relation of physical property data allows the designation of two main intervals at Site U1389, divided by a boundary at ~720 mbsf. Above this depth, a coherent relation exists between high magnetic susceptibility and GRA densities, correlating with coarser intervals. This relationship becomes more complex below ~720 mbsf, where both parameters are occasionally inversely correlated. The interval boundary determined by means of physical properties is close to a major lithostratigraphic change at ~710 mbsf and the Pliocene/Pleistocene boundary at ~700 mbsf. The reason for the offset between lithologic changes in relation of physical property parameters remains unclear and warrants further research. Color reflectance indicates that sand layers are mostly more reddish in color, whereas muddy intervals tend to be more greenish.

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