Measurements of physical properties at Site 1127 followed the procedures outlined in "Physical Properties" in the "Explanatory Notes" chapter. These included nondestructive measurements of P-wave velocity (every 4 cm; Table T8, also in ASCII format), gamma-ray attenuation (GRA) bulk density (every 4 cm; Table T9), MS (every 8 cm; Table T10, also in ASCII format), and natural gamma radiation (NGR) (every 16 cm; Table T11, also in ASCII format) using the MST. The P-wave logger was activated only on APC cores. Thermal conductivity (Table T12, also in ASCII format) was measured in unconsolidated sediment at a frequency of one determination per core. A minimum of two discrete P-wave velocity measurements per section were made on the working half of the split cores (Table T13, also in ASCII format), although measurements could not be undertaken in lithified sediments because of the malfunction of the PWS3 contact probe system. Consequently, there are no determinations of P-wave velocity below 290 mbsf. Standard index properties (Table T14, also in ASCII format) and undrained shear strength (only in unconsolidated sediments) (Table T15, also in ASCII format were measured at a frequency of one per section. No in situ measurements of formation temperature were made. Magnetic susceptibility data are discussed in "Paleomagnetism".
As described elsewhere (see "Operations" and "Organic Geochemistry"), very high concentrations of H2S were found through much of the cored interval. Exsolution of gas from recovered sediment, which in some cases caused expulsion of core onto the rig floor, greatly altered sediment physical properties. As NGR measurements are integrated over a larger depth interval than other measurements, these data proved to be the most useful for defining physical properties units (PP units).
Three major PP units are recognized in the NGR data. Physical properties Unit 1 (0-135 mbsf) is characterized by the presence of three high-amplitude (>5 cps), ~50-m wavelength cycles in NGR superimposed on a rising downcore trend (Fig. F20). Gamma-ray attenuation bulk density (~1.6-1.8 g/cm3) and P-wave velocity (~1.7-1.4 km/s) decrease with depth in this unit (Fig. F20). Physical properties Unit 1 corresponds to lithostratigraphic Unit I and Subunit IIA and the upper part of seismic Sequence 2A.
Physical properties Unit 2 (135-466 mbsf) is characterized by lower amplitude (<4 cps) and higher frequency (~20-m wavelength) cyclicity in NGR activity, superimposed onto a slight rising trend with depth. The latter may be an artifact of the greater bulk density, which increases to 350 mbsf (Fig. F20). Thereafter, there is no significant change in GRA bulk density in this unit. P-wave velocity increases in parallel with GRA bulk density, although measurements at depth are limited to unlithified sediments and cannot be considered representative of the full range of in situ values (Fig. F20). Physical properties Unit 2 corresponds to lithostratigraphic Subunits IIB (lower part) to IIE and seismic Sequences 2A (lower part), 2B, and 2C.
The distinctive change from a predominantly ~50-m to a ~20-m cyclicity between PP Units 1 and 2 may represent the widely recognized mid-Pleistocene change in the response on the ocean/climate system to Milankovitch forcing (Prell et al., 1992; Imbrie et al., 1984). The 100-ka component, dominant in the upper Pleistocene, may correlate with the ~50-m cycle, whereas the ~20-m cycle lower in the core may correlate with the 41-ka obliquity signal.
Physical properties Unit 3 (466-508 mbsf) consists of Pliocene sediments that are correlated to lithostratigraphic Unit III. The upper boundary of this unit is marked by a decrease in NGR and an increase in GRA bulk density (Fig. F20). Inconsistencies in thermal conductivity data at Site 1127 as a result of gas disturbance of sediment physical properties make the data suspect.
The NGR data from the MST matches well with in situ wireline logging data, and there is an obvious correlation between specific peaks and the frequency of NGR cycles. However, log data show a decline in the amplitude of the ~20-m-wavelength cycles present in PP Unit 2 with depth, whereas the MST data show a trend to increasing amplitude (Fig. F20). Downhole logging spectral data suggest that the NGR signal is dominated by changes in uranium concentration (see "Downhole Measurements"). Although these changes are possibly driven by changes in the sedimentary aragonite, preliminary XRD data show no obvious association with NGR data (see "Inorganic Geochemistry").
The effects of degassing on the physical properties of sediments at Site 1127 can be assessed by comparison of the mean and standard deviation of GRA bulk density per core with point measurements of bulk density from index properties determinations (Fig. F21). Between 0 and 86 mbsf, GRA bulk density decreases (Fig. F21) with a significant increase in the standard deviation from 0.04 to 0.07 g/cm3. Observations in sectioned cores suggest that this pattern was related to increasing delamination of the unlithified fine-grained sediment along primary bedding planes. Index properties samples from the same interval show a progressive increase of density with depth, as would normally be expected. Index properties bulk density more accurately reflects true sediment density, as it measures only the mass and volume of the solid and liquid phases and is not affected by artificial porosity resulting from gas disturbance. The difference between these two density measurements indicates a minimum core expansion of 12.5% at 100 mbsf, equivalent to 1 m for every 9 m cored. From 100 to 250 mbsf, the standard deviation of GRA bulk density readings increases (Fig. F21). This reflects the development of relatively widely spaced transverse cracks and some larger voids by degassing responding to the increased strength of the sediments in this interval (Fig. F22). Below 200 mbsf, GRA and moisture and density determinations increase and converge by a depth of 300 mbsf in response to increased lithification (Fig. F21). The higher strength of the lithified sediments precludes both delamination and cracking, and gas loss occurs predominantly by pore-scale exhalation.