The investigation of magnetic properties at Site 1116 included (1) the measurement of bulk susceptibility of whole core sections, (2) point susceptibility and remanent magnetization of archive half core sections, and (3) susceptibility and its anisotropy and remanent magnetization of discrete samples.
Magnetic susceptibility measurements were made on whole core sections as part of the multisensor track (MST) analysis (see "Magnetic Susceptibility"), and on half core sections as part of the archive multisensor track (AMST) analysis. The MST and AMST susceptibilities (uncorrected for volume) values ranged between the order of 10-3 and 10-2 SI (Fig. F31).
The mean susceptibility, the degree of anisotropy (Pj), and the shape parameter (T) of the susceptibility ellipsoid (Jelinek, 1981) of discrete samples are shown in Figure F32A. The inclinations of the maximum (Kmax) and minimum (Kmin) susceptibility axes of the ellipsoid before and after tilt correction are also shown, along with the dips used for the bedding correction.
Mean volume susceptibilities ranged between ~5 × 10-3 and 2 × 10-3 SI. These values were similar to or slightly higher than those at Hole 1114A. The Pj values ranged between ~1.05 and 1.15. The T values were dominantly higher than 0.5, indicating the predominance of oblate magnetic fabrics. Below ~100 mbsf, tilt-corrected inclinations of Kmax axes had inclinations shallower than 30º and Kmin axes steeper than ~60º. The shape and direction of the magnetic fabrics may reflect the degree of sediment compaction.
Magnetic fabrics were corrected for bedding tilt and for core orientation estimated from the declinations of the stable remanence from discrete samples (Fig. F32B). This correction assumes a geocentric axial dipole field in which the declination was 0º during normal polarity states and 180º during reversed polarity states. The polarity for each sample was determined from the sign of the tilt-corrected inclination after AF demagnetization at 25 mT. After corrections for bedding tilt and core orientation, Kmax axes below 100 mbsf were aligned roughly northeast-southwest (Fig. F32B). Kmin axes were slightly aligned to the northeast. In general, Kmax axes in sedimentary rocks indicate a direction that may be either parallel or perpendicular to the paleocurrent direction (Tarling and Hrouda, 1993); therefore, the orientation of the magnetic fabrics below 100 mbsf may reflect a northeast-southwest or northwest-southeast paleocurrent during sedimentation. The alignment of Kmin axes to the northeast, with a clear separation of Kint (K intermediate) aligned roughly perpendicular to both Kmax and Kint axes, suggests an imbrication of the magnetic fabric. These observations suggest a paleocurrent direction to the southwest. Alternatively, Kmin axes appear somewhat spread across a northwest-southeast girdle that includes Kint axes, which suggests a northwest-southeast paleocurrent direction. Additional data may help to resolve which paleocurrent direction is correct.
Measurements of remanent magnetization were made on relatively undisturbed sections from archive half cores and on discrete samples taken from working half core sections. Results are shown in Figures F33 and F34 and are listed in Table T5.
A total of 11 discrete samples were measured. Initial natural remanent magnetization (NRM) intensities were on the order of 10-2 A·m-1. Demagnetization behavior of discrete samples showed the presence of two or more magnetic components. A soft component removed between 0 and 5 mT generally showed a downward direction. After removal of the soft component, all samples showed a stable component that decayed approximately linearly toward the origin of the vector demagnetization plot between 15 and 25 mT (Fig. F33); this component is referred to as the characteristic remanent magnetization (ChRM).
Intensity of remanent magnetization of long cores after AF demagnetization at 25 mT was on the order of 10-3 to 10-2 A·m-1 (Fig. F34A, F34B). Inclinations corrected for bedding tilt showed normal and reversed polarities. Declinations were highly scattered as a result of the RCB drilling process. Discrete sample results were generally consistent with the long core data.
Because of poor recovery and discontinuous data sets, no attempt was made to interpret the data in terms of the magnetic polarity time scale.