The investigation of magnetic properties at Site 1118 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. MST and AMST susceptibilities (uncorrected for volume) ranged dominantly between values on the order of 10-4 and 10-3 SI; however, near the bottom of the hole values as low as 10-6 SI and as high as 10-2 SI occurred (Fig. F55A). In general, susceptibility data from the MST and AMST analyses agreed; differences in magnitude can be attributed to volume differences for the uncorrected data.
The trends of susceptibility and remanent intensity data after AF demagnetization at 20 mT are broadly similar (Fig. F55B). Increasing susceptibility values up the recovered section suggest that the contribution of remanence-carrying ferromagnetic minerals to the susceptibility also increases upward. The very high values (10-2-10-1 SI) below ~860 mbsf indicate that ferromagnetic minerals dominate the susceptibility near the bottom of the hole, probably related to the occurrence of volcanics (see "Lithostratigraphy" and "Igneous and Metamorphic Petrology").
Results of the measurement of the susceptibility and its anisotropy (AMS) on 119 discrete samples are shown in Figure F56A and F56B. The mean magnetic susceptibility, the degree of anisotropy (Pj) and the shape parameter (T) for the susceptibility ellipsoid (Jelinek, 1981), and the inclinations of the maximum (Kmax) and minimum (Kmin) axes of the susceptibility ellipsoid are shown vs. depth in Figure F56A. Stereonet plots in Figure F56B show the orientations of the principal axes of the susceptibility ellipsoids after correction for bedding tilt and for core orientation using the declinations of the stable remanent magnetization.
Mean susceptibilities from discrete samples generally agreed with long core susceptibilities. Between ~380 and 840 mbsf, Pj values were dominantly >1.05, with high values up to ~1.8 found between ~580 and 620 mbsf. These high Pj values are probably related to faulting observed in this interval (structural Subdomain Ic; see "Subdomain Ic"). The wide range of Pj values between ~380 and 680 mbsf was associated with lithostratigraphic Units II and III (see "Lithostratigraphic Unit II" and "Lithostratigraphic Unit III"). The high scatter in Pj values between ~260 and 300 mbsf and low values between ~300 and 380 mbsf are difficult to explain; there is no apparent correlation with the lithology (see "Lithostratigraphy").
The T values throughout the hole were dominantly positive and >0.5, which indicates the predominance of oblate fabrics. The relatively high scatter between ~260 and 340 mbsf may be related to soft-sediment deformation observed in this interval (see "Subdomain Ic").
Inclinations of Kmax and Kmin axes were strongly clustered at ~0º and 90º, respectively (Fig. F56A). Scattering of Kmax axes toward relatively steeper values and of Kmin axes toward relatively shallower values between ~260 and 340 mbsf correlates with the scatter in T values observed for this interval and may be related to soft-sediment deformation (see ""Subdomain Ic"). After correction for core orientation, based on the polarity of the stable remanence and assuming a geocentric axial dipole field, Kmax axes for samples from lithostratigraphic Unit III (see "Lithostratigraphic Unit III") are preferentially aligned east-west to east-southeast to west-northwest with Kmin axes clustered about vertical, and horizontally aligned Kint axes grouped about south-southwest (Fig. F56B). These data suggest an east-west to east-southeast-west-northwest paleocurrent direction. Orientations of the principal axes associated with the other lithostratigraphic units (see "Lithostratigraphy") may reflect paleocurrent information, but the data are not as clear as those from lithostratigraphic Unit III. Therefore, interpretation of paleocurrent information based on AMS results from the other lithostratigraphic units is not attempted in this report.
In summary, compaction effects are evident from the AMS data throughout the recovered section from Hole 1118A. An east-west to east-southeast to west-southwest paleocurrent direction is inferred for lithostratigraphic Unit III; the other lithostratigraphic units may contain paleocurrent information, but the data are equivocal without additional study.
Measurements of remanent magnetization were made on all but the most disturbed sections from archive half cores and on discrete samples from working half core sections. Results are shown in Figures F57 and F58.
A total of 121 discrete samples were subjected to AF demagnetization experiments to assess the stability of the natural remanent magnetization (NRM). The AF demagnetization results indicate that the NRMs of the samples generally consist of two magnetic components. A soft component, probably drilling-induced, with a shallow to steep downward direction was generally removed by demagnetization levels up to 15 mT. After removal of the soft component, most samples yielded a stable component that decayed linearly toward the origin of vector plots between 15 and 25 mT (Fig. F57A, F57B, F57C, F57D); this component is referred to as the characteristic remanent magnetization (ChRM). A few samples showed erratic behavior during AF demagnetization, which indicated a large contribution of a probable drilling-induced component (Fig. F57E, F57F); the ChRM from the samples was not isolated.
In Hole 1118A, intensity of remanent magnetization of long cores after AF demagnetization at 20 mT ranged from values on the order of 10-4 A·m-1 up to values on the order of 10-2 A·m-1, with values generally increasing up the hole (Fig. F58A).
The polarity of the remanent magnetization after AF demagnetization at 20 mT for Site 1118 was determined from the inclinations. Declinations were highly scattered, which precluded their use for magnetostratigraphic interpretation. Directions were corroborated by discrete sample analysis.
Figure F58A shows downcore variations of intensity, inclination, and declination from long core and discrete sample measurements. Magnetostratigraphic interpretation based on the inclinations is shown in Figure F58B.
The polarity change at ~387.5 mbsf represents the Matuyama/Gauss boundary (2.58 Ma; Berggren et al., 1995) and is consistent with the paleontologic data (see "Biostratigraphy").
The Gauss/Gilbert boundary (3.58 Ma; Berggren et al., 1995) is at ~846-849.5 mbsf. Using an estimated sedimentation rate of ~435 m/m.y. for the time period between 2.58 and 3.04 Ma, and a rate of ~485 m/m.y. for the time period between 3.04 and 3.58 Ma, the Gauss Chron should span ~462 m of section, which is consistent with the observed span of 458.5 to 462 m.
The onset of the Olduvai Subchron (C2n; 1.77-1.95 Ma; Berggren et al., 1995) is at ~288 mbsf; the termination of the Olduvai was not recovered from this site.
The termination and beginning of the Kaena Subchron (C2An.1r; 3.04-3.11 Ma; Berggren et al., 1995) are at 599 and 623 mbsf, respectively. Using an estimated sedimentation rate of ~319 m/m.y. based on the paleontologic datums at ~550 and 608 mbsf (see Fig. F54), the Kaena should span ~22 m of section, which is consistent with the observed span of ~24 m.
The Mammoth Subchron (C2An.2r; 3.22-3.33 Ma; Berggren et al., 1995) spans the interval between ~662.5 and 744.5 mbsf. Using an estimated sedimentation rate of ~485 m/m.y. (see "Sediment Accumulation Rate"), the Mammoth should span ~53 m of section, which is not consistent with the observed span of ~82 m. The termination of the Mammoth is consistent with the paleontologic data; however, there is a discrepancy between the stratigraphic positions of the first occurrence of G. tosaensis (3.35 Ma) and the beginning of the Mammoth (see "Biostratigraphy").
Evidence for the Reunion Subchron (C2r.1n; 2.14-2.15 Ma; Berggren et al., 1995) is between ~332 and 334.5 mbsf. Using an estimated sedimentation rate of ~155 m/m.y., the Reunion should span ~1.6 m of section, which is reasonably consistent with the observed span of ~2.5 m.