The investigation of magnetic properties at Site 1108 included the measurement of bulk susceptibility of whole core sections, point susceptibility and remanent magnetization of archive-half core sections, and magnetic susceptibility and its anisotropy and remanent magnetization of discrete samples. The RCB drilling throughout the hole prevented orientation of cores using the tensor tool. There was no recovery from Hole 1108A; only data from Hole 1108B are reported here.
Magnetic susceptibility measurements were made on whole core sections as part of the MST analysis (see "Magnetic Susceptibility"). Susceptibilities (bulk volume; uncorrected) were relatively high, on the order of 10-3 SI. Some horizons showed higher values on the order of 10-2 SI. Susceptibility spikes throughout the hole correlated well with medium- to coarse-grained layers (see "Physical Properties"). In general, susceptibility data from the MST and AMST analyses agreed; differences in magnitude can be attributed to volume differences for the uncorrected data (Fig. F31).
Initial mean susceptibilities of discrete samples were generally on the order of 10-3 SI, consistent with the MST and AMST data on long cores. The mean susceptibility, the degree of anisotropy (Pj) and the shape parameter (T) for the susceptibility ellipsoid (Jelinek, 1981), and the inclination of the minimum axis of the susceptibility ellipsoid (Kmin) with bedding dips are shown vs. depth in Figure F32. Above ~140 mbsf, Pj values were ~1.05 and increased slightly with depth. Below ~200 mbsf, Pj values remained relatively constant, ranging between 1.05 and 1.20, which reflected a relatively high degree of anisotropy. Nearly all samples showed positive T values higher than ~0.5, which indicated an oblate ellipsoid. Prolate ellipsoids were indicated for four samples with negative or near-zero T values. Two of the samples with prolate ellipsoids were from sections showing drilling-related deformation (flow-in) near the top of the cores. The remaining two from ~160 and 390 mbsf, respectively, were located within fault zones that were interpreted from structural data (see "Structural Geology"). Bedding dips and inclinations of Kmin axes without structural correction showed a strong correlation: Kmin was very steep where bedding was horizontal, and shallowed where bedding steepened (Fig. F32). All samples between ~200 and 400 mbsf showed very steep Kmin axes and, with the exception of the anomalous ones at 160 mbsf and 390 mbsf, showed oblate ellipsoids, which suggested that the anisotropy data dominantly reflected a primary magnetic fabric related to sediment compaction (Tarling and Hrouda, 1993).
Measurement of remanent magnetization was made on all but the most disturbed archive-half core sections recovered from Hole 1108B. Forty-four discrete samples from working-half sections were demagnetized by alternating field (AF) in six steps up to 25 mT.
Demagnetization behavior of discrete samples generally showed two components of magnetization (Fig. F33). The soft component, which was removed by 20 mT AF demagnetization, showed inclinations that were either steep downward or shallow downward, which probably represents an overprint acquired from the drill string (Fig. F33A, F33B, F33D). For some samples with a reversed polarity characteristic remanent magnetization (ChRM) (i.e., downward inclination; Fig. F33C), an overprint from the present dipole field (PDF; I ~-32º based on the 1980 isocline chart shown in Merrill and McElhinny, 1983, p. 19) or from the geocentric axial dipole (GAD) field (I ~-19º) could have contributed to the shallow and down direction of the soft component.
Intensity of remanent magnetization of long cores after AF demagnetization at 20 mT increases with depth between ~120 and 200 mbsf, from values on the order of 10-3 A·m-1 up to values on the order of 10-2 A·m-1; after which the intensity decreases to values on the order of 10-3 A·m-1 with a few narrow horizons showing higher values. Intensities of discrete samples were consistent with those from long cores (Fig. F34A). A similar trend was not observed in the magnetic susceptibility, which decreased from ~120 to 175 mbsf, and then abruptly increased around 180 mbsf and again around 220 mbsf, punctuating lower susceptibilities between these depths (Fig. F31). Other horizons below 220 mbsf also show abrupt susceptibility increases, most of which seem to correlate with the occurrence of coarser grained horizons (see "Magnetic Susceptibility"). The disagreement between the trends of remanent intensity and magnetic susceptibility suggests that the magnetic minerals that carry the remanent magnetization differ from those that dominate the magnetic susceptibility.
Although intensities were high, poor recovery from an unstable depositional environment (i.e., turbidites and faults), coupled with extensive core disturbance related to RCB drilling, precluded the compilation of a complete magnetostratigraphy. However, relatively undisturbed segments of lithified sediment from many core sections yielded a stable magnetic component after AF demagnetization that allowed magnetic polarities to be determined.
Inclinations of the remanent magnetization show relatively large scatter, particularly between sections and occasionally within sections. In addition, inclinations expected at the site from a GAD field are shallow (~19º) so that incomplete removal of secondary components or tilting of bedding could affect the polarity interpretation. Declinations are highly scattered, primarily as the result of the RCB drilling process, which precluded their use for magnetostratigraphic interpretation.
The polarity of the remanent magnetization after AF demagnetization at 20 mT was determined primarily from the inclinations. Although scatter between sections was relatively high, biases within the inclination data from long cores, corroborated by discrete sample analysis, facilitated the polarity interpretation. The time scale used was that of Berggren et al. (1995).
Interpreting polarities based on inclinations was hampered because of many factors: (1) the low latitude of this site (an inclination of ±19º is expected from the geocentric axial dipole model); (2) the possible incomplete removal of a steep downward magnetic overprint; (3) the unstable nature of the depositional environment (i.e., turbidites and faults); (4) the core disturbance caused by RCB drilling; (5) the lack of transition data exacerbated by generally poor recovery; and (6) the lack of dip azimuth and core orientation information. In spite of these obstacles, polarity zones could be assigned to much of the section below 150 mbsf, based primarily on inclination data in conjunction with the paleontologic data (see "Biostratigraphy").
The Brunhes/Matuyama polarity transition (0.78 Ma) was not preserved in these sediments. Limited data between 0 and 120 mbsf was due to extremely low core recovery. Paleomagnetic data showed steep negative directions attributed to flow-in disturbance throughout Core 180-1108B-1R. Therefore, interpretation of a small portion of the Brunhes at the top of the hole was based solely on the paleontologic data (Figs. F34A, F34B).
Above ~140 mbsf, polarity interpretation was generally unclear (Fig. F34A); however, reversed polarities were observed between ~82 and 84 mbsf. In conjunction with the paleontologic data, the interval between ~30 and 140 mbsf was interpreted as part of the Matuyama reversed polarity chron, which ranges in age between 0.78 and 1.77 Ma (C1r). The base of the upper Matuyama Chron, however, was not clearly defined. The normal polarity zone between 142 and 159-162 mbsf probably represents part of the Olduvai Subchron (C2n; 1.77 to 1.95 Ma), which is consistent with the paleontologic data. The termination of the Olduvai Subchron, however, is not clearly defined. Reversed polarities occur between 159-162 and 172 mbsf, which lies within the zone of a normal fault (see "Structural Geology"). The simplest interpretation based on the paleomagnetic data and the paleontologic data suggests that this region lies within the lower part of the Matuyama reversed polarity chron (C2r), which ranges in age between 1.95 and 2.58 Ma. Unexpectedly steep inclinations in the 159-172 mbsf interval might be related to tilting associated with faulting.
Between ~172 and 345 mbsf, a dominantly normal polarity interval occurs. The paleontologic data are sporadic throughout this interval, but they indicate an extended N21 zonation (see "Biostratigraphy"). Of key importance for interpreting the magnetostratigraphy is the occurrence of nannofossil Zone NN18 near the top of the 172-345 mbsf interval, with Zones NN19A-B directly above Zone NN18. Consequently, this interval probably represents the upper part of the Gauss normal polarity chron (C2An.1n; 2.58 to 3.04 Ma).
Shallow reversed polarities are found between ~360 and 400 mbsf, which lies partially within and directly above another fault zone (see "Structural Geology"); however, bedding attitudes in this zone were horizontal, suggesting that little deformation has occurred. The top of this interval may represent the termination of the Kaena Subchron (C2An.1r; 3.04 Ma), which is consistent with the paleontologic data. At ~428 mbsf, normal- to reversed-polarity transitional data were observed; this may represent the transition between C2An.2n and C2An.2r (termination of the Mammoth Subchron; 3.22 Ma), below which apparently reversed polarities occur.