PALEOMAGNETISM

Introduction

After alternating-field (AF) demagnetization to 20 mT, the natural remanent magnetization (NRM) of whole-round sections from Site 1170 was measured at 5-cm intervals using the pass-through cryogenic magnetometer. An exception was made for cores whose liners or end caps were deformed, because these could damage the magnetometer. These cores were measured as archive-half cores. The nonmagnetic core barrel assembly was used for even-numbered cores in Holes 1170A and 1170C and for odd-numbered cores in Hole 1170B, starting with Core 3H. The comparison between results from cores collected with the nonmagnetic corer and with those from the standard corer is discussed the "Appendix" chapter, as are results of experiments investigating the effect of core splitting on magnetization and other coring-related magnetic experiments. However, it is noted here that (1) comparison of the NRM and 20-mT demagnetized whole-core results from Section 189-1170A-9R-1 revealed a strong downward-inclined drilling overprint and (2) comparison of Section 189-1170D-5R-1 before and after sawing revealed that the split archive-half had picked up a substantial moment that was not completely demagnetized by 20 mT. The Tensor tool is usually employed to orient the APC cores beginning with the third core at each hole, but the poor determination of the declination of the cores at this site precluded the orientation of cores.

Discrete oriented samples were routinely collected from Holes 1170A and 1170D. These were used to aid in the interpretation of the long-core record of magnetization by providing additional measurements of polarity and basic magnetic characterization. Most of the samples were demagnetized at 5, 10, 15, 20, 30, 40, and 50 mT to permit principal component analysis. For rock-magnetic characterization, anhysteretic remanent magnetization (ARM) was given in 0.2-mT DC and 200-mT AC fields and isothermal remanent magnetization (IRM) in a DC field of 1 T. Some discrete samples were progressively saturated up to 1.0 T to study the hardness of the IRM.

Results

Long-Core Measurements

The long-core measurements for the APC cores are presented in Figures F16, F17, and F18, which show inclination and intensity for Holes 1170A, 1170B, and 1170C. Strongly disturbed sections are indicated and can be recognized by their anomalously high negative inclination. There are substantial lengths with an almost unbroken record of these high negative inclinations, which limited magnetostratigraphic work. The intensity of magnetization was between 10-5 and 10-4 mA/m. Even though measurements approached the background noise generated by core liners and the instrument's measurement threshold, a polarity record could be interpreted from distinctive patterns.

Hole 1170A was the least disturbed in the top 100 mbsf and afforded the best record of reversals. It was from this hole that the bulk of the magnetostratigraphy was interpreted. The Brunhes/Matuyama Chron boundary, the Jaramillo Subchron, the Olduvai Subchron, and the beginning of the Gauss Chron were all identifiable and provided a probable magnetostratigraphy for this first 100 mbsf (Fig. F16). Although individual features could be recognized in more than one hole, no consistent correlation between holes was achieved.

Below the first 100 mbsf in the APC cored interval, reversed inclinations in undisturbed sediments are seen in Cores 189-1170B-14H through 16H (Fig. F17). They appear to record the Gilbert Chron from 124 to 138 mbsf and below that the C3r Chron. Core 189-1170B-16H appears to contain the sequence of Subchrons C3An.1n to C3An.2n, which allowed the magnetostratigraphy to be established at this depth.

Below the APC cores, ~100 m of disturbed sediment with poor recovery precluded any further interpretation of magnetostratigraphy until a depth of ~250 mbsf was reached. There was better recovery below this depth with the improved coring conditions; hence, identification of a sequence of magnetozones from Subchron C5A.4n at 264 mbsf down to Chron C6n at 341 mbsf (Fig. F18) was possible. However, gaps in the record prevented the precise location of some chron boundaries. Near 400 mbsf, another section with recognizable magnetozones was found (Fig. F18). In Cores 189-1170A-43X and 44X, Subchrons C7n.1n, C7n.2n, C7An, and the termination of Subchron C8n.1n were identifiable.

Below 400 mbsf, identifiable chron boundaries were not observed. The core is dominantly normally magnetized over an interval of nearly 400 m. This suggests that either major remagnetization has taken place or there was a very high sedimentation rate. Comparison of the NRM and demagnetized results from Section 189-1170D-9R-1 demonstrated that the RCB coring imparts a steeply downward inclined moment, which is largely removed by 20-mT demagnetization (Fig. F19). Hence, the normal magnetization is not caused by the standard drill overprint. Another potential problem lies in the moment from the disturbed soft material between biscuits. To minimize this effect, the data were filtered and only sections which ranked <2 in the 0-5 scale of soft intervening material between the biscuits (see "Lithostratigraphy") are shown in Figure F20. Even after this treatment, it was not possible to interpret this interval magnetostratigraphically. The origin of the normal moment bias remains unclear, but it is likely to come from the soft disturbed material between the biscuits, which carries a shallow normal magnetization.

Discrete Samples

The rock magnetic properties of the nannofossil oozes were similar over the depth of the APC coring. IRM intensities were on the order of 10-2 A/m, ARM intensities about one order less, and the NRM initially another order of magnitude lower at 10-4 A/m (Fig. F21). The ratio of ARM:IRM is not typical of single-domain grain sizes in magnetite; hence, we interpret that there is an important contribution from a detrital source rather than the dominantly biogenic magnetite found at Site 1168. The NRM was initially a downward-oriented drill moment, which was largely removed by 20 mT, revealing the polarity of the magnetization, although precise determinations of directions were precluded by the weak signal.

The rock-magnetic characteristics of the chalks obtained with XCB drilling were similar to those of the nannofossil ooze (Fig. F21), suggesting that lithification had not affected important changes in the magnetic fraction. The NRM was variable, with few samples showing analyzable results—largely because the small volume of the samples precluded good measurements.

The discrete samples from the silty claystone of Hole 1170D have consistent rock magnetic properties downcore. Moreover, they are similar to the characteristics of the nannofossil oozes and chalks (Fig. F21). This result suggests that they all have similar magnetic fractions, which in turn implies they all come from similar sources. It also suggests that the magnetic mineralogy is not likely to be varying substantially downhole, although this will have to be investigated during postcruise analysis.

There is some variation downhole that correlates roughly with magnetic susceptibility (see "Physical Properties"). Figure F22A shows the variation of ARM and IRM with depth, which peaks in lithostratigraphic Units IV and V. Parameters that serve as proxies for the coarse-and fine-grained magnetic fractions are shown as a function of depth in Figure F22B. The normalized fraction of IRM demagnetized between 0 and 20 mT is a measure of the coarsest fraction. In contrast, the normalized IRM acquired between 200 and 500 mT is a measure of the finest fraction. These two vary downhole, and the values cross at 300 m, reflecting a finer magnetic fraction in the upper part and a coarser fraction below.

Magnetostratigraphy and Age-Depth Estimates

Using the reliable sections from Holes 1170A, 1170B, and 1170D, a magnetostratigraphy has been established (Table T14; Figs. F16, F17, F18). This has permitted an age-depth curve to be constructed (Fig. F23) that gives an estimate of sedimentation rates. It appears to indicate two different sedimentation rates—one from the Holocene down to the late Pliocene of 33 m/m.y. and a second of ~11 m/m.y. from the late Pliocene to the late Oligocene. A third, far greater sedimentation rate of 100 m/m.y. or more is likely in the Eocene section at the bottom of Hole 1170D, where we recorded a constant normal polarity corresponding presumably to a single polarity chron. From a purely magnetic viewpoint, this long magnetozone can be either Chron 20n or Chron 21n at ~45 Ma. Without a better sequence of reversals, we cannot determine the age reliably. If we accept the biostratigraphic constraints, it appears that the long normal magnetization is either Chron C20n or C21n. No matter which choice is made, it appears that sedimentation of ~100 m/m.y. is required. The final estimates of these sedimentation rates will be made in conjunction with the biostratigraphic data (see "Biostratigraphy").

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