Tensor tool orientations were successfully used to correct the magnetic declinations from 16.70 to 205.30 mbsf (Cores 190-1175A-3H through 22H). After measuring the natural remanent magnetization (NRM), all sections of the archive half of the core were partially demagnetized using AF magnetization at 30 mT at 5-cm intervals to remove magnetic overprints. Inclination data obtained after AF demagnetization provide useful information for interpreting late Pliocene to Pleistocene geomagnetic polarity reversals. However, identification of geomagnetic polarity intervals in the lower part of Hole 1175A was difficult because of poor core recovery.
To gain a better understanding of core disturbance caused by drilling, an experiment was conducted to compare the NRM of whole-round cores, working-half cores, and archive-half cores.
The declinations of cores from 16.70 to 205.30 mbsf (Cores 190-1175A-3H through 22H) were corrected using Tensor tool orientation data. The gradual change in declination in the range from -90° to 90° reflects the secular variation of the geomagnetic field, although many short intervals with reversed polarity inclinations in this secular variation curve are considered to be possible short geomagnetic events (Fig. F15). Scattered declinations below 205.30 mbsf indicate that several pieces of core rotated individually during XCB coring.
The majority of NRM for all sections is strongly biased toward steep inclinations of ~60°-80°. These overprints were successfully removed by AF demagnetization at 30 mT. Stable positive inclinations were continuously observed from 0 to 298.8 mbsf (Fig. F15). However, short intervals of reversed polarity inclinations were identified. These reversed inclinations probably reflect geomagnetic excursions during this normal chron. This is especially true for the seven reversed polarity inclination peaks that correspond to declination peaks (Table T11). To distinguish between magnetic excursions and core disturbance, detailed postcruise investigations of these inclination anomalies and comparisons with other magnetic records are required.
Magnetic intensity shows slightly low values with some high-intensity peaks from 0 to 311.38 mbsf (Section 190-1175A-33X-CC, 30 cm). These high-intensity peaks closely correspond to high-susceptibility peaks measured with the multisensor track (MST) (see "Physical Properties") and likely reflect the presence of magnetic minerals in sediments such as ash layers. Higher intensity was observed below 311.38 mbsf, although it is difficult to interpret intensity changes at this depth because of poor core recovery.
Site 1175 magnetostratigraphy is based on polarity changes determined by measuring the inclination of the archive half of the core after AF demagnetization at 30 mT. A magnetic polarity change from normal to reversed at 298.80 mbsf (Section 190-1175A-32X-5, 80 cm) is interpreted as the Brunhes/Matuyama Chron boundary dated at 0.78 Ma (Cande and Kent, 1995) (Fig. F16). Seven short reversed polarity events are also observed in the Brunhes Chron at 17.00 mbsf (Section 190-1175A-3H-1, 30 cm), 24.00 mbsf (Section 3H-5, 130 cm), 35.73 mbsf (Section 4H-CC, 5 cm), 73.00 mbsf (Section 8H-6, 130 cm), 92.75 mbsf (Section 11H-1, 5 cm), 158.00 mbsf (Section 18H-1, 20 cm), and 205.35 mbsf (Section 23X-1, 5 cm). These short intervals are thought to represent geomagnetic excursions. Based on previous studies, eight major excursions have been identified in this chron (Champion and Lanphere, 1988). However, identification of excursions is sometimes difficult because they are usually very short events. Furthermore, the possibility of core disturbance remains. In order to identify the excursions, comparison with paleomagnetic results from other locations must be made. The reversed polarity at 158.00 mbsf (Section 190-1175A-18H-1, 20 cm) may be an excursion corresponding to C1n-1 of Cande and Kent (1995).
The Matuyama Chron (0.780-2.581 Ma), characterized by a predominantly reversed polarity, is interpreted to extend from 298.80 mbsf to the bottom of the hole at 435.65 mbsf. In this reversed polarity chron, scattered intensity changes and poor core recovery make identification of short normal polarity geomagnetic events difficult. However, the nearly continuous normal polarity interval below 309.8 mbsf (Section 190-1175A-33X-6, 70 cm) within the Matuyama Chron is considered to be the Jaramillo Event (0.99-1.07 Ma).
Based on the depth and age of the Brunhes/Matuyama boundary, the sedimentation rate of lithostratigraphic Units I and II is estimated at 38.31 cm/k.y. (Fig. F17). The sedimentation rate within Units I and II corresponds closely to the biostratigraphic sedimentation rate curve (see "Biostratigraphy"). The age-depth point of the positive inclination zone (Jaramillo Event) within the Matuyama Chron also corresponds closely to the sedimentation curve estimated by biostratigraphic analysis. The sedimentation rate of Unit III could not be estimated using magnetostratigraphy because of poor core recovery.
To investigate the effect of core disturbance and the drilling-induced magnetic overprint during APC coring, we conducted an experiment to compare the magnetic remanence of whole- and half-core sections. Cores 190-1175A-3H, 7H, and 12H were selected to determine the influence of drilling, core cutting, and core splitting on magnetic measurements.
Whole-round core NRM measurements were taken with the cryogenic magnetometer after susceptibility measurements using the MST. NRM measurements of the working half were taken just after splitting the core with a wire splitter. NRM and magnetic remanence after AF demagnetization at 30 mT were measured on the archive half of the core after lithostratigraphic observation and susceptibility measurements using the AMST (see "Physical Properties").
NRM declination directions of working and archive halves followed similar curves from the top to the bottom of Cores 190-1175A-3H, 7H, and 12H. In contrast, the NRM declination curves of working and archive halves do not correspond to the NRM declinations of whole-round cores (Fig. F18). In Core 190-1175A-3H, declinations from 16.75 mbsf (Section 3H-1, 5 cm) to ~19.50 mbsf (Section 3H-2, 130 cm) and from ~24.00 mbsf (Section 3H-5, 130 cm) to 26.1 mbsf (Section 3H-7, 40 cm) show rapid changes in the range from -180° to 180°. Inclinations of archive and working halves show similar curves as the whole-round cores. A gradual increase of inclinations from about -40° to ~85° was observed from 16.75 mbsf (Section 190-1175A-3H-1, 5 cm) to ~19.50 mbsf (Section 3H-2, 130 cm).
Archive-half declinations after 30-mT demagnetization followed a curve similar to the NRM declination curve of the whole-round cores, although scattered declinations were still observed from 16.75 mbsf (Section 190-1175A-3H-1, 5 cm) to ~19.50 mbsf (Section 3H-2, 130 cm) and from ~24.00 mbsf (Section 3H-5, 130 cm) to 26.1 mbsf (Section 3H-7, 40 cm). After demagnetization, the archive half showed a shallowing of inclinations from NRM inclination values of 90°-80° to 50°-60°. Rapid changes of inclination from -45° to 70° were also observed from 16.75 mbsf (Section 190-1175A-3H-1, 5 cm) to ~19.50 mbsf (Section 3H-2, 130 cm) and from ~24.00 mbsf (Section 3H-5, 130 cm) to 26.1 mbsf (Section 3H-7, 40 cm). The changes in inclination between the NRM of whole-round cores and the magnetic remanence of archive halves after AF demagnetization at 30 mT correspond to those in declination (Fig. F18).
Correspondence of working- and archive-half NRM suggests that sediments in the core have not undergone strong disturbance during APC drilling in this particular case. This is indicated by similar declination and inclination curves between the working and archive halves of the core (Fig. F18). Rapid declination changes in the top and bottom of the core, from 16.75 mbsf (Section 190-1175A-3H-1, 5 cm) to ~19.50 mbsf (Section 3H-2, 130 cm) and from ~24.00 mbsf (Section 3H-5, 130 cm) to 26.1 mbsf (Section 3H-7, 40 cm), indicate that the core top and bottom have probably undergone disturbance during moving on deck after drilling.
The difference in declination between whole-round and halved cores is considered to be secondary magnetic noise acquired during splitting into half sections. Similar NRM declinations between working and archive halves also suggest the sediments acquired a noisy magnetization during splitting.
After AF demagnetization, archive-half declinations correspond to whole-round NRM declinations (Fig. F19), indicating that the magnetic noise was successfully removed from the split half. Archive-half inclinations after AF demagnetization show shallowing from the steep NRM inclination of ~80°-90° to 50°-60°. This shallow inclination corresponds to the expected axial dipole field inclination of 52° at Site 1175. Inclination changes after demagnetization suggest that the magnetic noise acquired by half sections can be removed by weak AF demagnetization.
Two significant discoveries were made during this experiment: (1) core sediments were probably not disturbed strongly during APC drilling in this case and (2) the wire splitting method of cutting a whole-round core creates a magnetic overprint that can be removed by weak AF demagnetization.