After measuring the natural remanent magnetization (NRM), all Hole 1178A sections of the archive-half of the core were partially demagnetized using alternating-field (AF) magnetization at 30 mT at 5-cm intervals to remove magnetic overprints acquired during XCB coring. At Hole 1178B, the AF demagnetization level was changed to 50 mT to remove the strong magnetic overprints acquired during RCB coring.
Two or three oriented discrete samples were routinely collected from each section of the working half of the core primarily for shore-based analysis of the anisotropy of magnetic susceptibility. Many discrete samples were taken for this purpose from the highly fractured Hole 1178B sediments.
Sediments were recovered from 0 to 410.8 mbsf in Hole 1178A and from 395.00 to 679.20 mbsf in Hole 1178B.
Magnetic declinations from Holes 1178A and 1178B show very scattered values, caused by rotation of small core pieces during XCB and RCB coring (Fig. F21). Identification of geomagnetic secular variation and/or geomagnetic polarity changes was therefore difficult, although the declinations were successfully used to reconstruct the structural orientation of sediments. After AF demagnetization at 30 mT, magnetic inclinations from Hole 1178A show a late Miocene to Pleistocene geomagnetic polarity record. However, noncontinuous polarity changes from the top to the bottom of both holes indicate the existence of age boundaries in the sediments. At Hole 1178B, most inclinations have continuous negative values (Fig. F21). These unanticipated inclinations may be related to repetitions in the sequence caused by thrust faults.
Sudden decreases and increases in magnetic intensity were also observed in both holes. Three steep changes in the magnetic intensity record were identified, one at 8.6 mbsf (Section 190-1178A-2H-4, 120 cm), one at 209.75 mbsf (Section 190-1178A-24X-1, 95 cm), and one at 660.05 mbsf (Section 190-1178B-30R-1, 5 cm).
All polarity reversals determined from inclinations after AF demagnetization at 30 and 50 mT were compared with the standard geomagnetic polarity time scale of Cande and Kent (1995). The identified chrons and subchrons at Holes 1178A and 1178B are given in Table T11.
A clear magnetic intensity boundary at 8.5 mbsf (Section 190-1178A-1H-1, 5 cm) corresponds to the age boundary at 8.5 mbsf identified by magnetostratigraphy using inclination changes.
Based on biostratigraphic results, inclinations from the top to the bottom of Hole 1178A are grouped into two different geomagnetic polarity periods. The stable normal inclinations from 0 to 8.5 mbsf (Section 190-1178A-1H-1, 5 cm) are considered to represent normal geomagnetic polarity of the Brunhes Chron (Fig. F19). Below 8.5 mbsf, slightly steep positive inclinations are continuously observed to 43.65 mbsf (Section 190-1178A-6H-2, 125 cm).
The correspondence between the horizons of magnetic inclination and intensity change suggests that a hiatus exists at 8.5 mbsf.
According to biostratigraphic results, inclination changes between 8.5 and 323.3 mbsf (Section 190-1178A-35X-6, 100 cm) should reflect Pliocene geomagnetic polarity changes, which are interpreted to be the Gauss (2.581-3.580 Ma) and Gilbert Chrons (3.580-5.894 Ma). A narrow reversed polarity interval from 43.65 (Section 190-1178A-6H-2, 125 cm) to 53.65 mbsf (Section 7H-3, 25 cm) may be the short geomagnetic event C2An.2r (3.220-3.330 Ma) within the Gauss Chron. The beginning of Gilbert Chron (3.580 Ma) is estimated at 89.05 mbsf (Section 190-1178A-11X-3, 125 cm). Relatively stable normal inclinations from 162 mbsf (Section 190-1178A-19X-1, 60 cm) to 203.75 mbsf (Section 23X-4, 5 cm) may represent normal events within Gilbert Chron.
Inclinations change to negative values below 209.75 mbsf. Based on biostratigraphic results, these negative inclinations may represent the reversed polarity within Gilbert Chron. At 323.3 mbsf (Section 190-1178A-35X-6, 100 cm), a normal inclination was clearly observed. This normal inclination is tentatively identified as the beginning of Chron C3A (5.894 Ma). A second normal polarity interval was observed at 353.50 mbsf (Section 190-1178A-39X-1, 30 cm) and may be correlated with the normal event at the beginning Chron C3B (6.935 Ma).
At the bottom of Hole 1178A, steep negative inclinations were seen. A short normal polarity interval from 405.42 mbsf (Section 190-1178A-44X-4, 40 cm) to 408.94 mbsf (Section 44X-CC, 5 cm) was also observed. Based on biostratigraphic results, these steep reversed and normal polarity inclinations may reflect the long reversed polarity period within Chron C4 considered to be Subchron C4r (8.072-8.699 Ma). Therefore, this short normal polarity interval may be the Subchron C4r.1n (8.225-8.257 Ma). This polarity identification suggests the existence of a hiatus at ~400 mbsf.
The magnetic inclinations of Hole 1178B show continuous steep negative values from the top to bottom of the hole except for some short neutral polarities (Fig. F19). Biostratigraphy results suggest that all reversed polarity inclinations may correspond to Subchron C4r (8.072-8.699 Ma). However, more detailed investigations such as rock magnetic tests are needed to verify these results.