Core archive halves from Hole 1125A and Cores 181-1125B-1H to 39X from Hole 1125B were measured on the shipboard pass-through cryogenic magnetometer. Declination, inclination, and intensity of natural remanent magnetization (NRM) and 10-mT (for a few cores only) and 20-mT alternating-field (AF) demagnetization steps were routinely measured at 5-cm intervals on core from Hole 1125A. For Hole 1125B, cores were only measured at the 20-mT step because of the time constraints. Measurements were stopped after Section 181-1125-39X-4 because the intensity of remanence had dropped below the noise level of the shipboard cryogenic magnetometer. In situ Tensor tool data were collected from APC cores from Hole 1125A (from Cores 181-1125A-3H through 22H and 181-1125B-4H through 20H). Only inclination could be used to determine magnetic polarity of Holes 1125A and 1125B. At least two oriented discrete samples were collected from the working half of each core interval in Holes 1125A and 1125B (up to Section 181-1125B-39X-4) for shore-based progressive AF and thermal demagnetization and rock magnetic studies. Whole-core magnetic susceptibility was routinely measured on all cores using a Bartington susceptibility loop on the automated multisensor track (MST).
The NRM intensity in Site 1125 averages between 10-4 and 10-5 A/m. Higher remanence spikes (10-2 A/m) occur in the vicinity of tephra layers (Fig. F10). As for other sites on Leg 181, a steep downcore drilling-induced remanent magnetization dominates the NRM (Fig. F11). No rock-magnetic investigation has been conducted because of the time constraint at this site.
AF demagnetization of 20 mT reduces the intensity of magnetization to 10-5-10-6 and mostly removes the steep downcore overprint. At such weak intensities, the inclination data are very noisy and polarity interpretation is difficult. However, between 50 and 140 mbsf, inclination directions in Holes 1125A and 1125B show some polarity changes. These are slightly better defined in Hole 1125A.
The inclination record between 0 and 14 mbsf is mostly normal. The LO of the nannofossils Helicosphaera inversa (0.16 Ma, 2.05 mbsf) and the LO of Pseudoemiliania lacunosa (0.42 Ma, 4.53 mbsf) suggest it most likely represents the Brunhes (C1n) Chron. Inclination between 14 and 39 mbsf is most likely positive (reversed polarity), but includes two possible normal events. Because it immediately underlies the Chron (C1n), and because it contains the LO of the nannofossils Reticulofenestra asanoi (0.85 Ma, 23.6 mbsf) and H. sellii (1.26 Ma), and the FO of the nannofossils Gephyrocapsa parallela (0.95 Ma, 23.6 mbsf) and Gephyrocapsa (medium) (1.67, 28.2 mbsf), it is most likely to be Chron C1r (upper Matuyama). The two normal polarity excursions are possibly the Jaramillo (C1r.1n) and Cobb Mountain (C1r.2r-1n) Subchrons. Another normal polarity is identified at ~40 mbsf, which might be the Olduvai (C2n) Chron. Between 42 and 48 mbsf, the LOs of the nannofossils Discoaster brouweri (1.96 Ma, 42 mbsf) and D. tamalis (2.76 Ma, 47.1 mbsf) suggest that this interval should be within Chron C2r; however, the inclinations are ambiguous and further investigation is needed to confirm the polarity.
Between 48 and 70.5 mbsf, polarity is dominantly normal and may represent the Gauss normal chron (C2An). Two short reversed subchrons exist within Chron C2An of the Geomagnetic Polarity Time Scale (Cande and Kent, 1995; Berggren et al., 1995). However, these are not obvious in the inclination record at Site 1125. Polarity is ambiguous between 70.5 and 93 mbsf. The inclination record between 93 and 203 mbsf is more variable, with alternating normal and reversed polarity. The acme of the nannofossil Gephyrocapsa (small) (3.88 Ma) and the FO of the nannofossil P. lacunosa (4.0 Ma) at 80.8 and 94.9 mbsf respectively and the LO of the diatom Hemidiscus triangularis (5.3 Ma, ~220 mbsf) suggest that the interval between 93 and 200 mbsf should be within the Gilbert Chron (C2Ar, C3n, and C3r). Several polarity changes may represent the subchrons of C3n but the inclination record is too ambiguous for this level of interpretation. These tentative chron assignments are summarized in Fig. F12.
Hole 1125B was advanced to 547 mbsf by XCB coring, but paleomagnetic interpretation was not possible beneath 200 mbsf as intensity of remanence was below the noise level of the shipboard cryogenic magnetometer. Further shore-based research is necessary before more detailed magnetic polarity interpretation can be made.