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doi:10.2204/iodp.proc.303306.108.2006

Paleomagnetism

The natural remanent magnetization (NRM) of all undisturbed archive-half core sections from Site U1308 was measured before and after alternating-field (AF) demagnetization in peak fields of 10 mT. Sections 303-U1308B-8H-6, 12H-3, 12H-4, 12H-5 and 303-U1308C-19H-2 and 20H-2 were additionally AF demagnetized in peak fields of 15 and 20 mT. Core 303-U1308F-24H was additionally AF demagnetized in peak field of 15 mT.

The NRM intensities, inclinations, and declinations from Site U1308 are shown in Figures F14, F15, and F16, respectively. Data associated with intervals identified as drilling slurry affected by drilling disturbance or exceptionally coarse grained deposits (see “Lithostratigraphy”) were culled. NRM intensities of Cores 303-U1308A-1H through 24H (lithologic Unit I and Subunit IIA; see “Lithostratigraphy”) range between 10^–2 and 10^–1 A/m. Below Core 303-U1308A-25H, the white nannofossil ooze of lithologic Subunit IIB has much lower intensities in the 10^–4 and 10^–3 A/m range (Fig. F14). After AF demagnetization at 10 mT, intensities are reduced by ~30% (Fig. F14). Further AF demagnetization at peak fields of 15 mT or 20 mT produced only minor intensity changes and was not routinely undertaken.

Steep positive inclinations observed prior to demagnetization and caused by the drill string magnetic overprint are generally removed by peak AF demagnetization of 10 mT, and inclinations at that level vary around the expected values (approximately ±68°) for a geocentric axial dipole. Tensor tool–corrected declinations are consistent with inclination-defined polarity interpretations. These results suggest that the characteristic remanent magnetization is usually adequately defined after AF demagnetization at 10 mT peak fields.

Holes U1308A–U1308C, U1308E, and U1308F (only one core was recovered from Hole U1308D) document an apparently continuous sequence of polarity transitions allowing correlation to the geomagnetic polarity timescale (Cande and Kent 1995; Channell et al., 2002; 2003) (Figs. F15, F16). Identification of the Brunhes, Matuyama, and Gauss Chronozones are unambiguous. Additionally, the lower part of Hole U1308A appears to extend into the Gilbert Chronozone. Within the Matuyama Chronozone, the Jaramillo, Cobb Mountain, Olduvai, and Reunion Subchronozones are identified, as are the Kaena and Mammoth Subchronozones within the Gauss Chronozone. The top transition of the Olduvai Subchronozone is truncated at the top and base of Cores 303-U1308A-24H and 303-U1308B-24H. A short interval of normal polarity below the Jaramillo Subchronozone is tentatively interpreted as the Punaruu geomagnetic excursion previously found in a volcanic section on Tahiti (Chauvin et al., 1990) and at ODP Leg 162 Sites 983 and 984 (Channell et al., 2002).

The Gauss/Matuyama transition is complicated by an interval of nonhorizontal bedding (see “Lithostratigraphy”) common in Cores 17H through 19H of the different holes. Inclinations above the interpreted Gauss/Matuyama boundary are shallower than expected, possibly as a result of structural deformation. In the lower part of Hole U1308A, a mixed polarity interval is tentatively interpreted as the Gilbert Chronozone. Table T26 summarizes the depths (mbsf and mcd) of the polarity transitions and events identified in the different holes at Site U1308 and the correlation to the geomagnetic polarity timescale (Cande and Kent 1995; Channell et al., 2002, 2003). Age interpretations are summarized in Table T27.

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