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doi:10.2204/iodp.proc.303306.107.2006

Paleomagnetism

The natural remanent magnetization (NRM) of all undisturbed Site U1307 archive-half core sections were measured and remeasured after alternating-field (AF) demagnetization in peak fields of 10 mT. Additionally, Sections 303-U1307B-6H-2, 12H-4, 12H-5, and 16H-1 to 16H-4 were demagnetized at 15 mT. Sections 303-U1307A-8H-2, 8H-4, 13H-1, 13H-2, 17H-6, and 18H-1 to 18H-6 and Sections 303-U1307B-8H-4, 9H-1 to 9H-3, and 10H-3 were demagnetized at 20 mT. Sections 303-U1307A-3H-3 to 3H-5, and 4H-2 and Sections 303-U1307B-4H-5, 5H-3 to 5H-6, 6H-6, 13H-5 to 13H-6, and 16H-5 to 16H-7 were demagnetized at both 15 and 20 mT. The number of demagnetization steps and the peak fields used reflected the magnetic characteristics of the sediments, the severity of the drill string magnetic overprint, the shipboard protocol of not exceeding peak fields of 20 mT, and the need to maintain core flow through the laboratory. Sections completely affected by drilling disturbance were not measured. The NRM intensities, inclinations, and declinations from Site U1307 are shown in Figures F14, F15, and F16, respectively. Data associated with intervals identified as drilling slurry, deformation, and exceptionally coarse deposits (see “Lithostratigraphy”) are not shown. Average NRM intensities are strong and variable both before and after demagnetization. Intensities prior to demagnetization are generally in the 10^–1 A/m range and occasionally as low as 1 × 10^–2 A/m. Intensity variations occur at both the meter and tens of meters scales with longer variations having greater amplitude than the shorter ones (Fig. F14). After AF demagnetization at peak fields of 10 or 20 mT, NRM intensities are reduced (low 10^–1 A/m range) and the pattern of variability changes. The amplitude of the meter-scale variability becomes greater, whereas the longer-period variability is reduced. Below ~25 mcd, average values are lower and the general character of the meter-scale intensity variability changes with a downhole trend of decreasing peak values (Fig. F14).

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 (Fig. F15). Demagnetization at higher peak fields (15 or 20 mT) results in little difference in either direction or intensity, indicating that the majority of the drill string overprint was successfully removed. Inclinations associated with normal and reversed polarities vary around the expected values (approximately ±73°) for a geocentric axial dipole. Declinations show within-core consistency and, when Tensor tool-corrected, facilitate polarity interpretations (Fig. F16). The directional record from Holes U1307A and U1307B document an almost-continuous sequence of polarity transitions, allowing correlation to the geomagnetic polarity timescale (Cande and Kent, 1995; Channell et al., 2002; 2003) (Figs. F15, F16, F17). Identification of the Brunhes, Matuyama, and Gauss Chronozones is unambiguous. Within the Matuyama Chronozone, the Jaramillo, Olduvai, and Reunion Subchronozones are identified. The lower Jaramillo polarity transition is truncated by a thick (1.1 m) sand layer in both holes (see “Lithostratigraphy”). A thinner sand layer was observed at the same stratigraphic interval at Site U1306 (see “Paleomagnetism” in the “Site U1306” chapter).

Below the Jaramillo Subchronozone, three normal polarity subchronozones are indicated by positive inclinations (Fig. F15) and reversed declinations (Fig. F16). Assuming constant sedimentation rates between the base of the Jaramillo and top of the Olduvai Subchronozones, a normal to reverse transition underlain by a core break at ~54 mcd is consistent with the top of the Cobb Mountain Subchronozone (Fig. F17). The missing base of the Cobb Mountain Subchronozone could reflect a hiatus, as suggested by nannofossil biostratigraphy. Two normal polarity subzones, each ~1.0 m thick, were observed at ~72.7 and 79.25 mcd in Hole U1307B (Figs. F15, F16). Based on constant sedimentation rates, these subzones are consistent with the Gardar (1.472–1.480 Ma) and Gilsa (1567–1575 Ma) magnetic excursions (Channell et al., 2002) (Fig. F17). Within the Gauss Chronozone, the Kaena and Mammoth Subchronozones are identified, though the normal polarity interval (C2An.2n) between them is thinner than would be expected based on constant sedimentation rates (Fig. F17). Figures F15 and F16 and Tables T12 and T13 document the polarity zonations and corresponding age interpretations.

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