Previous   |    Next

doi:10.2204/iodp.proc.303306.105.2006

Paleomagnetism

The natural remanent magnetization (NRM) of the archive-half core sections of Site U1305 were measured and remeasured after AF demagnetization in peak fields of up to 20 mT. Cores 303-U1305A-1H through 19H, Sections 303-U1305A-20H-3 and 20H-4, Cores 303-U1305B-1H to 16H, and Cores 303-U1302C-1H to 18H were AF demagnetized at 10 mT. Section 303-U1305C-17H-3 was demagnetized at 5 and 10 mT. Core 303-U1305B-17H and Sections 303-U1305C-21H-1 and 21H-6 were demagnetized at 10, 15, and 20 mT. Sections 303-U1305C-21H-2 through 21H-5 were demagnetized at 10 and 15 mT. Sections 303-U1305A-20H-1, 20H-2, 20H-5, 20H-6, and 20H-7 and Cores 303-U1305A-21H through 30H, 303-U1305B-18H through 28H, 303-U1305C-20H and 22H through 31H were demagnetized at 10 and 20 mT. The number of demagnetization steps and the peak field used reflected the demagnetization 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. Data associated with intervals identified as drilling slurry, deformation, and exceptionally coarse deposits (see “Lithostratigraphy”) were culled.

The NRM intensities at Site U1305 are strong both before and after demagnetization. The intensities show variability at both the meter scale and throughout the sequence. Intensities before demagnetization range from the high 10^–1 to >1 A/m. After demagnetization at peak fields of 10 or 20 mT, intensities are reduced to the low 10^–1 A/m range. Below 166 mcd, mean intensities are ~50% of what they are above (Fig. F18). Meter-scale variations of a factor of 3 or 4 are observed throughout the sequence.

Steep positive inclinations observed prior to demagnetization and due to the drill string magnetic overprint are removed by peak AF demagnetization of 10 mT for the upper 160–200 mcd (Fig. F19). Below this depth, higher demagnetization levels were required to remove the drill string overprint. The highest peak fields used (20 mT) were only partially successful below 210 mcd (Fig. F19). Inclinations associated with normal and reversed polarities of the upper 200 mcd vary around the expected values (approximately ±72) for a geocentric axial dipole. Declinations show within-core consistency and, when Tensor tool-corrected, facilitate polarity interpretation (Fig. F20). The directional record from Holes U1305A, U1305B, and U1305C document an almost-continuous sequence allowing correlation to the geomagnetic polarity timescale (Cande and Kent, 1995; Channell et al., 2002) (Fig. F19). The Brunhes and the upper Matuyama Chronozones including the Jaramillo Subchronozone are clearly identified. The Cobb Mountain Subchronozone and part of the Olduvai Subchronozone are also identifiable, though these polarity zones are less clear because of the incomplete removal of the normal polarity drill string magnetic overprint. Figures F19 and F20 and Tables T16 and T17 document the polarity zones and corresponding age interpretations.

Top of page   |    Previous   |    Next