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doi:10.2204/iodp.proc.303306.106.2006

Paleomagnetism

Natural remanent magnetization (NRM) of the archive-half core sections of all cores was measured before and after alternating-field (AF) demagnetization in a peak field of 10 mT. One or two additional steps of AF demagnetization were conducted for some core sections from Holes U1306A and U1306B. Core 303-U1306A-13H and Sections 303-U1306A-22H-2 and 22H-3 were demagnetized at AF peak fields of 10, 15, and 20 mT. Sections 303-U1306A-23H-3 to 24H-7, 303-U1306A-25H-4 to 33H-7, and 303-U1306B-15H-2, 15H-5, 16H-1 to 17H-3, 17H-6 to 17H-7, 19H-2, 22H-1 to 27H-5, 31H-4, 31H-6 to 31H-7, and 32H-1 to 33H-7 were demagnetized at AF peak fields of 10 and 20 mT.

The magnetic intensities, inclinations, and declinations before and after AF demagnetization are shown in Figures F19, F20, and F21. Data associated with intervals identified as drilling slurry, disturbance, and exceptionally coarse grained deposits (see “Lithostratigraphy”) were culled. Intensities of the NRM are in the 10^–1 to 1 A/m range for most intervals (Fig. F19). AF demagnetization at 10 mT peak field significantly reduced the intensities. Further AF demagnetization up to 20 mT peak field produced only minor changes (Fig. F19).

The polarities of the characteristic remanent magnetization (ChRM) were not discernible in the magnetic directions measured before AF demagnetization because of a magnetic overprint acquired during the coring process. ChRM became visible after AF demagnetization at peak field of 10 mT. Little difference is observed between the magnetic directions obtained at AF demagnetization of 10 and 20 mT, so most of the overprint and viscous magnetic components were removed by 10 mT AF peak field demagnetization. Inclinations after AF demagnetization at 10 mT peak field vary around the expected values (~±73°) for a geocentric axial dipole (GAD) during normal and reversed polarity intervals. Declinations calculated with available Tensor-tool corrections are consistent with the polarity zones defined by inclinations. These results indicate that the ChRM is usually adequately defined after AF demagnetization at 10 mT peak field. A few sections, particularly from the lower part of the sections, required additional AF demagnetization at peak field up to 20 mT to clearly identify the polarity of their magnetization.

The four holes document an apparently continuous sequence including the Brunhes Chronozone and much of the Matuyama Chronozone. The Jaramillo, Cobb Mountain, and Olduvai Subchronozones are clearly identified. The lower Jaramillo polarity transition is truncated by a sand layer at a depth of ~165 mcd. The polarity transition near the base of Holes U1306A and U1306B at a depth of ~311 mcd (Table T18) represents the base of the Olduvai Subchronozone. The Iceland Basin Event (Channell et al., 1997) is observed in sediments of Holes U1306A, U1306C, and U1306D at a depth of ~32 mcd in the Brunhes Chronozone. A short interval of normal polarity below the Cobb Mountain Subchronozone is tentatively interpreted as the Bjorn geomagnetic event previously identified at Ocean Drilling Program (ODP) Leg 162 Sites 983 and 984 (Channell et al., 2002). Tables T18 and T19 summarize the depths (mbsf and mcd) of the polarity transitions at Site U1306 and their correlation to the geomagnetic magnetic polarity timescale of Cande and Kent (1995).

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