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doi:10.2204/iodp.proc.303306.103.2006

Paleomagnetism

The natural remanent magnetization (NRM) of the archive half-core sections of Sites U1302 and U1303 were measured and remeasured after alternating-field (AF) demagnetization in peak fields of up to 20 mT. Cores 303-U1302A-1H through 7H and 303-U1302C-1H were AF demagnetized at 5, 10, 15, and 20 mT. Cores 303-U1302A-8H through 9H were AF demagnetized at 10, 15, and 20 mT. Cores 303-U1302A-10H, 303-U1302B-1H through 10H, and 303-U1302C-2H through 11H and core from Holes U1302D, U1302E, U1303A, and U1303B were AF demagnetized at 10 and 20 mT. Cores 303-U1303A-11H through 13H and 303-U1303B-11H were demagnetized at 20 mT. The number of demagnetization steps was influenced by the magnetic characteristics of the sediments 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 affected by drilling disturbance and exceptionally coarse deposits (see “Lithostratigraphy”) were culled.

The NRM intensities at both Sites U1302 and U1303 are strong with meter-scale variations of up to a factor of 3. Intensity values after demagnetization with 20 mT peak fields ranged from ~1.5 × 10^–1 to 5 × 10^–2 A/m in lithologic Subunit IA (down to ~106 mcd) (Fig. F21). NRM intensities are lower in lithologic Subunit IB (below ~106 mcd) with values after demagnetization in 20 mT peak fields varying around 1 × 10^–2 A/m (Fig. F21).

Steep positive inclinations observed prior to demagnetization and due to the drill string magnetic overprint are removed by 10 mT peak AF demagnetization. Intensity continues to decrease between 10 and 20 mT demagnetization, although inclinations and declinations show little difference. Inclinations in lithologic Subunit IA are consistent with those expected from a geocentric axial dipole (GAD) (~67°) for the 50°10′N latitude of these sites (Fig. F22). Declinations (Fig. F23) show within-core consistency. Inclinations and declinations of Subunit IB are more variable, likely reflecting its origin as a debris flow (see “Lithostratigraphy”). The positive inclinations of lithologic Subunit IA are interpreted as a primary magnetization indicating normal polarity deposition entirely in the Brunhes Chron (see also “Biostratigraphy”). A short interval of negative inclinations and reversed declinations was observed in correlative sediments in Sections 303-U1302C-3H-5, 303-U1302B-3H-2, and 303-U1303A-3H-2. Based on shipboard stratigraphic estimates, this feature likely reflects the Iceland Basin Event with an estimated age of 187 ka (Channell et al., 1997). The polarity zones from lithologic Subunit IB are not clearly defined. Sediments with a more tranquil depositional origin were found below the debris flow deposits at the base of Subunit IB in Hole U1302A. These show reversed inclinations with relatively well defined declination over two sections underlain by positive inclination and a 180° declination shift to the base of the cored section. Biostratigraphic interpretation suggesting a late Matuyama age for these sediments is consistent with the reversed inclinations. Whether the transition to positive inclination observed in Core 303-U1302C-12H represents a true geomagnetic reversal (possibly the upper Jaramillo) is not unequivocally supported by the biostratigraphic data or the overall quality of the paleomagnetic data in Subunit IB.

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