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doi:10.2204/iodp.proc.303306.104.2006

Paleomagnetism

The natural remanent magnetization of the archive half-core sections of Site U1304 were measured and then remeasured after alternating-field (AF) demagnetization in peak fields of up to 20 mT. Cores 303-U1304A-1H through 7H and 17H through 26H were AF demagnetized at peak fields of 10 and 20 mT. Cores 303-U1304A-8H through 13H were AF demagnetized at peak fields of 20 mT. Cores 303-U1304A-14H through 16H were AF demagnetized at 10 mT. It appears that the viscous magnetization component was removed and the characteristic magnetization revealed at peak fields of 10 mT; therefore, all subsequent sections from Holes U1304B, U1304C, and U1304D were only demagnetized to that level.

The magnetization intensity before and after AF demagnetization and the inclination and declination obtained after AF demagnetization are shown in Figures F17, F18, F19. Data associated with intervals identified as drilling slurry, affected by drilling disturbance, or exceptionally coarse grained deposits (see “Lithostratigraphy”) were culled. Intensities are in the 10^–1 A/m range for most intervals (Fig. F18). Intervals rich in diatom oozes (see “Lithostratigraphy”) are, however, less strongly magnetized, with intensities in the 10^–3 A/m range. Based on the differences in both intensity and direction before and after demagnetization, it appears that viscous magnetization components are removed after AF demagnetization at peak fields of 10 mT. Little difference in magnetization direction or intensity is observed between 10 and 20 mT, suggesting that the characteristic magnetic directions are generally adequately defined by the 10 mT step. Inclinations associated with both normal and reversed polarity intervals vary around the expected values (approximately ±69°) for a geocentric axial dipole (Fig. F18). Declinations are consistent within core. Tensor tool-corrected declinations reveal clear polarity zone boundaries (Fig. F19). Holes U1304A and U1304B document an almost continuous sequence recording the Brunhes and much of the Matuyama Chronozones, the Jaramillo Subchronozone and Cobb Mountain Subchronozones, and a fraction of the Olduvai Subchronozone. The polarity transition at the top of the Olduvai polarity subchronozone is observed near the base of the recovered sections at ~260–265 mcd (Table T18).

For a few intervals, polarity interpretations are ambiguous. In the diatom ooze at 195–204 mcd, below the Cobb Mountain Subchronozone, AF demagnetization at peak fields of 10 or 20 mT did not succeed in removing a normal polarity magnetic overprint (Fig. F18). In Hole U1304A, the lower Jaramillo polarity transition is apparent in declination (Fig. F19) but is poorly defined in inclination. Hole U1304B clearly shows the transition from normal to reversed polarity in both declination and inclination. Overall, Holes U1304A and U1304B give very consistent records of past polarity changes when plotted versus meters composite depth.

Hole U1304C only penetrated part of the Brunhes Chronozone with poor core quality due to heavy swell (see “Operations” and “Lithostratigraphy”). Drilling in Hole U1304D continued through to the Olduvai normal subchronozone. Holes U1304C and U1304D document a discontinuous record due to a number of disturbed sections caused by poor weather conditions.

No geomagnetic excursions were recorded in sediments from the Brunhes Chronozone. Short intervals of normal polarity are recognized in Holes U1304A and U1304B during the Matuyama Chronozone below the Cobb Mountain Subchronozone. These might reflect the Gardar and/or the Gilsa geomagnetic events previously identified at Ocean Drilling Program Leg 162 Sites 983 and 984 (Channell et al., 2002). Tables T18 and T19 summarize the depths (mbsf and mcd) of polarity zone boundaries identified in the different holes at Site U1304 and their correlation to the geomagnetic polarity timescale (Cande and Kent, 1995).

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