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doi:10.2204/iodp.proc.323.106.2011

Paleomagnetism

The archive halves of all sediment cores recovered at Site U1342 were measured on the three-axis cryogenic magnetometer at 2.5 cm intervals. Natural remanent magnetization (NRM) was measured before (NRM step) and/or after (demagnetization step) stepwise alternating-field (AF) demagnetization in peak fields of up to 20 mT. Cores from Hole U1342A were measured at NRM step and 20 mT demagnetization step; other Site U1342 cores were measured only at 20 mT demagnetization step to keep up with core flow.

Inclination and intensity after 20 mT AF demagnetization from Holes U1342A, U1342C, and U1342D are plotted in Figure F19. The average inclination values are nearly 70° over the normal polarity intervals, which are close to the site axial dipole inclination (~72°), indicating that we can effectively remove overprint magnetization caused by the drill pipe and/or core barrel from the NRM records. All inclination records for cores from Holes U1342A, U1342C, and U1342D as well as the 60 point averages are plotted in Figure F20. A polarity zonation was defined from the inclination record and correlated to the polarity timescale based on micropaleontology datums (see "Biostratigraphy"). The Brunhes/Matuyama boundary and the base of the Jaramillo Subchron are clearly identified in Holes U1342A, U1342C, and U1342D. The top of the Jaramillo Subchron and both the top and the base of the Cobb Mountain Subchron are identified in Holes U1342C and U1342D but not in Hole U1342A. Three excursions were tentatively noted in Holes U1342A, U1342C, and U1342D: the Kamikatsura, Santa Rosa, and Punaruu excursions, the depths and ages of which are shown in Table T11.

Relative paleointensity estimates from Site U1341 were compared to estimates from Site U1342. Both estimates are based on normalizing the NRM after 20 mT AF demagnetization by magnetic susceptibility. Magnetic susceptibility was used as a correlation tool to determine if a reproducible relative paleointensity record could be developed for these two sites. Magnetic susceptibility and relative paleointensity for Site U1341 are plotted in Figure F21. Magnetic susceptibility and relative paleointensity for Site U1342 are plotted in Figure F22. The comparison was limited to the Brunhes Chron (last 781,000 y). It is possible to see a correlative pattern of relative paleointensity at the two sites, as indicated by the relative numbering scheme in Figures F21 and F22. However, the relative paleointensity estimates are still significantly influenced by lithologic variability and should not be considered high-resolution estimates of true geomagnetic field intensity variations.

The age-depth relationship at Site U1342 was estimated by combining the information recovered from our estimates of polarity boundaries, excursions, and relative paleointensity (Brunhes only). The ages of excursions and relative paleointensity features are assigned using estimates of how each one is related to a specific marine isotope stage (Fig. F22). The final time-depth curve is shown in Figure F23. The fine-grained hemipelagic marine sediments are estimated to have been deposited during the last ~1.5 m.y., with essentially a constant sediment accumulation rate during the last 1 m.y.

Figure F24 summarizes rock magnetic variability in Hole U1342A. Magnetic susceptibility is shown at the top, and magnetic remanence (after 20 mT AF demagnetization) is shown at the bottom. Note the discrete intervals where both magnetic parameters undergo more than order-of-magnitude decreases that indicate intervals of significant magnetic mineral dissolution. These intervals appear to be closely related to laminated sediment intervals, and both are probably related to enhanced rates of reduction diagenesis during those selected time intervals. The major dissolved intervals are noted in Table T12. Our chronostratigraphic estimates suggest that these dissolved intervals (and their associated laminations) occur in interglacial sediments (Table T12). A few narrow intervals of significantly stronger remanence (Table T13) are tentatively associated with authigenic greigite or ice-rafted pebbles, which always occur in glacial-stage sediments with lower porosity.

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