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doi:10.2204/iodp.proc.323.107.2011

Paleomagnetism

The archive halves of all cores recovered at Site U1343 were measured on the three-axis cryogenic magnetometer. All measurements were done at 2.5 cm intervals for APC cores and 5–20 cm intervals for XCB cores. Natural remanent magnetization (NRM) was measured before (NRM step) and/or after (demagnetization step) stepwise alternating-field (AF) demagnetization in peak fields up to 20 mT. Cores from Hole U1343A were measured at NRM step and 20 mT demagnetization step; other cores from Site U1343 were measured only at 20 mT demagnetization step to keep up with core flow.

Inclination and intensity after 20 mT AF demagnetization step from Holes U1343A, U1343C, and U1343D are plotted in Figure F16; data from Hole U1343E are plotted in Figure F17. Average inclination values are nearly 70° for the normal polarity intervals, which is 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. Inclination values from Hole U1343E cores and the 60 point averages are plotted in Figure F18. 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 was clearly identified in Holes U1343A, U1343C, and U1343E between 180 and 185 mbsf. Both the termination and the onset of the Jaramillo Subchron were identified in Hole U1343E. Below this depth, inclination tends to cluster around normal polarity values, which makes it hard to identify polarity zonation. The top boundary of the normal polarity zone at ~292 mbsf is tentatively identified as the termination of the Cobb Mountain Subchron. These depths and ages are listed in Table T16.

The relative paleointensity of cores from Site U1343 was estimated to examine potential geochronological information during the Brunhes Chron. Figure F19 shows relative paleointensity estimates based on normalizing NRM after 20 mT AF demagnetization by magnetic susceptibility for the uppermost 100 m CCSF-A. The paleointensity variation has large amplitude and obviously shows a coherent change with magnetic susceptibility (Fig. F19), suggesting that NRM intensity was largely influenced by environmental change. Figure F19 also shows that lower (higher) NRM intensities are always associated with lower (higher) magnetic susceptibility values. Figure F20 shows NRM intensity after 20 mT demagnetization plotted against magnetic susceptibility for Site U1343 cores. If magnetic grains in the sediments were unaffected by magnetic mineral dissolution or authigenesis and paleointensity were constant, the data in Figure F20 would be distributed linearly along a trend through the origin. A higher slope would indicate stronger paleointensity during deposition and/or finer magnetic grain size in the sediments. A lower slope would indicate weaker paleointensity and/or coarser magnetic grains in the sediments. The data distribution shows that high NRM intensities are absent in the low magnetic susceptibility range (about <75 × 10^–5 SI), which means that the lower magnetic susceptibility sediments lack finer magnetic grains. This observation suggests that intervals of low magnetic susceptibility (Fig. F19) lack fine magnetic grains, possibly because anaerobic conditions in the sedimentary column cause fine magnetic grains to dissolve much faster than coarse magnetic grains. Consequently, NRM data from Site U1343 cores are not suitable for paleointensity reconstruction but may be useful for the study of paleoenvironmental changes.

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