Previous   |   Next

doi:10.2204/iodp.proc.323.109.2011

Paleomagnetism

The archive halves of all cores recovered at Site U1345 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 up to 20 mT. All cores from Hole U1345A and Cores 323-U1345C-1H through 7H were measured at NRM step and 20 mT demagnetization step; other Site U1345 cores were measured only at 20 mT demagnetization step to keep up with core flow.

Inclination, declination, and intensity of NRM after 20 mT AF demagnetization with magnetic susceptibility for Hole U1345A are plotted in Figure F18. Inclination and intensity of NRM after 20 mT AF demagnetization with magnetic susceptibility for Holes U1345C–U1345E are plotted in Figure F19. In Figure F18, declination is plotted with values corrected using the FlexIt orientation tool (black dots) and with raw values (blue dots). The corrected declinations cluster well around 0°, indicating that the tool functionally worked for this site. The average inclination values are nearly 70° for the normal polarity intervals observed in Site U1345 cores. These values are close to the site axial dipole inclination (~72°), indicating that overprint magnetization caused by the drill pipe and/or core barrel can be effectively removed from the NRM records. Because there are no apparent polarity reversals in the cores at this site, the entire sequence from Site U1345 is assigned to the Brunhes normal polarity zone.

Relative paleointensity of cores was estimated from Site U1345 to examine potential geochronological information during the Brunhes Chron. Figure F20 shows magnetic susceptibility (top) and relative paleointensity (bottom) for Hole U1345A based on normalizing NRM after 20 mT AF demagnetization by magnetic susceptibility. The paleointensity variation has a large amplitude and obviously shows a coherent change with magnetic susceptibility (Fig. F20), suggesting that NRM intensity was largely influenced by environmental change. However, there are differences, and the relative paleointensity pattern is consistent with that seen at all other sites. On the basis of those correlations, MIS 1–12 are tentatively identified in Figure F20.

The dramatic changes in NRM shown in Figure F18 indicate notable effects of early sediment diagenesis, which were also noted at the previous sites. Significant magnetic mineral dissolution starts within 5 mbsf because of anaerobic oxidation of methane (AOM)–sulfate reduction processes (see "Geochemistry and microbiology"). This is also evident at Sites U1344, U1343, and U1339. The active zone of dissolution appears to be limited from the seafloor to ~10 m depth interval, so magnetization does not change significantly at deeper depths. This dissolution complicates our ability to make relative paleointensity estimates but does not appear to hinder us from gaining a reliable sense of directional changes, both in secular variation and in polarity.

Top of page   |   Previous   |   Next