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doi:10.2204/iodp.proc.323.108.2011

Paleomagnetism

The archive halves of all cores recovered at Site U1344 were measured on the three-axis cryogenic magnetometer. All measurements were done at 2.5 cm intervals for APC cores and 2.5–5 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. All APC cores from Holes U1344A and U1344B as well as Cores 323-U1344D-1H through 6H and 323-U1344E-1H and 2H were measured at NRM step and 20 mT demagnetization step; other cores from Site U1344 were measured only at 20 mT demagnetization step to keep up with core flow.

Inclination, declination, and intensity after 20 mT AF demagnetization with magnetic susceptibility for Hole U1344A are plotted in Figure F20. Inclination and intensity of NRM after 20 mT AF demagnetization with magnetic susceptibility for Holes U1344D and U1344E are plotted in Figure F21. In Figure F20, the declination values of Hole U1344A are plotted as black dots for corrected values using the FlexIt orientation tool and as blue dots for raw values. The corrected declinations cluster well around 0°, indicating that the tool functionally worked at this site. The average inclination values are nearly 70° over the normal polarity intervals observed in Site U1344 cores, which 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.

Both raw and smoothed inclination records from Hole U1344A cores are plotted in Figure F22. The inclination records are so scattered that it is difficult to identify polarity zonations for Hole U1344A cores. However, the inclinations cluster around the site's expected value of 72° in the uppermost 260 m, which can be assigned to the Brunhes normal polarity zone, as well as in some other intervals below this depth. We used the following procedure to extract "well clustering" inclinations from the original data set without subjectivity. According to paleosecular variation analysis (Merrill and McElhinny, 1983), virtual geomagnetic poles (VGPs) observed at the latitude of this site (~60°N) should have an angular dispersion of ~20° that is converted to ~13° in an angular dispersion of inclination. Average inclinations within 72° ± 13° were extracted and interpreted to be normal polarity. The results are shown on the barcode-like polarity chart next to the inclination diagram in Figure F22. Normal polarity zones were defined from the polarity chart and the zonation was correlated to the polarity timescale based on micropaleontology datums (see "Biostratigraphy"). The Brunhes/Matuyama boundary is clearly identifiable at ~280 mbsf. The Jaramillo, Cobb Mountain, and Olduvai subchrons may correlate with the extracted normal polarity zones placed ~380, 420, and 680 mbsf, respectively. These depths and ages are listed in Table T18 with estimated error depth ranges.

The relative paleointensity of Site U1344 cores was estimated to examine potential geochronological information during the Brunhes Chron. Figure F23 displays magnetic susceptibility (top) and relative paleointensity (bottom) for Hole U1344A. These values are based on the normalization of NRM after 20 mT AF demagnetization step by magnetic susceptibility. The paleointensity variation has large amplitude and obviously shows a coherent change with magnetic susceptibility (Fig. F23), suggesting that the NRM intensity has been largely influenced by environmental change. However, there are differences, and the relative paleointensity pattern is consistent with that seen at Sites U1340, U1341, U1342, and U1343. On the basis of those correlations, we estimated marine isotope Stages (MIS) 1–19 in Figure F23.

The dramatic changes in NRM shown in Figure F20 indicate notable effects of early sediment diagenesis, also noted at previous sites. Significant magnetic mineral dissolution starts within 10 mbsf due to anaerobic oxidation of methane (AOM)–sulfate reduction processes (see "Geochemistry and microbiology"). This is also evident at Sites U1343 and U1339. The active zone of dissolution appears to be limited from the seafloor to a ~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.

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