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doi:10.2204/iodp.proc.341.107.2014

Paleomagnetism

The natural remanent magnetization (NRM) of Site U1421 archive-half cores was measured before and after alternating field (AF) demagnetization. Peak AFs were generally restricted to a maximum of 10 or 20 mT for most sections recovered using the APC system with standard half-length and nonmagnetic full-length core barrels (see “Operations”) and the XCB system (Table T11). Peak AFs of 30 mT were used on Section 341-U1421A-62X-1A. A two-step measurement sequence with peak AFs of 10 mT, a three-step measurement sequence with peak AFs of 10 and 20 mT, and a five-step measurement sequence with peak AFs of 5, 10, 15, and 20 mT were all employed. In addition to the typical measurement interval of 2.5 cm, a 1 cm interval was used when the recovery of suitable material for polarity determinations within a section was limited. The number of demagnetization steps, the peak field used, and the resolution of measurements reflected the demagnetization characteristics of the sediment, the severity of the drill string magnetic overprint, the desire to use low peak fields to preserve the magnetization for future shore-based studies, and the quality of the section recovered. Sections completely disturbed by drilling, as noted by the Lithostratigraphy and/or Paleomagnetism groups, were not measured. Lonestones were carefully removed prior to section measurement without disturbing the sediment. Data associated with intervals affected by obvious drilling deformation or large clasts were culled during data processing.

NRM intensities at Site U1421 were relatively strong before AF demagnetization, ranging from 10^–1 A/m in the APC-recovered intervals to 10^–2 A/m in the XCB-recovered intervals (Fig. F28). Whether this results from differences in material recovered or magnetic properties of the core type is not initially obvious. In the APC-recovered sections, the steel core barrels resulted in slightly higher intensities than in the nonmagnetic barrels prior to demagnetization. After AF demagnetization at peak fields of 10 or 20 mT, intensities are generally in the 10^–3 to 10^–2 A/m range and consistent throughout the recovered interval. Intensities from the APC-recovered intervals compared on their CSF-A depth scales are consistent between holes, varying at both the meter and decameter scales (Fig. F29).

Steep, positive inclinations observed in the APC section prior to demagnetization, likely due to the drill string magnetic overprint, were generally removed by peak AF demagnetization of 10 mT (Fig. F29). After AF demagnetization, inclinations average around values expected (approximately ±73.5°) for a geocentric axial dipole (GAD) at the site latitude for much of the recovered interval, although values significantly steeper or shallower than GAD predictions are not uncommon (Fig. F28). These steeper or shallower values are not an unexpected observation considering the clast-rich facies drilled. Almost exclusively positive inclinations are consistent with normal polarity throughout the recovered sequence, and it is therefore considered to be entirely within the Brunhes Chronozone (0–0.781 Ma; Cande et al., 1995; Hilgen et al., 2012), which is consistent with biostratigraphic evidence (see “Paleontology and biostratigraphy”).

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