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doi:10.2204/iodp.proc.341.106.2014

Paleomagnetism

The natural remanent magnetization (NRM) of Site U1420 archive-half cores was measured before and after alternating field (AF) demagnetization. Peak AFs were restricted to a maximum of 20 mT for most sections recovered using the RCB system, although peak AFs of 40 mT were occasionally used (Table T9). Reduced core recovery allowed a five-step demagnetization procedure with peak AFs of 0, 5, 10, 15, and 20 mT to be routinely employed. In addition to the typical measurement interval of 2.5 cm, a 1 cm interval was used on sections with limited recovery of undisturbed sediment. Sections completely disturbed by drilling, as noted by the Lithostratigraphy and/or Paleomagnetism groups, were not measured. Lonestones were removed prior to measurement when possible to avoid their undue influence on the magnetic signal. Data associated with intervals affected by obvious drilling deformation were culled during data processing.

The NRM intensities in Hole U1420A are relatively strong before (10^–2 ~ 10^–1 A/m) and after AF demagnetization at peak fields of 20 mT (10^–3 to 10^–2 A/m). Intensities systematically drop through AF demagnetization using peak AFs from 5 to 20 mT and are consistent throughout the drilled interval (Fig. F25). Intervals of clast-rich diamict usually have higher intensities than those associated with clast-poor facies (see “Lithostratigraphy”).

After AF demagnetization, inclinations average around values expected (approximately ±73.5°) for a geocentric axial dipole at the site latitude, but values significantly steeper or shallower than expected are not uncommon. These variations are likely associated with heterolithic facies that are common at this site. The magnetic moment of sand, gravel, and cobbles is likely to be randomly oriented, compared with the finer sediment matrix that appears, when recovered without abundant clasts, to be well aligned with the geomagnetic field.

Because inclinations at Site U1420 are exclusively positive, indicative of normal polarity (Fig. F25), it is thought that the sediment recovered over the 1014 m CSF-A drilled interval is exclusively within the Brunhes Chronozone and younger than 0.781 Ma (Cande and Kent, 1995; Hilgen et al., 2012). The assignment of this normal polarity interval to the Brunhes Chronozone is consistent with the radiolarian biostratigraphy (see “Paleontology and biostratigraphy”).

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