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doi:10.2204/iodp.proc.341.105.2014

Paleomagnetism

The natural remanent magnetization (NRM) of Site U1419 archive-half cores was measured and remeasured after alternating field (AF) demagnetization. Peak AFs were restricted to a maximum of 20 mT for all sections recovered using the APC system with full and half-length, nonmagnetic and steel core barrels (see “Operations”) and recovered using the XCB system (Table T11). The number of demagnetization steps and the peak field used reflect the demagnetization characteristics of the sediments, 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 need to maintain core flow through the laboratory. When time permitted, additional demagnetization steps were added for the uppermost few cores of Holes U1419B and U1419C to facilitate magnetic interpretation. Sections completely affected by drilling disturbance, as noted by the Lithostratigraphy and/or Paleomagnetism groups, were not measured. Data associated with intervals affected by obvious drilling deformation or measurement error (flux jumps; e.g., Richter et al., 2007) were culled prior to uploading or during data processing.

The NRM intensities of materials recovered with the APC (Holes U1419A–U1419E) and XCB (Cores 341-U1419A-21X through 29X) systems were strong before AF demagnetization (10^–1 A/m), but in contrast to Sites U1417 and U1418, there was a larger reduction in intensity after demagnetization (10^–2 to 10^–4 A/m). Transformation of depths to the CCSF-B scale (see “Stratigraphic correlation”) using the multiple APC-drilled holes allows results to be compared between holes. On the CCSF-B depth scale, intensities are consistent between holes and vary at both the meter and decameter scales (Fig. F35). Discrete intervals of very low intensities observed during intervals with biosiliceous ooze and diatom-rich mud suggest a relationship between magnetic mineralogy, lithology (see “Lithostratigraphy”), and potentially redox chemistry (see “Geochemistry”). No clear correlation between core barrel type (full length nonmagnetic, full-length steel, or half-length steel) and magnetization was observed.

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. F36). Inclinations are near values expected for a geocentric axial dipole (approximately ±73.5°) at the site’s latitude, although intervals of shallower than expected or even reversed inclination are observed. These are not thought to reflect a change in polarity but instead either core disturbance in the clast-rich facies or potentially geomagnetic events. As a result, the sediment recovered is thought to be exclusively within the Brunhes Chronozone and younger than 0.781 Ma (Cande et al., 1995; Hilgen et al., 2012).

Shore-based analyses will allow significant refinement of these interpretations, especially within the APC-recovered section where lithologic variability and core disturbance influenced the shipboard magnetic record. Much of the ambiguity could result from the low level of AF demagnetization used that did not fully remove the drill string overprint and the inclusion of sections with minor deformation. Focusing on the splice that contains the best-recovered intervals, sampling within the most pristine central part of the core, and targeting better preserved and specific lithologies with discrete samples would likely lead to more reliable results.

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