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doi:10.2204/iodp.proc.341.104.2014

Paleomagnetism

The natural remanent magnetization (NRM) of the Site U1418 archive-half cores was measured before and after alternating field (AF) demagnetization. Peak AFs were restricted to a maximum of 20 mT for all sections recovered using the APC system with standard (full and half length) and nonmagnetic core barrels (see “Operations”), the XCB system, and all but one section recovered using the RCB system (Table T14). A peak AF of 30 mT was used on Section 341-U1418F-68R-1A. The number of demagnetization steps and the peak AF used reflected 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 sections capturing polarity transitions and the uppermost few cores of each hole to facilitate magnetic interpretation. Sections completely disturbed by drilling, as identified 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 recovered materials were strong before (10^–1 A/m) and after (10^–2 to 10^–3 A/m) demagnetization (Fig. F45). The amplitude of the higher frequency variability prior to demagnetization is significantly reduced in lithostratigraphic Unit II (~250–800 m CCSF-B) in Hole U1418F (Fig. F20), likely reflecting the reduction of coarse material in the lower part of the sequence (see “Lithostratigraphy”). In contrast, the character of the demagnetized NRM remains essentially constant through the recovered interval, reflecting the removal of the magnetic influence of the coarsest (low coercivity) material on the paleomagnetic record. No clear correlation between core barrel type (full, half, or nonmagnetic) employed and magnetization was observed.

Transformation of depths to the CCSF-B depth scale (see “Stratigraphic correlation”) allows for comparison between multiple APC-drilled holes. On the CCSF-B depth scale, intensities are consistent between holes, showing variability at both the meter and decameter scales. (Fig. F45). Steep positive inclinations observed in the APC section prior to demagnetization, consistent with a drill string magnetic overprint, are generally removed by peak AF demagnetization of 10 mT. After peak AF of 20 mT, inclinations are generally consistent with those expected (approximately ±73°) for a geocentric axial dipole at the site latitude, with intervals of shallower than expected inclination also commonly observed (Fig. F46). These shallow inclinations likely reflect the influence of subtle core deformation or sediment fabric of the coarse-grained facies (see “Lithostratigraphy”). Declinations in some cores show serial correlation consistent with paleomagnetic secular variation, and in others, variability is too large and inconsistent to be geomagnetic in origin (Fig. F47).

Hole U1418F was rotary drilled to 941.18 m CCSF-B (see “Operations”). As noted above, magnetic intensities in the RCB-recovered section prior to demagnetization are lower than those in the APC-recovered section, whereas those after demagnetization are similar to, and consistent with, NRM intensities observed in the APC-recovered section (Fig. F20). A slight decrease in intensity to 10^–3 A/m is observed at depths deeper than 800 m CCSF-B (Fig. F20), consistent with the transition to lithostratigraphic Unit III (see “Lithostratigraphy”), although lower recovery in this interval may be a contributing factor. Inclinations within the RCB-recovered interval are consistent with expected values for the site latitude. The reduction of the coarse component within the lower part of the sequence (deeper than 250 m CCSF-B) allows for a high-quality paleomagnetic record. Inclinations of the RCB sections reveal an almost continuous sequence, allowing correlation to the geomagnetic polarity timescale (Cande and Kent 1995; Hilgen et al., 2012) (Fig. F20). Even though the Matuyama–Brunhes polarity transition was not completely recovered, the transition from reversed polarity in the next core deeper (Core 341-U1418F-43R) to normal polarity in the next core shallower (Core 41R) is clear. The polarity transitions into and out of the Jaramillo (1r.1n) Subchronozone are clearly observed within Cores 57R/58R and 62R (Table T12). Two short intervals of normal polarity are observed deeper than the interval correlated with the Jaramillo Subchronozone. The deeper intervals are tentatively interpreted as the Cobb Mountain Subchronozone (C1r.2n) (Fig. F20), which is consistent with the biostratigraphic datums (see “Paleontology and biostratigraphy”).

Shore-based analyses may allow refinement of these interpretations, especially within the APC-recovered section where sedimentary noise and core disturbance influenced the shipboard magnetic record. Much of the ambiguity could also result from the inability of the low level of AF demagnetization used to fully remove the drill string overprint and the inclusion of sections with minor deformation. Focusing on the splice that contains the best-recovered intervals and sampling the most pristine central part of the core should result in a noticeably improved record. Additionally, discrete sampling through polarity transitions observed within the RCB-recovered interval may improve the understanding of these features.

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