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doi:10.2204/iodp.proc.341.103.2014

Paleomagnetism

The natural remanent magnetization (NRM) of Site U1417 archive-half cores was measured before and after alternating field (AF) demagnetization. Peak AFs were restricted to a maximum of 20 mT for sections recovered using the APC system with standard (full and half length) and nonmagnetic core barrels (see “Operations”). Higher AFs of up to 40 mT were used for a few sections recovered using the XCB and RCB systems (Table T14). The number of demagnetization steps and the peak field 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. Sections completely disturbed by drilling, 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 interpretation.

For sections recovered through APC drilling, the NRM intensities at Site U1417 are strong both before (10^–1 A/m, with occasional decimeter-scale intervals of >25) and after (10^–2 A/m) demagnetization (Fig. F47). Intensities show variability at both the meter and decameter scales through this part of the sequence, reflecting lithology (see “Lithostratigraphy”) and, for the demagnetized intensity, geomagnetic field variability. High intensities of up to >0.25 A/m before demagnetization are more commonly observed in Holes U1417C and U1417D and could reflect a stress-induced progressive increase in the magnetization of the drill string or bottom-hole assembly (e.g., Richter et al., 2007). No clear correlation was observed between core barrel type (full, half, or nonmagnetic) employed and magnetization.

In the XCB- and RCB-cored materials, magnetic intensity shows greater variability, reflecting lithologic changes and variable core quality (Fig. F48; see “Lithostratigraphy”). NRM intensities after demagnetization continue to be in the 10^–2 A/m range, transitioning to lower values through the ~300–350 m CSF-A interval, corresponding to lithostratigraphic Unit IV. Intensities are variable between ~350 and 475 m CSF-A, corresponding to lithostratigraphic Subunits VA and VC, ranging from 10^–4 to 10^–2 A/m. Deeper than ~475 m CSF-A, intensities are consistently lower, in the 10^–3 to 10^–4 A/m range, and vary on an ~50 m scale.

In the APC-recovered interval, transformation of depths to CCSF-B (see “Stratigraphic correlation”) allows comparison between holes that facilitates polarity interpretation. After demagnetization at peak fields of 10 or 20 mT, intensities are reduced to the 10^–2 A/m range and are consistent between all holes (Fig. F47). Steep positive inclinations observed in the APC section prior to demagnetization consistent with a low-coercivity drill string magnetic overprint are removed by peak AF demagnetization of 10 mT in the upper ~160 m CCSF-B (Fig. F49). Within this interval, polarity can be unambiguously determined. Inclinations associated with normal and reversed polarities vary around the values expected (approximately ±72°) for a geocentric axial dipole at the site latitude. Declinations show serial correlation within-core. Inclination from Holes U1417A–U1417D documents a continuous sequence (see “Stratigraphic correlation”), allowing correlation to the geomagnetic polarity timescale (GPTS; Cande and Kent 1995) on the geologic timescale (Hilgen et al., 2012). When polarity transitions occur within a core, declination changes are consistent with inclination-based polarity interpretations (Fig. F49). The Matuyama/Brunhes boundary and the upper Matuyama Chronozone (C1r) containing the Jaramillo Subchronozone (C1r.1n) are clearly identified. A short interval of normal polarity deeper than the Jaramillo polarity zone is observed in multiple holes and is correlated with the Cobb Mountain Subchronozone (C1r.2n). Deeper than ~160 m CCSF-B, unambiguous recognition of polarity transitions is more difficult, even in the APC-recovered sections (Fig. F49). We do partially observe the Olduvai (C2n) and Reunion (C2r.1n) Subchronozones and correlate their apparent polarity boundaries to the GPTS. Figure F49 and Tables T12 and T15 document these polarity zonations and corresponding age interpretations.

In the XCB- and RCB-recovered intervals, the Gauss (2An) to Matuyama (2r) polarity transition is clearly observed in correlative sections of Holes U1417B, U1417D, and U1417E (Fig. F50; Table T15). At greater depths, correlations are more ambiguous. Although both negative and positive inclinations were recovered deeper than ~300 m CCSF-B, with apparent transitions often captured in a single XCB/RCB core section, interpretation of directional data is complicated by low core recovery and in particular by biscuiting. The quality of the paleomagnetic record within these few recovered meters suggests either that the extremely condensed section around the observed polarity reversals is recovered from the top, middle, or bottom of the drilled interval or that the recovered interval characterized by hard biscuits and softer slurry represents coherently recovered pieces spread through each drilled interval. The options that limit the stratigraphic emplacement of the coherent biscuits within the softer slurry in the XCB/RCB cores include either a narrow episode of recovery near the top or bottom of the drilled interval or evenly distributed recovery through the entire core length. To improve recognition of polarity zonations and to assess the nature of material within the XCB and RCB cores, we assume the latter option of distributed emplacement and linearly expand each measurement made at 2.5 cm intervals on the recovered core evenly through the drilled interval depth. This simplistic approach assumes that recovered material is derived equally from within the cored interval. An alternative approach using the biscuit boundaries to guide expansion is beyond the time constraints of shipboard research. The results of this exercise, constrained by biostratigraphy, are shown in Figure F50, revealing a recognizable pattern of polarity reversals in the XCB/RCB portions of Holes U1417B, U1417D, and U1417E. Much of the Gauss (C2An) and Gilbert (C2Ar) Chronozones can be recognized and correlated to the GPTS in Holes U1417B, U1417D, and U1417E (Tables T12, T15). However, whether the lowest normal polarity interval observed at ~425 m CCSF-B reflects the Thvera (C3n.4n) or Sidufjall (C3n.3n) is unclear. At deeper intervals, guided by biostratigraphic datums (see “Paleontology and biostratigraphy”), correlations are made to the top of Chron C3An at ~475 m CCSF-B and to Chron C3Bn at ~585 m CCSF-B. Deeper than ~600 m CCSF-B, the placement of polarity boundaries is equivocal.

Shore-based analyses will allow significant refinement of these interpretations. Much of the ambiguity could result from the inability of the low level of AF demagnetization to fully remove the drill string overprint, especially in intervals where XCB and RCB drilling was applied. Additionally, targeting better preserved and more specific lithologies by discrete sampling will likely lead to more reliable results.

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