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doi:10.2204/iodp.proc.340.103.2013

Paleomagnetism

Core 340-U1393A-1H was recovered using a standard steel APC core barrel; Core 2H was recovered using a nonmagnetic barrel. The FlexIt orientation tool was not used to orient declination at Site U1393 because of seafloor conditions. Expected inclination for the site is 31.0° during normal polarity and –31.0° during reversed polarity, assuming a geocentric axial dipole (GAD). Archive halves of cores from Hole U1393A were measured on the three-axis superconducting rock magnetometer (SRM) at 2.5 cm intervals. NRM was measured before (NRM₀) and after alternating field (AF) demagnetization (NRM_{FIELD in mT}) in peak fields as high as 20 mT. Section 340-U1393A-1H-1A was measured at NRM_{0, 5, 10, 15, 20}. Sections 340-U1393A-1H-2A through 4A were measured at NRM_0, 10, 20.

Results

NRM₀ intensity shows a similar signature to magnetic susceptibility (see “Physical properties”). These parameters, particularly magnetic susceptibility, are strongly related to the concentration of ferrimagnetic minerals (e.g., magnetite) and correlate well with depositional units of volcanic origin (see “Lithostratigraphy”).

Inclination, declination, and intensity of measured sections are shown in Figure F3. NRM₀ intensity reaches a peak of 5 A/m and inclines steeply and downward between 80° and 90°. This steep inclination is characteristic of a viscous remanent magnetization (VRM) overprint caused by the drill string. Typically this overprint is removed by demagnetization in alternating fields of 5 or 10 mT, accompanied by a large reduction in intensity. The similarity between NRM₁₀ and NRM₂₀ inclination suggests removal of this overprint. NRM₂₀ intensities are approximately two orders of magnitude lower with median destructive fields typically <10 mT, indicating the soft coercivity of the sample and removal of the VRM. This overprinting explains the coherence between magnetic susceptibility and NRM₀, as both are related to the concentration of ferrimagnetic minerals.

Sand dominates the composition of the recovered material (see “Lithostratigraphy”). These grains are often too large to orient to Earth’s magnetic field and are highly sensitive to disturbance during coring because of their largely unconsolidated nature. NRM₂₀ inclination and declination are highly variable over several tens and even hundreds of degrees with little coherence to values predicted by a GAD. Paleomagnetic results from Hole U1393A are consistent with chaotic deposition of large clasts incapable of recording geomagnetic field variations and/or those that have experienced significant coring disturbance. In summary, no meaningful paleomagnetic interpretations can be made about behavior of the geomagnetic field at Site U1393.

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