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doi:10.2204/iodp.proc.339.106.2013

Paleomagnetism

Paleomagnetic investigation of the APC, XCB, and RCB cores collected at Site U1388 included measurement of magnetic susceptibility of whole-core and archive-half split-core sections and the natural remanent magnetization (NRM) of archive-half split-core sections. NRM was measured before and after alternating field (AF) demagnetization with 20 mT peak field for all studied cores of the site. Stepwise AF demagnetization of 10 selected discrete samples was performed at successive peak fields of 0, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 70, and 80 mT to verify the reliability of the split core measurements and to determine the demagnetization behavior of the recovered sediment. The depth levels where the measured discrete samples were taken are indicated by blue triangles in the first panel of Figure F22. We processed data extracted from the Laboratory Information Management System (LIMS) database by removing all measurements collected from disturbed and void intervals, which are listed in Table T9, and all measurements that were made within 10 cm of the section ends, which are slightly biased by measurement edge effects. The processed NRM inclination, declination, and intensity data after 20 mT peak field AF demagnetization are listed in Tables T10, T11, and T12.

Natural remanent magnetization and magnetic susceptibility

The intensity of NRM after 20 mT peak field AF demagnetization is in the range of ~10^–4 to ~10^–2 A/m (Fig. F22, third panel). The sands from lithologic Unit I (Core 339-U1388A-1H; see “Lithostratigraphy”) exhibit very low intensities (~10^–4 A/m) and are underlain by sediment with moderately high intensity values (~10^–3–10^–2 A/m).

Despite the XCB-induced coring disturbance and the drill string overprint, a very stable magnetic component was preserved in the sediment. A magnetic overprint with steep, positive inclination, which was probably acquired during drilling, was usually removed by up to 20 mT peak field AF demagnetization (Fig. F23). Although the XCB cores are heavily biscuited and frequently contain as much of the disturbed matrix as the intact material, the quality of the resulting paleomagnetic data is excellent and sufficient to determine the magnetic polarity.

Except for two discrete samples from Hole U1388A that consist of mainly sands from lithologic Unit I (see “Lithostratigraphy”) and failed to yield stable NRM, progressive AF demagnetization of eight discrete samples from Hole U1388B revealed stable NRM with positive inclinations (Fig. F23). Several samples exhibit a steep, normal overprint that was generally removed after AF demagnetization at peak field of ~15–20 mT, demonstrating that the 20 mT magnetic cleaning level is, in general, sufficient to eliminate the overprint. A few samples also appear to have acquired a significant amount of anhysteretic remanent magnetization at high peak field (>55 mT) AF demagnetization steps, especially for one sample from 160.86 mbsf in Hole U1388B (Fig. F23). The acquired anhysteretic remanent magnetization is possibly because of bias caused by ambient magnetic field during AF demagnetization at high peak fields. We calculated component NRM directions of the eight discrete samples from data from 25–50 mT demagnetization steps using principal component analysis (Kirschvink, 1980) and the UPmag software (Xuan and Channell, 2009). Maximum angular deviations associated with the principal component analysis are mostly <10°, suggesting the component NRM directions are reasonably well defined. Component NRM inclinations of these discrete samples vary between ~50° and 65° (Fig. F22, yellow circles on inclination panel) and are generally consistent with the archive-half section measurements.

Magnetic susceptibility measurements were made on whole cores from all three holes as part of the Whole-Round Multi Sensor Logger (WRMSL) analysis and on archive-half split core sections using the Section Half Multi Sensor Logger (SHMSL) (see “Physical properties”). Magnetic susceptibility is consistent between the two instruments and, in general, parallels the intensity of magnetic remanence. The WRMSL acquired susceptibility was stored in the database in raw meter units. These were multiplied by a factor of 0.68 × 10^–5 to convert to the dimensionless volume SI unit (Blum, 1997). A factor of (67/80) × 10^–5 was multiplied by the SHMSL acquired susceptibility stored in the database. Magnetic susceptibility varies between 5 × 10^–5 and 40 × 10^–5 SI (Fig. F22, fourth panel). Note that in Figure F22, a constant of 25 × 10^–5 SI was added to the SHMSL measurements (gray lines) to facilitate the comparison with the WRMSL measurements (black lines).

Magnetostratigraphy

The geomagnetic field at the latitude of Site U1388 (36.27°N) has an expected inclination of 55.73°, assuming a geocentric axial dipole model, which is sufficiently steep to determine magnetic polarity in cores that lack a horizontal orientation. NRM inclination data (after 20 mT peak field AF demagnetization) from all three holes indicate that only the Brunhes (C1n) normal polarity chron is recorded in these sediments (Fig. F22). This interpretation is supported by the discrete sample measurements. A discrete sample from the base of Hole U1388B (Sample 339-U1388B-24X-7, 44–46 cm) clearly carries a positive inclination and is <0.781 Ma. Whether the Brunhes Chron is complete or incomplete cannot be determined from the magnetostratigraphy. However, preliminary biostratigraphic datums (see “Biostratigraphy”) indicate an age between 600 and 900 ka at the base of Hole U1388B.

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