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doi:10.2204/iodp.proc.339.108.2013

Paleomagnetism

Paleomagnetic investigation of the 78 APC and XCB cores collected at Site U1390 included the measurement of magnetic susceptibility of whole-core and archive-half split-core sections and the 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. The FlexIt tool was used to orient 29 cores in the APC sections of Holes U1390A and U1390B starting with Core 4H and in Hole U1390C starting with Core 9H. However, as was the case at Site U1389, the tool failed to properly orient the cores in Hole U1390A. The technical issue was resolved subsequently and the APC core orientations for Holes U1390B and U1390C are provided in Table T11 and used for APC core reorientation (Fig. F22). We processed data extracted from the Laboratory Information Management System database by removing all measurements collected from disturbed and void intervals, which are listed in Table T12 (see “Stratigraphic correlation”), 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 (including the FlexIt tool corrected declination), and intensity data after 20 mT peak field AF demagnetization are listed in Tables T13, T14, and T15.

Natural remanent magnetization and magnetic susceptibility

The intensity of NRM after 20 mT demagnetization is similar in magnitude in the overlapping parts of Holes U1390A, U1390B, and U1390C, ranging from ~10^–5 to ~10^–2 A/m (Fig. F22, third panel). Sediment from the uppermost ~20 mbsf exhibits the highest NRM intensities, on the order of 10^–2 A/m, with a mean of ~0.017 A/m. Below 20 mbsf, magnetic intensities are variable (mean value is ~0.0046 A/m) but generally lower than those in the top part of the section. NRM intensity data also show a few spikes where black or gray color mottling (burrows) is present (see “Lithostratigraphy”). The high values of NRM intensity could be related to diagenetic growth of fine-grained magnetic minerals (possibly iron sulfides) in the burrows.

Despite the coring disturbance and drill string overprint in the XCB-cored sections, a relatively stable magnetic component was preserved in sediment from all holes, allowing for the determination of magnetic polarity for most parts of the recovered sedimentary sequences. The XCB sections in Hole U1390A are often heavily biscuited and frequently contain as much of the disturbed matrix as the intact material, compromising the quality of the resulting paleomagnetic data.

Magnetic susceptibility measurements were made on whole cores from all three holes as part of the Whole-Round Multisensor Logger (WRMSL) analysis and on archive-half split-core sections using the Section Half Multisensor Logger (SHMSL) (see “Physical properties”). 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 is consistent between the two instruments and, in general, parallels the intensity of magnetic remanence. 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

We use magnetic inclinations and FlexIt tool corrected declinations when available to interpret magnetostratigraphy for the APC-cored sediment sequences. The lack of core orientation and the significant coring disturbance, as well as drill string overprint in the XCB cores, limit our magnetostratigraphic interpretation for the XCB-cored sediment in Hole U1390A to relying on magnetic inclination changes. The geomagnetic field at the latitude of Site U1390 (36.32°N) has an expected inclination of 55.78°, assuming a geocentric axial dipole field model, which is sufficiently steep to determine magnetic polarity in cores that lack horizontal orientation.

NRM inclination data (after 20 mT peak field AF demagnetization) in all three holes indicate that the Brunhes (C1n) normal polarity chron is recorded in the uppermost ~230 m of sediment (Fig. F22). This interpretation is consistent with the LO of G. omega (0.6 Ma) at 228.69 mbsf. Biostratigraphic evidence (see “Biostratigraphy”) suggests the existence of two hiatuses in Hole U1390A, with the older one at ~290 mbsf (0.7–1.2 Ma) and the younger one at ~230 mbsf (0.6–0.47 Ma), which makes a straightforward magnetostratigraphic interpretation difficult. Although a very clear normal and reversed magnetic polarity pattern seems to exist (Fig. F22), an assignment of the normal and reversed polarity intervals to known parts of the geomagnetic polarity timescale is impossible without additional constraints from discrete sample measurements.

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