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doi:10.2204/iodp.proc.346.110.2015

Paleomagnetism

Paleomagnetic samples and measurements

Paleomagnetic investigations at Site U1430 included measurement of magnetic susceptibility of whole-core and archive-half split-core sections and natural remanent magnetization (NRM) of archive-half sections. NRM was measured before and after alternating field (AF) demagnetization with a 20 mT peak field for archive-half sections of APC-collected Cores 346-U1430A-1H through 28H at 5 cm intervals. NRM of archive-half sections of APC Cores 346-U1430B-1H through 28H and APC Cores 346-U1430C-1H through 22H was measured only after 20 mT AF demagnetization at 5 cm intervals. The FlexIT core orientation tool (see “Paleomagnetism” in the “Methods” chapter [Tada et al., 2015b]) was used to orient APC Cores 346-U1430A-2H through 25H. The APC core orientation data for Hole U1430A are reported in Table T13.

We collected a total of 29 paleomagnetic discrete cube samples (see “Paleomagnetism” in the “Methods” chapter [Tada et al., 2015b]) from Cores 346-U1430A-1H through 28H and 32X. The discrete samples were typically taken from the first section of each core and occasionally from other sections of the cores (triangles in Fig. F41A, F41B). Stepwise AF demagnetization on 28 discrete samples (orange triangles in figure) collected from Hole U1430A was performed at successive peak fields of 0, 5, 10, 15, 20, 25, 30, 40, 50, and 60 mT to verify the reliability of the split-core measurements and to determine the demagnetization behavior of the recovered sediment. Following each demagnetization step, NRM of the discrete samples was measured with the samples placed in the “top-toward” or “+z-axis toward magnetometer” orientation (see “Paleomagnetism” in the “Methods” chapter [Tada et al., 2015b]) on the discrete sample tray.

We processed data extracted from the shipboard Laboratory Information Management System (LIMS) database by removing all measurements collected from disturbed and void intervals and all measurements that were made within 10 cm of the section ends, which are slightly biased by measurement edge effects. For declination data from cores in Hole U1430A where FlexIT tool data are available, we corrected the declination values for each core using the estimated orientation angles. A modified version of the UPmag software (Xuan and Channell, 2009) was used to analyze the NRM data of both the split-core section and the discrete cube samples. The disturbed and void intervals used in this process are reported in Table T14. The processed NRM inclination, declination, and intensity data after 20 mT AF demagnetization are reported in Table T15 and shown in Figure F41.

Natural remanent magnetization and magnetic susceptibility

NRM intensity after 20 mT AF demagnetization in the three measured holes at Site U1430 is similar in magnitude for overlapping intervals, mostly ranging between ~10^–5 and 10^–2 A/m. For the uppermost ~40 m of the recovered sediments from Holes U1430A and U1430B, NRM intensity of the measured core sections after 20 mT demagnetization is mostly on the order of 10^–3 A/m and occasionally reaches 10^–2 A/m. Between ~40 and ~60 m CSF-A, NRM intensity decreases downcore from 10^–3 A/m to ~10^–5 A/m. Deeper than ~60 m CSF-A until the bottom of the holes, NRM intensity is low and mostly on the order of 10^–5 to 10^–4 A/m. The low NRM intensity deeper than ~60 m CSF-A is accompanied by large scatter in paleomagnetic direction. For many measured cores, NRM intensity near the top part of the cores appears to show high intensity that gradually decreases downcore to about average values of the cores within ~1 m. We think this is most likely related to stronger drilling overprints in the upper part of the cores.

The AF demagnetization behavior of the 28 measured discrete samples is illustrated in Figure F42. Declination and inclination values acquired from the discrete sample measurement generally agree well with the split-core measurement after 20 mT AF demagnetization. All samples exhibit a steep, normal overprint that was generally removed after AF demagnetization at peak fields of ~15–20 mT, demonstrating that the 20 mT AF demagnetization is, in general, sufficient to eliminate the drilling overprint. Discrete samples from the top ~30 m of Hole U1430A show well-defined component directions with positive inclinations (Fig. F42A–F42D). For measured discrete samples from deeper than ~30 m CSF-A, NRM intensity before and after stepwise AF demagnetization is about one or two magnitudes lower than that from shallower than ~30 m CSF-A. NRM measurements of discrete samples from the greater depth with weak intensity often appear to have lower coercivity and are affected by an anhysteretic remanent magnetization (ARM), possibly acquired because of bias caused by ambient magnetic field during demagnetization.

Magnetic susceptibility measurements were taken on whole cores from all holes as part of the Whole-Round Multisensor Logger (WRMSL) analysis and on archive-half 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 measurement is consistent between the two instruments and across the different holes for overlapping intervals and varies mostly between 1 × 10^–5 and 20 × 10^–5 SI (Fig. F41, fourth panel). For the uppermost ~60 m of the recovered sediment, magnetic susceptibility is generally close to 10 × 10^–5 SI. Deeper than ~60 m CSF-A until the bottom of the holes, magnetic susceptibility largely varies between ~1 × 10^–5 and 10 × 10^–5 SI. Magnetic susceptibility of sediment in all holes, in general, mimics NRM intensity, suggesting that the magnetic minerals that carry NRM are the same or at least coexist with those that dominate magnetic susceptibility.

Magnetostratigraphy

Paleomagnetic inclination and declination data of the holes show patterns that allow for the determination of magnetic polarity for at least the uppermost ~40 m of recovered sediment. Both magnetic declination and inclination after 20 mT AF demagnetization were used when possible for the magnetostratigraphic interpretation at Site U1430. The geomagnetic field at the latitude of Site U1430 (37.90°N) has an expected inclination of 57.29°, assuming a geocentric axial dipole field model, which is sufficiently steep to determine magnetic polarity in APC cores that lack horizontal orientation. We identified the Brunhes/Matuyama (B/M) boundary (0.781 Ma) at ~33.5 m CSF-A in Hole U1430A, ~34 m CSF-A in Hole U1430B, and ~37 m CSF-A (where close to slump are found) in Hole U1430C (Table T16).

Above the identified B/M boundary, inclinations after 20 mT AF demagnetization from all three holes vary around the expected normal polarity dipole inclination at Site U1430. In Hole U1430A, the FlexIT-corrected declinations (green dots in Fig. F41A) are mostly stable and vary around 0° or 360°. Right below the B/M boundary, inclinations from both holes are apparently dominated by shallow and negative values. The interpreted B/M boundary is consistent with the stepwise demagnetization data from the measured discrete samples. Discrete samples from above the boundary (Fig. F42A–F42D) show well-defined characteristic remanence with positive inclinations, whereas discrete samples from right below the B/M boundary (Fig. F42E) apparently have negative inclinations at high demagnetization steps. The depth level of the B/M boundary in Hole U1430A agrees well with many of the identified biostratigraphic events (see “Biostratigraphy”).

Below the B/M boundary, NRM intensity of the sediment after 20 mT AF demagnetization is generally weak and on the order of 10^–5 to 10^–4 A/m, whereas NRM inclination and declination after 20 mT demagnetization show large scatter. The weak NRM intensity, increased coring disturbance, strong drill string overprint, and the large scatter in paleomagnetic declinations makes magnetostratigraphic interpretations difficult for the deep part of sediments recovered at Site U1430.

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