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doi:10.2204/iodp.proc.346.109.2015

Paleomagnetism

Paleomagnetic samples and measurements

Paleomagnetic investigations at Sites U1428 and U1429 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 most archive-half sections from Holes U1428A and U1429A at 5 cm intervals. Because of increased core flow and limited measurement time available at the paleomagnetism station, NRM of archive-half core sections from the rest of the holes at Sites U1428 and U1429 were measured only after 20 mT peak field AF demagnetization at every 5 cm interval resolution. NRM of a list of archive-half core sections dominated by disturbance, sand, or soupy ash were not measured (see Table T19). The FlexIT core orientation tool (“Paleomagnetism” in the “Methods” chapter [Tada et al., 2015b]) was used to orient Cores 346-U1428A-2H through 16H and 18H, as well as Cores 346-U1429A-2H through 13H, 15H through 18H, and 20H through 23H. The APC-collected core orientation data for Holes U1428A and U1429A are reported in Table T20.

A total of 15 paleomagnetic discrete cube samples were collected from Hole U1428A, and a total of 20 discrete cubes were taken from Hole U1429A. Discrete samples (see “Paleomagnetism” in the “Methods” chapter [Tada et al., 2015b]) were typically collected from the first section of each core in Holes U1428A and U1429A and occasionally from deep sections when the first section is not suitable for collecting a discrete cube sample (triangles in Fig. F47A, F47C). Stepwise AF demagnetization on all collected discrete samples 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” (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 Holes U1428A and U1429A 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 T19. The processed NRM inclination, declination, and intensity data after 20 mT peak field AF demagnetization are reported in Table T21 and shown in Figure F47.

Natural remanent magnetization and magnetic susceptibility

NRM intensity after 20 mT peak field AF demagnetization in the two measured holes at Site U1428 and three holes at Site U1429 is similar in magnitude for overlapping intervals, mostly ranging between ~10^–4 and 10^–3 A/m. For the uppermost ~80 m of the recovered sediment at Site U1428 and uppermost ~110 m of sediment at Site U1429, NRM intensity of the measured core sections after 20 mT peak field AF demagnetization is mostly on the order of 10^–3 A/m. Deeper than ~80 m CSF-A until the bottom of the two holes at Site U1428, NRM intensity drops to the order of 10^–4 A/m except for Core 346-U1428A-23H, which has NRM intensity on the order of 10^–3 A/m. For Site U1429, NRM intensity after 20 mT peak field AF demagnetization appears to be largely varying between ~10^–4 and 10^–3 A/m deeper than ~110 m CSF-A. Many of the drilled cores appear to have steep inclinations and unusually high intensity near the core tops that gradually decrease to around the expected normal polarity inclination and average intensity within a few tens of centimeters. This may be related to stronger deformation and overprint in part of the core top sediment during coring.

The AF demagnetization behavior of all measured discrete samples is illustrated in Figures F48 and F49. 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 peak field AF demagnetization is, in general, sufficient to eliminate the drilling overprint. All measured discrete samples of Sites U1428 and U1429 show a well-defined characteristic NRM component with positive inclinations. Most samples appear to acquire an anhysteretic remanent magnetization at high demagnetization steps (>50 mT), possibly due to bias caused by ambient magnetic field during demagnetization.

The steep inclinations observed in pass-through measurement near the core tops are not seen in measurement of discrete samples collected near the core tops. As discrete samples are taken from the less disturbed center part of the core sections, the absence of steep inclination (after 15–20 mT peak field AF demagnetization) in discrete sample measurements indicates that the steep inclinations observed in pass-through measurements near the core tops are mainly due to stronger overprint in the outer part of the cores. During the coring process, the upper part of each core must slide further along the inside wall of the core liner than the lower portion of the core, which could cause more deformation for the upper part of the cores (e.g., Acton et al., 2002). Further study is needed to understand why the usual steep inclination and high intensity near the core tops are more apparent in some cores such as those cored at Sites U1428 and U1429.

Magnetic susceptibility measurements were made on whole cores from all holes as part of the 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. The magnetic susceptibility measurement is consistent between the two instruments and across the different holes for overlapping intervals, and varies mostly between 5 × 10^–5 and 20 × 10^–5 SI (Fig. F47, fourth panel). Magnetic susceptibility of sediment from both sites, 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. For sediment recovered deeper than ~95 m CSF-A at Site U1428, NRM intensity of the core sections after 20 mT AF demagnetization shows changes that agree less well with variations in magnetic susceptibility.

Magnetostratigraphy

Both magnetic declination and inclination after 20 mT peak field AF demagnetization were used when possible for the magnetostratigraphic interpretation at Sites U1428 and U1429. The geomagnetic field at the latitudes of Sites U1428 (31.68°N) and U1429 (31.62°N) have expected inclinations of 50.99° and 50.92°, respectively, assuming a geocentric axial dipole field model, which is sufficiently steep to determine magnetic polarity in APC cores that lack horizontal orientation. Paleomagnetic inclination of all holes at Sites U1428 and U1429 is dominated by positive values around the expected normal polarity dipole inclinations, suggesting that all recovered sediment from the two sites was deposited during the Brunhes Chron (C1n, 0–0.781 Ma). Except Core 346-U1429A-21H where the FlexIT tool appears to fail in orienting the core, the FlexIT-corrected declinations for all other oriented cores in Holes U1428A and U1429A (green dots in Fig. F47A, F47C) vary mostly around 0° or 360°, agreeing with our interpretation of Brunhes age for the recovered sediment. This interpretation is also consistent with the discrete sample measurement results and agrees with many identified biostratigraphic events (see “Biostratigraphy”).

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