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doi:10.2204/iodp.proc.346.107.2015

Paleomagnetism

Paleomagnetic samples and measurements

Paleomagnetic investigations at Site U1426 included the measurement of magnetic susceptibility of whole-round and archive-half split-core sections, and the 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 all archive-half sections from Hole U1426A at every 5 cm interval. Because of increased core flow and limited measurement time available at the paleomagnetism station, NRM of archive-half sections from Holes U1426B to U1426D was measured only after (not before) 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 a total of 25 APC-collected cores in Hole U1426A, starting from Core 346-U1426A-2H (Table T16).

We collected one paleomagnetic discrete cube sample (see “Paleomagnetism” in the “Methods” chapter [Tada et al., 2015b]) from the first section of each of the 59 cores recovered in Hole U1426A (light blue triangles in Fig. F41A, F41B). Stepwise AF demagnetization on 16 discrete samples collected from Hole U1426A (orange triangles in figure) 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 U1426A 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 the split-core section samples. The disturbed and void intervals used in this process are reported in Table T17. The processed NRM inclination, declination, and intensity data after 20 mT AF demagnetization are reported in Table T18 and shown in Figure F41.

Natural remanent magnetization and magnetic susceptibility

NRM intensity after 20 mT AF demagnetization in all four measured holes at Site U1426 is similar in magnitude for overlapping intervals, mostly ranging between ~10^–4 and 10^–2 A/m. For the uppermost ~90 m of the recovered sediment, NRM intensity of the measured core sections after 20 mT demagnetization is close to ~10^–2 A/m, except the ~60–80 m CSF-A interval where NRM intensity is on the order of 10^–4 to 10^–3 A/m. Between ~90 and 256 m CSF-A, NRM intensity varies frequently between ~10^–4 and ~10^–3 A/m. Deeper than ~256 m CSF-A until the bottom of the holes, NRM intensity appears to be more stable and is mostly close to ~10^–3 A/m.

The AF demagnetization behavior of the 16 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. NRM measurement of discrete samples with weak intensity (e.g., Fig. F42B, F42D, F42F, F42I, F42M, F42O, F42P) often appears to have lower coercivity and acquire an anhysteretic remanent magnetization (ARM), possibly due to 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 2 × 10^–5 and 20 × 10^–5 SI (Fig. F41, fourth panel). Except for the 300–400 m CSF-A interval of recovered sediment in Hole U1426A, where the large-scale cyclic variation of magnetic susceptibility from ~1 × 10^–5 to ~10 × 10^–5 SI seems absent in NRM intensity, magnetic susceptibility of sediment in all holes, in general, mimics NRM intensity, suggesting that the magnetic minerals carrying 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 the top ~250 m of recovered sediment. Both magnetic declination and inclination after 20 mT AF demagnetization were used when possible for the magnetostratigraphic interpretation at Site U1426. The geomagnetic field at the latitude of Site U1426 (37.03°N) has an expected inclination of 56.47°, 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 boundary (0.781 Ma) and the Jaramillo Subchron (0.988–1.072 Ma) in Holes U1426A, U1426C, and U1426D. The Olduvai Subchron (1.778–1.945 Ma) was recorded in Holes U1426A and U1426C. In Hole U1426A, we also identified the Matuyama/Gauss boundary (2.581 Ma) (Table T19).

Inclination of the four APC cores drilled in Hole U1426B after 20 mT AF demagnetization clearly varies around the expected normal polarity dipole value at the site location (Fig. F41C), and sediment recovered from Hole U1426B should have been deposited during the Brunhes Chron. The Brunhes/Matuyama boundary (0.781 Ma) is recorded at ~81.7 m CSF-A in Hole U1426A, ~76.4 m CSF-A in Hole U1426C, and ~80.9 m CSF-A in Hole U1426D. Above the Brunhes/Matuyama boundary, inclination values in Holes U1426A, U1426C, and U1426D after 20 mT AF demagnetization vary closely around the expected normal polarity dipole value at the site location (Fig. F41A, F41D, F41E). In Hole U1426A, the FlexIT-corrected declinations show a shift from values mostly around 0° to values around 180° at the interpreted Brunhes/Matuyama boundary (in Core 346-U1426A-10H). This interpretation is consistent with the stepwise demagnetization data of discrete samples. The discrete sample from 41.58 m CSF-A shows well-defined component directions with positive inclination (Fig. F42A), and the discrete sample from 80.42 m CSF-A appears to have transitional or negative inclination (Fig. F42B).

We interpret the positive inclination intervals from ~85.94 to 98.5 m CSF-A in Hole U1426A and from ~85 to 99.4 m CSF-A in Hole U1426C as the Jaramillo Subchron (0.988–1.072 Ma). Hole U1426D appears to have recorded the top of the Jaramillo Subchron (0.988) at ~86.1 m CSF-A. In Hole U1426A, the positive inclination interval is accompanied by FlexIT-corrected declinations varying around 0°. Declinations near the top and bottom of this interval in the two holes both show an apparent ~180° shift, supporting the two depth levels as polarity boundaries suggested by the inclination changes. In addition, the discrete sample from 90.56 m CSF-A has well-defined characteristic remanence with positive inclination (Fig. F42C), agreeing with the archive-half section measurement results.

The Olduvai Subchron (C2n, 1.778–1.945 Ma) is recorded from ~169.45 to 191.6 m CSF-A in Hole U1426A and from ~170.5 to 189.6 m CSF-A in Hole U1426C. During the interpreted Olduvai Subchron, inclinations in the two holes after 20 mT AF demagnetization are mostly positive and close to the expected normal polarity dipole value. Declinations near the top and bottom of this interval in the two holes both show an apparent ~180° shift, and FlexIT-corrected declinations in Hole U1426A are mostly close to 0° or 360° during this interval. The depth levels of the Olduvai Subchron in the holes are consistent with the discrete sample measurement results. The discrete sample from 182.71 m CSF-A recorded well-defined characteristic remanence with positive inclination (Fig. F42G), whereas the discrete samples from 163.31 m CSF-A (Fig. F42F) and from 201.36 m CSF-A (Fig. F42H) appear to have remanence with primarily negative inclinations. Our interpretation agrees well with the LO of radiolarian A. acquilonium (1.2–1.7 Ma) between ~172.43 and 182.25 m CSF-A and the LO of calcareous nannofossil C. macintyrei (~1.6 Ma) at ~173.67–175.04 m CSF-A in Hole U1426A (see “Biostratigraphy”).

The Matuyama/Gauss boundary (2.581 Ma) was recorded at ~254.6 m CSF-A (Core 346-U1426A-28H) in Hole U1426A. Inclination of Hole U1426A after 20 mT AF demagnetization apparently switches from dominantly negative values above this boundary to mostly positive values right below the boundary (Fig. F41B). The depth at which the Matuyama/Gauss boundary is found in Hole U1426A is consistent with a list of biostratigraphic events identified in the hole (see “Biostratigraphy”), including the FO of radiolarian C. davisiana (~2.7 Ma) at ~256.98–261.15 m CSF-A. Below the Matuyama/Gauss boundary, NRM inclination after 20 mT AF demagnetization shows mostly positive values that are apparently steeper than the expected normal polarity dipole inclination at the site location. Increased coring disturbance, strong drill string overprint, the lack of core orientation, and the large scatter in paleomagnetic declinations makes magnetostratigraphic interpretations difficult for the ~270 to ~396 m CSF-A interval of sediment recovered at Site U1426.

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