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doi:10.2204/iodp.proc.346.106.2015

Paleomagnetism

Paleomagnetic samples and measurements

Paleomagnetic investigations at Site U1425 included measurement of magnetic susceptibility of whole-core and archive-half split-core sections and of 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 U1425B at 5 cm intervals. Because of increased core flow and limited measurement time available at the paleomagnetism station, NRM of archive-half sections from Hole U1425D were 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 Cores 346-U1425B-2H through 12H and 14H. The APC-collected core orientation data for Hole U1425B that are reported in Table T15.

We collected paleomagnetic discrete cube samples (see “Paleomagnetism” in the “Methods” chapter [Tada et al., 2015b]) from the first section of 52 cores in Hole U1425B and occasionally from deep sections when the first section was not suitable for collecting discrete cube sample (Fig. F53). Stepwise AF demagnetization on 35 discrete samples collected from Hole U1425B 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. Depth levels where the measured discrete samples were collected are marked as orange triangles along the inclination data column in Figure F53. To avoid potential acquisition of anhysteretic remanent magnetization (ARM) during AF demagnetization, which was observed in discrete sample measurements from Sites U1422 and U1423, especially at high AF peak fields (see “Paleomagnetism” in the “Site U1422” chapter and “Paleomagnetism” in the “Site U1423” chapter [Tada et al., 2015c, 2015d]), we followed the Expedition 318 Scientists (2011) protocol to demagnetize and measure the discrete samples multiple times at each demagnetization level. The measurements acquired at each demagnetization step were then averaged to cancel out potential ARM acquisition during AF demagnetization (see “Paleomagnetism” in the “Methods” chapter [Tada et al., 2015b]).

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. The XCB-cored archive-half sections of Holes U1425B and U1425D frequently contain drilling “biscuits” surrounded by as much disturbed material as intact material, strongly compromising the quality of the resulting paleomagnetic data. We removed data collected within those disturbed intervals of the 11 measured XCB-cored archive-half sections. For declination data from cores in Hole U1425B where FlexIT tool data are available, we corrected the declination values for each core using 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 T16. The processed NRM inclination, declination, and intensity data after 20 mT AF demagnetization are reported in Table T17 and shown in Figure F53.

Natural remanent magnetization and magnetic susceptibility

NRM intensity after 20 mT AF demagnetization in the two measured holes at Site U1425 is similar in magnitude for overlapping intervals, mostly ranging between ~10^–5 and 10^–2 A/m. For the top ~50 m of the recovered sediment, NRM intensity of the measured core sections after 20 mT demagnetization is on the order of 10^–3 to 10^–2 A/m. NRM intensity gradually decreases downcore to ~10^–4 to 10^–3 A/m between ~50 and ~125 m CSF-A, and to ~10^–5 to 10^–4 A/m between ~125 and ~250 m CSF-A. Deeper than ~250 m CSF-A until the bottom of the holes, NRM intensity appears to gradually increase from below 10^–4 to ~10^–3 A/m. For some cores in Hole U1425B (e.g., Cores 346-U1425B-34H and 35H) and a few cores in Hole U1425D (e.g., Cores 346-U1425D-16H to 25H and Core 32H), NRM intensity of the top part of the cores appears to be dramatically (one to two magnitudes) higher than that of the bottom part. These intensity changes are not accompanied by similar changes in magnetic susceptibility and are more likely artifacts related to remanence acquired during the coring process.

The typical well-behaved AF demagnetization behavior of 8 out of the 35 measured discrete samples is illustrated in Figure F54. 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 sample measurements acquired using the Expedition 318 Scientists (2011) protocol show generally reduced ARM acquisition. However, some measured samples still appear to acquire remanence during AF demagnetization especially at high peak field demagnetization steps (>40–50 mT) (e.g., Fig. F54B–F54E). This is possibly related to changes in ambient field on the ship during the measurements and the fact that some of the samples are too weak to be measured accurately using the shipboard superconducting rock magnetometer.

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 varies between 5 × 10^–5 and 20 × 10^–5 SI for sediment from the top ~80 m of the holes, and are generally <5 × 10^–5 SI for sediment from between ~80 and ~250 m CSF-A (Fig. F53, fourth panel). Deeper than ~250 m CSF-A to the bottom of the holes, magnetic susceptibility appears to gradually increase from <5 × 10^–5 SI to ~10 × 10^–5 SI. Except for the intervals where NRM intensity is abnormally high in the top part of the cores (e.g., Cores 346-U1425B-34H and 35H and Cores 346-U1425D-16H through 25H and 32H), magnetic susceptibility, 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 from Site U1425 show patterns that allow for the determination of magnetic polarity for at least the uppermost ~90 m of recovered sediment. Compared with Hole U1425D, paleomagnetic directions appear to be more stable and better preserved in Hole U1425B. Both magnetic declination and inclination after 20 mT AF demagnetization were used when possible for the magnetostratigraphic interpretation at Site U1425. The geomagnetic field at the latitude of Site U1425 (39.49°N) has an expected inclination of 58.75°, 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 (0.988–1.072 Ma) and Olduvai (1.778–1.945 Ma) Subchrons, as well as the Matuyama/Gauss boundary (2.581 Ma) in both Holes U1425B and U1425D (Table T18).

The Brunhes/Matuyama boundary (0.781 Ma) is recorded at ~33.7 m CSF-A in Hole U1425B and at ~34.2 m CSF-A in Hole U1425D. Above this boundary, inclination values after 20 mT AF demagnetization vary closely around the expected positive inclination at the site location in both holes (Fig. F53A, F53C). In Hole U1425B, the FlexIT-corrected declination values show a shift from ~0° to ~180° at this boundary (in Core 346-U1425B-4H). Our interpretation agrees well with the LO of planktonic foraminifer N. kagaensis (0.7 Ma) at ~27.04–37.58 m CSF-A in Hole U1425B (see “Biostratigraphy”) and is consistent with the results of discrete samples from 20.33 and 38.06 m CSF-A in Hole U1425B. Stepwise demagnetization of discrete samples from 20.33 m CSF-A in Hole U1425B (Fig. F54A) shows a stable and positive inclination that was likely acquired during the Brunhes Chron. The results of a discrete sample from 38.06 m CSF-A in Hole U1425B (Fig. F54B) appears to record negative inclination that is consistent with the interpreted Matuyama age.

We interpret the positive inclination intervals between ~39.2 and 41.4 m CSF-A in Hole U1425B and between ~38.5 and 40.5 m CSF-A in Hole U1425D as recording the Jaramillo Subchron (0.988–1.072 Ma) in both holes. In Hole U1425B, this 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 data.

Between ~57.2 and 69.50 m CSF-A in Hole U1425B, the FlexIT-corrected declination values vary mostly around 0° or 360°, with inclination dominated by positive values close to the expected normal polarity dipole value of the site. We interpret this interval as the Olduvai Subchron (C2n; 1.778–1.945 Ma) recorded in Hole U1425B. In Hole U1425D, the Olduvai Subchron is identified between ~54.2 and 68.4 m CSF-A, during which inclination after 20 mT AF demagnetization appears to be mostly positive and close to the expected normal polarity dipole value. The depth levels of the Olduvai Subchron in the holes are consistent with the LO of radiolarian A. acquilonium (1.2–1.7 Ma) at ~46.96–56.53 m CSF-A and the LO of calcareous nannofossil C. macintyrei (~1.6 Ma) at ~54.03–56.53 m CSF-A in Hole U1425B (see “Biostratigraphy”).

The Matuyama/Gauss boundary (2.581 Ma) was recorded at ~87.8 m CSF-A in Hole U1425B and at ~85.2 m CSF-A in Hole U1425D. The paleomagnetic inclination of samples from the holes after 20 mT AF demagnetization apparently switches from dominantly negative values above this boundary to mostly positive values right below the boundary. In Hole U1425B, the inclination pattern change is accompanied by FlexIT-corrected declination values switching from an average of ~180° to ~0°. Declination of samples from Hole U1425D, although not oriented, also shows an ~180° shift around this boundary in Core 346-U1425D-10H. The depth level of the Matuyama/Gauss boundary in the holes is consistent with a list of biostratigraphic events identified in Hole U1425B (see “Biostratigraphy”).

Deeper than ~90 m CSF-A in both holes, NRM intensity is weak and is generally on the order of 10^–5 to 10^–4 A/m. NRM inclination after 20 mT AF demagnetization shows mostly positive values that are apparently steeper than the expected dipole inclination. Increased coring disturbance, strong drill string overprint, and the largely scattered paleomagnetic directions makes magnetostratigraphic interpretations difficult for the deep part of the holes at Site U1425.

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