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doi:10.2204/iodp.proc.346.104.2015

Paleomagnetism

Paleomagnetic samples and measurements

Paleomagnetic investigations for cores collected at Site U1423 included the 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 U1423A. Because of increased speed of core flow, NRM of archive-half sections from Holes U1423B and U1423C was measured only after 20 mT AF demagnetization at every 5 cm interval. The FlexIT core orientation tool (see “Paleomagnetism” in the “Methods” chapter [Tada et al., 2015b]) was used to orient 20 APC cores in Hole U1423A, starting from Core 346-U1423A-2H. Core orientation data collected in Hole U1423A are reported in Table T13.

We typically collected one paleomagnetic discrete cube sample (see “Paleomagnetism” in the “Methods” chapter [Tada et al., 2015b]) from the first section of each core in Hole U1423A, and occasionally from deep sections when the first section was not suitable for collecting a discrete cube sample. Four discrete samples were collected at deep depths from the bottom four cores in Hole U1423B, and two additional discrete samples were collected from Hole U1423C. Depth levels where the discrete samples were taken are marked by triangles along the left side of the paleomagnetic inclination data column in Figure F31. Stepwise AF demagnetization on 12 discrete samples from Hole U1423A, 1 discrete sample from Hole U1423B, and 2 discrete samples from Hole U1423C was performed at successive peak fields of 0, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 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 sample 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. 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 F31.

Natural remanent magnetization and magnetic susceptibility

The intensity of NRM after 20 mT AF demagnetization in all three holes is similar in magnitude for overlapping intervals, mostly ranging from ~10^–5 to 10^–2 A/m. For core sections from the uppermost ~25 m, NRM intensity is on the order of 10^–2 A/m. NRM intensity drops downcore to the order of 10^–4 to 10^–3 A/m between ~25 and ~50 m CSF-A. Deeper than ~50 m CSF-A until the bottom of the holes, NRM intensities are mostly on the order of ~10^–4 A/m.

The AF demagnetization behavior of eight discrete samples from normal and reversed polarity intervals at varying depths is illustrated in Figure F32. 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 ~10–15 mT, demonstrating that the 20 mT AF demagnetization is, in general, sufficient to eliminate the overprint. For measured discrete samples from deeper than ~50 m CSF-A, NRM intensities before and after stepwise demagnetizations are generally one magnitude lower than those from shallower than this level. NRM measurement of discrete samples from the deep depth with weak intensity values often appears to be significantly affected by an anhysteretic remanent magnetization, possibly acquired because of bias caused by ambient magnetic field during AF demagnetization.

Magnetic susceptibility measurements were taken 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 varies between 10 × 10^–5 and 90 × 10^–5 SI for sediment from the uppermost ~25 m of the holes and is generally <10 × 10^–5 SI for sediment from below ~25 m CSF-A (Fig. F31, fourth panel). Magnetic susceptibility measurements are consistent between the two instruments and, in general, mimic NRM intensity, suggesting that the magnetic minerals that carry NRM are the same or at least coexist with those that dominate magnetic susceptibility.

Magnetostratigraphy

In spite of the drill string overprint and generally low NRM intensity, paleomagnetic inclination and declination data of the holes appear to show patterns that allow for the determination of magnetic polarity for the uppermost ~110 m of recovered sediment in Holes U1423A and U1423B. Both paleomagnetic declination and inclination after 20 mT AF demagnetization were used when possible for magnetostratigraphic interpretation at Site U1423. The geomagnetic field at the latitude of Site U1423 (41.6992°N) has an expected inclination of ~60.7°, 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), the Jaramillo (0.988–1.072 Ma) and Olduvai (1.778–1.945 Ma) Subchrons, and the Matuyama/Gauss boundary (2.581 Ma) at Site U1423 (Table T16).

The Brunhes/Matuyama boundary is recorded at ~52.5 m CSF-A in Hole U1423A and at ~51.7 m CSF-A in Hole U1423B. Shallower than this boundary, inclination values after 20 mT AF demagnetization vary around the expected dipole inclination value of ~60.7°. Just deeper than this boundary, inclination is dominated by shallow and negative values. The two discrete samples from 17.69 and 46.27 m CSF-A measured with stepwise AF demagnetization show stable and positive inclinations around the expected dipole value of ~60° (Fig. F32A, F32B). In Hole U1423A around this boundary, the FlexIT-corrected declination appears to change from values close to 0° to values mostly ~180°. The depth of the Brunhes/Matuyama boundary is consistent with the appearance of a tephra layer at ~45 m CSF-A, recognized as the “Hkd-ku,” and with a known age of ~0.78 Ma (see “Lithostratigraphy”).

Between ~57.3 and ~61.2 m CSF-A in Hole U1423A, FlexIT-corrected declination shows values mainly around 0°, and inclination appears to be dominated by the expected normal polarity dipole value of the site. We interpret this ~4 m interval as the Jaramillo Subchron (C1r.1n, 0.988–1.072 Ma). In Hole U1423B, the Jaramillo Subchron is recognized between ~58.4 and ~62.3 m CSF-A with inclination varying mostly around the expected normal polarity dipole value (Fig. F31B).

Between ~76.4 and ~94.5 m CSF-A in Hole U1423A, the FlexIT-corrected declination values clearly vary around 0°, with inclination values dominated by the expected normal polarity dipole value. We interpret this interval as the Olduvai Subchron (C2n, 1.778–1.945 Ma). The declination of Core 346-U1423B-9H, although not oriented, shows an ~180° shift at ~76 m CSF-A, whereas inclination appears to change from shallow and negative values to mostly positive dipole values. This depth level is recognized as the top of the Olduvai Subchron recorded in Hole U1423B. The bottom of the Olduvai Subchron is not clearly recorded in Hole U1423B. The depth levels of the Olduvai Subchron in the holes are consistent with the LO of Axoprunum acquilonium (1.2–1.7 Ma) at ~74.14–93.17 m CSF-A in Hole U1423A (see “Biostratigraphy”).

At ~112 m CSF-A in Hole U1423A and ~113.6 m CSF-A in Hole U1423B, inclination after 20 mT AF demagnetization in both holes appears to change from shallow and negative values to dominant steep positive values. In Hole U1423A just deeper than this level, FlexIT-corrected declinations vary around 0°. We tend to interpret this level as the Matuyama/Gauss boundary (2.581 Ma). This interpretation agrees with the FO of Cycladophora davisiana (2.7 Ma) at ~112.06–121.7 m CSF-A in Hole U1423A. Deeper than ~140 m CSF-A in Holes U1423A and U1423B, inclination and declination both show large scatter, and it is difficult to make any reliable magnetostratigraphic interpretations. For the six measured APC cores recovered from Hole U1423C, inclination after 20 mT AF demagnetization shows mostly positive values that are apparently steeper than the expected dipole value. The lack of orientation and significant changes in paleomagnetic directions make it difficult for magnetostratigraphic interpretations for Hole U1423C.

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