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doi:10.2204/iodp.proc.346.103.2015

Paleomagnetism

Paleomagnetic samples and measurements

Paleomagnetic investigation for cores collected at Site U1422 included the measurement of magnetic susceptibility of whole-round and archive-half split-core sections and of natural remanent magnetization (NRM) of archive-half sections. NRM of all cores from Holes U1422A–U1422C and Cores 346-U1422D-1H through 3H was measured before and after alternating field (AF) demagnetization with a 20 mT peak field at every 5 cm interval. Because of increased core flow through the paleomagnetism station, NRM of Cores 346-U1422D-4H through 16H and of all cores from Hole U1422E was measured only after 20 mT AF demagnetization. The FlexIT core orientation tool (see “Paleomagnetism” in the “Methods” chapter [Tada et al., 2015b]) was successfully deployed to orient 13 APC-collected cores in Hole U1422C, starting from Core 346-U1422C-2H. Core orientation data collected in Hole U1422C are reported in Table T15.

We 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 U1422C and occasionally from deep sections when the first section was not suitable for taking a discrete sample. Depth levels where discrete samples were taken are marked by triangles along the left side of the paleomagnetic inclination data column in Figure F32C. Stepwise AF demagnetization of 12 discrete samples from Hole U1422C was performed at successive peak fields of 0, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 70, and 80 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 F32C. Following each demagnetization step, NRM of the discrete samples was usually measured with the sample placed in three different orientations on the discrete sample tray (i.e., “away-up,” “top-right,” and “top-toward”; see“Paleomagnetism” in the “Methods” chapter [Tada et al., 2015b]) and then averaged to acquire statistically more reliable results. Because of increased core flow, we sometimes only performed stepwise AF demagnetization up to 60 mT peak field and measured the samples in only one orientation (top-toward).

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 taken 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 T16. The processed NRM inclination, declination, and intensity data after 20 mT AF demagnetization are reported in Table T17 and shown in Figure F32.

Natural remanent magnetization and magnetic susceptibility

NRM intensity after 20 mT AF demagnetization in all five holes is similar in magnitude for overlapping intervals, mostly ranging between ~10^–5 and 10^–2 A/m. For core sections from the uppermost ~7 m, NRM intensity is on the order of 10^–2 A/m. NRM intensity gradually decreases downcore to the order of 10^–4 to 10^–3 A/m between ~7 and ~35 m CSF-A. NRM intensity appears to highly fluctuate between 10^–5 and 10^–3 A/m from ~40 to ~80 m CSF-A and is mostly on the order of 10^–4 A/m from ~80 to ~140 m CSF-A. Between ~140 and ~205 m CSF-A in Hole U1422C, NRM intensity is on the order of 10^–4 to 10^–3 A/m.

The AF demagnetization behavior of eight discrete samples from normal and reversed polarity intervals at varying depths is illustrated in Figure F33. 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 overprint. For measured discrete samples from below ~20 m CSF-A, NRM intensities before and after stepwise demagnetizations are generally one or two magnitudes lower than those from above this level. NRM measurement of discrete samples from deep depths (deeper than ~30 m CSF-A) 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 five 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”). 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, 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. Magnetic susceptibility varies between 10 × 10^–5 and 50 × 10^–5 SI for sediment from the uppermost ~7–10 m of the holes and is generally <10 × 10^–5 SI for sediment from below 7–10 m CSF-A (Fig. F32, fourth panel).

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 determination of magnetic polarity for at least the uppermost ~80 m of recovered sediment. Both magnetic declination and inclination after 20 mT AF demagnetization were used when possible for magnetostratigraphic interpretation at this site. The geomagnetic field at the latitude of Site U1422 (43.77°N) has an expected inclination of 62.44°, 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 at Site U1422 (Table T18).

Inclinations of the two APC cores (~10 m long) recovered in Holes U1422A and U1422B vary closely around the expected positive inclination at the site location (Fig. F32A, F32B), suggesting the uppermost ~10 m of sediment from the two holes was deposited during the Brunhes Chron. The Brunhes/Matuyama boundary was identified at ~33.5 m CSF-A in Hole U1422C, ~32.3 m CSF-A in Hole U1422D, and ~32.5 m CSF-A in Hole U1422E. Inclination values above the identified boundary vary around the expected dipole inclination value of ~62.44°. In Hole U1422C, the FlexIT-corrected declination shows values mostly around 0°, which is expected for a normal polarity at the site for the uppermost ~33.5 m. This interpretation is consistent with the FO of calcareous nannofossil E. huxleyi (0.29 Ma) at 19.87–24.19 m CSF-A in Hole U1422C (see “Biostratigraphy”).

The second significant inclination pattern change was observed at ~77.5 m CSF-A in Hole U1422C, ~82.5 m CSF-A in Hole U1422D, and ~76.8 m CSF-A in Hole U1422E. In Hole U1422C, this inclination pattern change is accompanied by FlexIT-corrected declination values changing from an average of ~180° to ~0°. We interpret this horizon as the top of the Olduvai Chron (C2n, 1.778 Ma). In Hole U1422C, where FlexIT tool orientation data are available, the corrected declination appears to change from an average of ~0° back to an average of ~180° at ~84.6 m CSF-A. As declination is less affected by the generally vertical drilling overprints, this could be interpreted as the bottom of the Olduvai Subchron (C2n, 1.945 Ma) recorded in Hole U1422C. These interpretations are consistent with the lithologic Unit I/II boundary (with an age of ~2.5 Ma; Tada, 1994) (see “Lithostratigraphy”) recognized at 90.52 m CSF-A in Hole U1422C, 89.71 m CSF-A in Hole U1422D, and 90.30 m CSF-A in Hole U1422E.

Between the Brunhes/Matuyama boundary and the top of the Olduvai Chron, a short interval with relatively stable positive inclinations around the expected dipole inclination value are identified between ~41.5 and 45.2 m CSF-A in Hole U1422C, ~41 and 45.6 m CSF-A in Hole U1422D, and ~41.8 and 45.7 m CSF-A in Hole U1422E. In Hole U1422C, FlexIT-corrected declinations also appear to vary mostly around 0°. We interpret this interval as the Jaramillo Subchron (0.988–1.072 Ma). Postcruise studies are needed to further refine the depth levels of these identified boundaries.

Below the interpreted bottom of the Olduvai Chron in Hole U1422C and top of the Olduvai Chron in Holes U1422D and U1422E, inclinations show mostly positive values that are apparently steeper than the expected dipole inclination, indicating drill string overprinting that may not have been efficiently removed. Strong overprint and the lack of orientation for the bottom cores, as well as the large scatter in declination makes it difficult for any reliable magnetostratigraphic interpretations.

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