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doi:10.2204/iodp.proc.339.103.2013

Paleomagnetism

Paleomagnetic investigation of the 67 cores collected at Site U1385 included the measurement of magnetic susceptibility of whole-round and archive-half split-core sections and the natural remanent magnetization (NRM) of archive-half split-core sections. NRM was measured before and after alternating field (AF) demagnetization with peak fields of 10 and 20 mT for archive-half sections from Holes U1385A, U1385B, and U1385C. For archive-half sections from Holes U1385D and U1385E, NRM was measured before and after AF demagnetization with 20 mT peak field. Data from the FlexIt tool were used to orient cores from all holes except Hole U1385C, from which only one core was retrieved. FlexIt tool data were collected starting with Core 4H in Holes U1385A, U1385B, and U1385D and with Core 5H in Hole U1385E (Table T13). Stepwise AF demagnetization on 16 selected discrete samples from Hole U1385A 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. The depth levels from which the measured discrete samples were taken in Hole U1385A are indicated by triangles in the first panel of Figure F18. We processed data extracted from the Laboratory Information Management System (LIMS) database by removing all measurements collected from disturbed and void intervals, which are listed in Table T14 (see “Stratigraphic correlation”), and all measurements that were made within 10 cm of the section ends, which are slightly biased by measurement edge effects. The processed NRM inclination, declination, and intensity data after 20 mT peak field AF demagnetization are listed in Tables T15, T16, T17, T18, and T19.

Natural remanent magnetization and magnetic susceptibility

The intensity of NRM after 20 mT peak field AF demagnetization in all five holes is similar in magnitude, ranging from ~10^–5 to ~10^–2 A/m (Fig. F19). For core sections from the uppermost ~50 mbsf, NRM intensity is on the order of ~10^–2 A/m. Between ~50 mbsf and the bottom of the holes, NRM intensity decreases to ~10^–3 to ~10^–5 A/m. The general agreement of the trend between the NRM intensity and magnetic susceptibility suggests that the magnetic minerals that carry the NRM are the same that dominate the magnetic susceptibility.

In spite of the overall low intensities of NRM below ~50 mbsf, a relatively stable magnetic component was preserved in sediment from all five holes, which allows for the determination of magnetic polarity. A magnetic overprint with steep positive inclinations, which was probably acquired during drilling, was usually removed by up to 20 mT peak field AF demagnetization (Fig. F19). However, NRM directions show relatively large scatter below ~50 mbsf. This suggests that secondary magnetizations still remain and are probably a viscous remanent magnetization and/or chemical remanent magnetization caused by diagenetic growth or dissolution of magnetic minerals.

We calculated component NRM directions of the discrete samples from the data from the 25–50 mT demagnetization steps using principal component analysis (PCA; Kirschvink, 1980) and UPmag software (Xuan and Channell, 2009). Three discrete samples from the uppermost 90 mbsf yielded reasonably good component directions, with maximum angular deviation of <15°. Component inclinations of these discrete samples are generally consistent with the archive-half section measurements (yellow circles in the first panel of Fig. F18). The demagnetization behavior of two discrete samples that yielded good PCA results from above 50 mbsf and of two samples from below 50 mbsf with poor demagnetization behavior is illustrated in Figure F19. The two discrete samples from above 50 mbsf display a soft magnetic overprint that was removed at 15–20 mT AF demagnetization, demonstrating that this magnetic cleaning level is sufficient to eliminate the overprint.

Magnetic susceptibility measurements were made on whole cores from all five 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”). Magnetic susceptibility is consistent between the two instruments and, in general, mimics the NRM intensity. 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 50 × 10^–5 SI (Fig. F18). Note that a constant of 25 × 10^–5 SI was added to the SHMSL measurements (gray lines) to facilitate the comparison with the WRMSL measurements (black lines).

Magnetostratigraphy

Both magnetic declination and inclination were used when possible for the magnetostratigraphic interpretation at this site. The geomagnetic field at the latitude of Site U1385 (37.57°N) has an expected inclination of 56.98°, assuming a geocentric axial dipole field model, which is sufficiently steep to determine magnetic polarity in APC cores that lack horizontal orientation.

The Brunhes–Matuyama polarity transition occurs at ~93.8 mbsf in Hole U1385A (interval 339-U1385A-11H-5, ~80 cm), ~91.8 mbsf in Hole U1385B, ~97.3 mbsf in Hole U1385D, and ~92 mbsf in Hole U1385E (Fig. F18; Table T20). The polarity transition should occur over an interval of <1 m, assuming a sedimentation rate of ~100 m/m.y. and a polarity transition with <10 k.y. duration. However, the relatively large scatter of the NRM directions made it difficult to determine the exact position of the boundary. The top and bottom of the Jaramillo Subchron (C1r.1n) occur, respectively, at ~106.8 and ~117.7 mbsf in Hole U1385A (intervals 339-U1385A-13H-1, ~80 cm, and 14H-2, ~68 cm), ~101.2 and ~113.4 mbsf in Hole U1385B, ~107.3 and ~116.3 mbsf in Hole U1385D, and ~101.5 and ~111.2 mbsf in Hole U1385E (Fig. F18; Table T20). We interpret the short normal polarity interval at ~131–132.4 mbsf in Hole U1385A, ~129–132.1 mbsf in Hole U1385B, ~127.4–130.3 mbsf in Hole U1385D, and ~130.8–132.3 mbsf in Hole U1385E (Fig. F18; Table T20) as the Cobb Mountain Subchron (C1r.2n), although the large scatter of the NRM directions makes this interpretation somewhat tentative. In spite of relatively high sedimentation rates of ~100 m/m.y., we obtained no evidence for short-duration polarity flips and/or excursions, such as the Blake event, in the Brunhes Chron.

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