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doi:10.2204/iodp.proc.339.109.2013

Paleomagnetism

Paleomagnetic investigation of the 112 APC, XCB, and RCB cores (excluding 1 wash core and 4 cores without recovery) collected at Site U1391 included the measurement of magnetic susceptibility of whole-core and archive-half split-core sections and natural remanent magnetization (NRM) of archive-half split-core sections before and after alternating field (AF) demagnetization with 20 mT peak field. NRM before demagnetization was measured on Cores 339-U1391A-1H through 38X and 339-U1391B-1H through 24X but was discontinued because of time constraints at the end of the expedition. The FlexIt tool was used to orient 32 cores in the APC sections of Holes U1391A and U1391B, starting with Core 4H in Hole U1391A and 3H in Hole U1391B. The APC core orientations for Holes U1391B and U1391C are provided in Table T9 and used for APC core reorientation (Fig. F20). We processed data extracted from the Laboratory Information Management System database by removing all measurements that were made within intervals with interstitial water samples, intervals that contain voids as summarized in the core descriptions, anomalous intervals noted during measurement, and 10 cm of the section ends, which are slightly biased by measurement edge effects. The processed NRM inclination, declination (including the FlexIt tool corrected declination for Holes U1391A and U1391B), and intensity data after 20 mT peak field AF demagnetization are listed in Tables T10, T11, and T12.

Natural remanent magnetization and magnetic susceptibility

The intensity of NRM after 20 mT peak field AF demagnetization is similar in magnitude in the overlapping parts of Holes U1391A–U1391C, ranging from ~10^–5 to ~10^–2 A/m (Fig. F20, third panel). Sediments from the uppermost ~105 mbsf exhibits the highest NRM intensities, on the order of 10^–2 A/m, with a mean of ~0.014 A/m. Below ~105 mbsf, magnetic intensities are variable but generally lower than those in the uppermost part of the section (mean value = ~0.0024 A/m).

Despite the coring disturbance and drill string overprint in the XCB-cored sections, a relatively stable magnetic component was preserved in sediment from all holes, allowing for the determination of magnetic polarity for most parts of the recovered sedimentary sequences. The XCB sections in Holes U1391A and U1391B are often heavily biscuited and frequently contain as much of the disturbed matrix as the intact material, severely compromising the quality of the resulting paleomagnetic data.

Magnetic susceptibility measurements were made 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”). Magnetic susceptibility is consistent between the two instruments and, in general, parallels the intensity of magnetic remanence. WRMSL 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 5 × 10^–5 and 40 × 10^–5 SI (Fig. F20, fourth panel). Susceptibility in the uppermost ~105 m of sediments (mean value is ~20 × 10^–5 SI) is higher than that of sediments below that level (mean value = ~10 × 10^–5 SI. Note that in Figure F20, 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

We used magnetic inclinations, and FlexIt tool corrected declinations when available, to interpret magnetostratigraphy for the APC-cored sediment sequences. The lack of core orientation and significant coring disturbance, as well as drill string overprint in the XCB and RCB cores, limit our magnetostratigraphic interpretation for the XCB- and RCB-cored sediments in Holes U1391A and U1391B to relying on magnetic inclination changes. The geomagnetic field at the latitude of Site U1391 (37.56°N) has an expected inclination of 56.78°, assuming a geocentric axial dipole field model, which is sufficiently steep to determine magnetic polarity in cores that lack horizontal orientation.

NRM inclination data (after 20 mT peak field AF demagnetization) from all three holes indicate that the uppermost ~170 m of sediment was deposited during the Brunhes Chron (C1n) (Fig. F20). This interpretation is consistent with the LO of P. lacunosa (0.47 Ma) at ~114.68 and ~118.75 mbsf and the LO of R. asanoi (0.9 Ma) at 229.28 and ~235.87 mbsf in Holes U1391A and U1391B, respectively (see “Biostratigraphy”). The remainder of the XCB sections in Holes U1391A and U1391B is heavily disturbed and overprinted, and although several intervals reveal a clear reversed or normal polarity pattern, it is impossible to assign them to parts of the geomagnetic polarity timescale without the analysis of discrete samples from the XCB biscuits.

The top of the RCB-cored section of Hole U1391C records the lower part of the Matuyama Chron (C1r.2r) and, surprisingly well, the top (~452–463 mbsf) and bottom (~486 mbsf) of the Olduvai Subchron (C2n). This interpretation is constrained by the LOs of C. macintyrei (1.66 Ma) and D. broweri (1.95 Ma) at ~406 and 474 mbsf, respectively (see “Biostratigraphy”). The long normal polarity interval between ~575 mbsf and the base of Hole U1391C is assigned to the Gauss normal chron. This interpretation is supported by the LO of D. surculus (2.53 Ma) at ~561 mbsf and the LO of D. tamalis (2.80 Ma) at ~604 mbsf. Shorter subchrons of the geomagnetic polarity timescale, such as the Reunion or the Kaena, could not be resolved. The resolved polarity boundaries at Site U1391 are summarized in Table T13.

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