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doi:10.2204/iodp.proc.339.104.2013

Paleomagnetism

Paleomagnetic investigation of the 101 APC, XCB, and RCB cores collected at Site U1386 (excluding the three wash cores from Hole U1386C and two empty cores from Holes U1386A and U1386B) included the measurement of magnetic susceptibility of whole-core 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 20 mT peak field for all studied cores of the site. The FlexIt tool was used to orient the cores in the APC section of Holes U1386A and U1386B starting with Core 4H (Table T13). Although the rubber knob holding the tool in place broke sometime during the operation in Hole U1386B, the results of the reorientation attempt look convincing and the data appear to be usable. Stepwise AF demagnetization on 24 selected discrete samples 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 help determine the magnetostratigraphy in the XCB-cored sections. The depth levels where the measured discrete samples were taken are indicated by triangles in the first panel of Figure F31. 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 Tables 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, and T17.

Natural remanent magnetization and magnetic susceptibility

The intensity of NRM after 20 mT demagnetization is similar in magnitude in the overlapping parts of Holes U1386A and U1386B, ranging from ~10^–4 to ~10^–2 A/m (Fig. F31). Below ~450 mbsf, NRM intensity decreases to ~10^–5 to ~10^–4 A/m.

In spite of the significant coring disturbance and drill string overprint in the XCB- and RCB-cored sections, a relatively stable magnetic component was preserved in sediment from all three holes, which allows for the determination of magnetic polarity for most parts of the recovered sediment sequences. 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. F32). However, NRM directions show relatively large scatter in the XCB and RCB sections below ~180 mbsf (Hole U1386A) and ~150 mbsf (Hole U1386B). The XCB cores are heavily biscuited and usually contain as much of the disturbed matrix as the intact material, strongly compromising the quality of the resulting paleomagnetic data. Only discrete samples taken from the biscuits will be able to reveal a better quality paleomagnetic signal.

The AF demagnetization behavior of eight discrete samples from normal and reversed polarity intervals is illustrated in Figure F32. All samples exhibit a steep, normal overprint that was generally removed after AF demagnetization at a peak field of ~15–20 mT, demonstrating that the 20 mT magnetic cleaning level is sufficient to eliminate the overprint. The samples also appear to acquire an anhysteretic remanent magnetization during AF demagnetization at high peak fields (>55 mT), possibly because of a bias caused by the ambient magnetic field during demagnetization. We calculated component NRM directions of the discrete samples from data from the 25–50 mT demagnetization steps using principal component analysis (Kirschvink, 1980) and the UPmag software (Xuan and Channell, 2009). Component inclinations of discrete samples with maximum angular deviation less than ~15° are shown as yellow circles in Figure F31 (first panel).

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 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 5 × 10^–5 and 40 × 10^–5 SI (Fig. F31, fourth panel). Note that in Figure F31, 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

Sedimentation rates at Site U1386 are high, on the order of 350 m/m.y., which result in the occurrence of all magnetic polarity reversals in the highly disturbed XCB and RCB part of the cored section. The lack of core orientation and the significant coring disturbance and drill string overprint in the XCB and RCB cores limit our magnetostratigraphic interpretation to rely entirely on changes in magnetic inclination and on measurements of discrete samples taken from the relatively undisturbed drilling biscuits. The geomagnetic field at the latitude of Site U1386 (36.83°N) has an expected inclination of 56.27°, assuming a geocentric axial dipole field model, which is sufficiently steep to determine magnetic polarity in APC, XCB, and RCB cores that lack horizontal orientation.

The Brunhes–Matuyama polarity transition occurs at ~274.6 mbsf in Hole U1386A (interval 339-U1386A-32X-2, ~90 cm) and at ~273.3 mbsf in Hole U1386B (interval 339-U1386B-30X-7, ~60 cm) (Fig. F31; Table T18). The relatively large scatter of the remanent directions in the XCB cores made it difficult to determine the exact position of the boundary; however, detailed paleomagnetic work on discrete samples from drilling biscuits should resolve the transition relatively well. The termination of the Jaramillo Subchron (C1r.1n) was identified with the help of discrete samples in Hole U1386A at ~344.6 mbsf (Section 339-U1386A-39X-4) and in Hole U1386B at ~347.9 mbsf (Section 339-U1386B-39X-6) (Fig. F31; Table T18). The onset of the Jaramillo Subchron at 1.072 Ma occurs at ~374 mbsf in Hole U1386B. This interpretation is supported by discrete sample results as well as a shift of split-core inclination to more negative values (Fig. F31, first panel). The top of the RCB-cored section in Hole U1386C seems to record part of the Matuyama Chron (C1r.2r). Below Core 339-U1386C-10R (~450 mbsf) magnetostratigraphic interpretation is not possible because of a combination of poor core recovery, very low NRM intensity, sedimentary features such as debris flows and slumps (see “Lithostratigraphy”), and an apparent hiatus (see “Biostratigraphy”).

With sedimentation rates of ~350 m/m.y. in the APC part of the section, NRM data show several intervals with shallow or negative inclinations that are often accompanied by large changes (~180°) in the FlexIt tool–corrected declination data. Some example intervals are at ~80 and ~168 mbsf in Hole U1386A and at ~17 mbsf in Hole U1386B. Visual inspection of the split-core images and core descriptions indicate no apparent disturbance in these intervals, suggesting these intervals may be associated with geomagnetic excursions. High-resolution measurements with full demagnetization sequence are needed to confirm these interpretations.

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