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doi:10.2204/iodp.proc.339.105.2013

Paleomagnetism

Paleomagnetic investigation of the 130 APC, XCB, and RCB cores (excluding one empty core in Hole U1387B and one wash core, two empty cores, and two short (<45 cm) cores in Hole U1387C) collected at Site U1387 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. In addition, Cores 339-U1387C-54R through 61R were measured after AF demagnetized with peak fields 10, 20, 25, and 30 mT. The FlexIt tool was successfully used to orient five cores in the APC section of Holes U1387A and U1387B starting with Core 4H (Table T12). Stepwise AF demagnetization of 32 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 determine the magnetostratigraphy in the strongly overprinted and disturbed XCB- and RCB-cored sections. The depth levels from which the measured discrete samples were taken are indicated by blue triangles in the first panel of Figure F25. We processed data extracted from the Laboratory Information Management System database by removing all measurements collected from disturbed and void intervals, which are listed in Table T13 (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 Table T14, T15, and T16.

Natural remanent magnetization and magnetic susceptibility

The intensity of NRM after 20 mT demagnetization is similar in magnitude in the overlapping parts of Holes U1387A, U1387B, and U1387C, ranging from ~10^–5 to ~10^–2 A/m (Fig. F27, third panel). The uppermost ~92 mbsf exhibits the highest NRM intensities, on the order of 10^–2 A/m. Below this level, NRM intensity drops one to three orders of magnitude, in the range of ~10^–5 to ~10^–3 A/m.

Despite the significant coring disturbance and drill string overprint in the XCB- and RCB-cored sections, a relatively stable magnetic component was preserved in sediments from all three holes that allows for the determination of magnetic polarity for some 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. F28). The uppermost few (~6) XCB cores in both Holes U1387A and U1387B appear to be less disturbed, and the NRM directions are less affected (Fig. F27). XCB sections from deeper parts of the holes are often 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 enable the extraction of a better-quality paleomagnetic signal. The RCB cores from Hole U1387C exhibit a relatively well preserved magnetic polarity record downhole to ~450 mbsf. Below this depth, weak NRM intensity combined with a significant overprint severely compromises the quality of the magnetic signal.

The AF demagnetization behavior of eight discrete samples from normal and reversed polarity intervals is illustrated in Figure F28. All samples exhibit a steep, normal overprint that was generally removed after AF demagnetization at peak field of ~15–20 mT, demonstrating that the 20 mT magnetic cleaning level is, in general, sufficient to eliminate the overprint. The samples also appear to acquire a significant amount of anhysteretic remanent magnetization at high peak field (>55 mT) AF demagnetization steps, possibly because of bias caused by 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 F27 (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, parallels the intensity of magnetic remanence. 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. F27, fourth panel). Note that in Figure F27, 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

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 NRM inclination and on measurements of discrete samples taken from the relatively undisturbed drilling biscuits. The geomagnetic field at the latitude of Site U1387 (36.81°N) has an expected inclination of 56.25°, assuming a geocentric axial dipole field model, which is sufficiently steep to determine magnetic polarity in cores that lack horizontal orientation.

The Brunhes–Matuyama polarity transition is constrained by discrete samples and occurs in Hole U1387A between ~182 mbsf (Sample 339-U1387A-21X-2W, 72–74 cm) and ~207 mbsf (Sample 23X-6W, 61–63 cm) (Fig. F28). A discrete sample from 192.17 mbsf in Hole U1387A (Sample 22X-3W, 64–66 cm) appears to carry an intermediate NRM inclination (Figs. F27, F28). We therefore place the Brunhes/Matuyama boundary at ~190 mbsf in Hole U1387A. Our best estimate for the position of the Brunhes–Matuyama transition in Hole U1387B at ~192 mbsf is based on comparison with the general inclination pattern in Hole U1387A and the occurrence of negative inclinations at this depth. The relatively large scatter of the remanent directions in the XCB cores made it difficult to determine the exact position of the boundary; however, more detailed paleomagnetic work on discrete samples from drilling biscuits should resolve the transition relatively well. The top and bottom of the Jaramillo Subchron (C1r.1n) cannot be determined in any of the three holes based on the core-section measurements or the limited number of discrete sample measurements. The top of the RCB-cored section of Hole U1387C records the lower part of the Matuyama Chron (C1r.2r) and, surprisingly well, the top of the Olduvai Chron (C2n) at ~412.5 mbsf. This interpretation is supported by at least two discrete sample measurements that yielded relatively stable characteristic remanent magnetizations. Discrete samples from 370.51 mbsf (Sample 339-U1387C-10R-3W, 70–72 cm) and 447.12 mbsf (Sample 18R-3W, 71–73 cm) clearly carry reversed and normal magnetization, respectively (Figs. F27, F28). The core-section inclination (after 20 mT peak AF demagnetization) exhibits a clear normal polarity pattern between ~412.5 and ~450 mbsf, but the onset of the Olduvai Chron (C2n) is obliterated by low magnetic intensities combined with coring disturbances and a strong coring-induced overprint. Below ~450 mbsf, magnetostratigraphic interpretation is not possible, and even discrete samples can rarely resolve reliable paleomagnetic information.

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