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doi:10.2204/iodp.proc.339.107.2013

Paleomagnetism

Paleomagnetic investigation of the 159 APC, XCB, and RCB cores (excluding 1 wash core and 10 short [<50 cm] cores from Hole U1389E) collected at Site U1389 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 fields for all studied cores of the site. The FlexIt tool was used in an attempt to orient 23 cores from the APC sections in Holes U1389A, U1389C, and U1389D starting with Core 4H. However, the tool failed to properly orient the cores and the corrected magnetic declination is directed toward ~180° instead of 0°. The results of this attempt are therefore not reported. Stepwise AF demagnetization of eight 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 some of the strongly overprinted and disturbed XCB- and RCB-cored sections. 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 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 demagnetization is similar in magnitude in the overlapping parts of Holes U1389A through U1389E, ranging from ~10^–5 to ~10^–2 A/m (Fig. F23, third panel). The uppermost ~80 mbsf exhibits the highest intensities, on the order of 10^–2 A/m, with a mean of ~0.01 A/m. In the XCB section between 80 mbsf and 185 mbsf, NRM intensities are variable (mean value is about 0.007 A/m) but generally lower than that in the uppermost part of the section. With the exception of a few intervals, sediment below ~185 mbsf exhibits the lowest intensities (mean value is about 0.002 A/m).

Despite the coring disturbance and drill string overprint in the XCB- and RCB-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 sediments. 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. F24). The XCB sections of Holes U1389A and U1389C frequently contain drilling biscuits surrounded by as much disturbed material as intact material, strongly compromising the quality of the resulting paleomagnetic data. The RCB cores from Hole U1389E exhibit a relatively well preserved magnetic polarity record down to the base of the hole at ~990 mbsf.

The AF demagnetization behavior of four discrete samples from normal and reversed polarity intervals is illustrated in Figure F24. 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 magnetic cleaning level is, in general, sufficient to eliminate the overprint. The samples also appear to acquire a significant amount of anhysteretic remanent magnetization (ARM) 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 F23 (first panel).

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 is, in general, parallel to 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. F23, fourth panel). Note that in Figure F23, 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 magnetic inclination and occasionally on measurements of discrete samples taken from the relatively undisturbed drilling biscuits. The geomagnetic field at the latitude of Site U1389 (36.43°N) has an expected inclination of 55.88°, assuming a geocentric axial dipole field model, which is sufficiently steep to determine magnetic polarity in cores that lack horizontal orientation.

The LO of nannofossil R. asanoi (0.905 Ma) in Hole U1389A at ~327.6 mbsf (see “Biostratigraphy”) suggests that the position of the Brunhes–Matuyama polarity transition is located at a slightly shallower depth. However, recovery in this part of the cored section is poor and the overprint appears to be relatively strong. Results from discrete samples from this interval intended to resolve the Brunhes–Matuyama transition are mostly inconclusive. Discrete sample from 335.12 mbsf in Hole U1389A (Sample 339-U1389A-37X-3W, 71–73 cm), exhibits reversed polarity and is clearly located within the Matuyama Chron (C1r) (Figs. F23, F24). Results from Hole U1389C are similar, with the LO of R. asanoi at ~327.17 mbsf, poor XCB recovery, and a significant coring-induced overprint. Core 339-U1389C-38X (~345–350 mbsf) contains reversed intervals and appears to be located in the Matuyama Chron.

The NRM inclination in the RCB section of Hole U1389E between ~335 and ~990 mbsf defines several long normal and reversed polarity chrons (Fig. F23; Table T20). The dominantly reversed polarity interval between ~335 and ~542 mbsf contains the FO of R. asanoi (1.08 Ma) and the LO of C. macintyrei (1.66 Ma) (see “Biostratigraphy”), which constrains this interval to the Matuyama Chron (C1r). Evidence for the Jaramillo and Cobb Mountain Subchrons in this overprinted and poorly recovered part of the section could not be found. The long normal polarity interval between ~542 mbsf and ~592 mbsf correlates to the Olduvai Chron (C2n) and is well constrained by the LO of C. macintyrei (1.66 Ma) and the FO of G. inflata (2.09 Ma). The reversed Subchrons C2r.1r and C2r.2r between ~592 and ~696 mbsf are well constrained by several nannofossil and planktonic foraminifer events, notably the LOs of G. punticulata (2.41 Ma) and D. surculus (2.53 Ma). A short normal polarity interval at ~636 mbsf is assigned to the Reunion Subchron (C2r.1n). The Matuyama/Gauss boundary appears well defined at ~696 mbsf. The Gauss normal chron is constrained by several biostratigraphic datums: the LOs of D. tamalis (2.9 Ma), D. altispira (3.17 Ma), and S. seminulina (3.19 Ma). The two reversed intervals within the Gauss normal chron are tentatively interpreted as the Kaena (C2An.1r) and Mammoth (C2An.2r) Subchrons. The temporal disappearance of the planktonic foraminifer G. punticulata (3.57 Ma) at ~939.96 mbsf suggests that the polarity transition to the termination of Chron C2Ar should occur near the base of the cored section, probably in Core 339-U1389E-67R. However, poor core recovery in the heavily overprinted and disturbed cores and an inconclusive discrete sample makes a sound magnetostratigraphic interpretation difficult. In the ~990 m composite Pliocene–Pleistocene record of Site U1389, 11 polarity reversals of the geomagnetic polarity timescale are present. Polarity boundaries recognized at Site U1389 are summarized in Table T20.

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