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doi:10.2204/iodp.proc.340.108.2013

Paleomagnetism

Cores 340-U1398A-1H, 2H, 5H through 7H, and 340-U1398B-1H through 11H were recovered using the APC and nonmagnetic core barrels. Cores 340-U1398A-8H through 13H and 340-U1398B-12H through 24H were recovered using the APC and standard steel core barrels. The FlexIt core orientation tool was used on all cores recovered with the APC and nonmagnetic barrels; thus, between 0 and 51 mbsf in Hole U1398A and between 0 and 83 mbsf in Hole U1398B, declination can be corrected to true north. Where FlexIt tool data were not available, declination was guided by the discrete inclination (orange data, Figs. F10, F11; see “Paleomagnetism” in the “Methods” chapter [Expedition 340 Scientists, 2013a]). Expected inclination for the site is 27° during normal polarity and –27° during reversed polarity, assuming a geocentric axial dipole (GAD). Archive halves of cores from Holes U1398A and U1398B were measured on the three-axis superconducting rock magnetometer (SRM) at 2.5 cm intervals (Table T5). NRM was measured before (NRM₀) and after stepwise alternating field demagnetization at 20 mT (NRM₂₀). Thirty-nine discrete samples were collected from the center of the working half of the core to compare to the SRM data (see “Paleomagnetism” in the “Methods” chapter [Expedition 340 Scientists, 2013a]). Three Hole U1398B samples (from 43, 186.7, and 246.2 mbsf) were stepwise demagnetized to 100 mT and also given an anhysteretic remanent magnetization (ARM) and an isothermal remanent magnetization (IRM) in fields of 20, 100, 300, and 1000 mT to assess their magnetic mineral composition (see “Paleomagnetism” in the “Methods” chapter [Expedition 340 Scientists, 2013a]).

Sediment recovered from Site U1398 was heterogeneous in composition and varied between layers of hemipelagic sediment and volcaniclastic tephras and turbidites (see “Lithostratigraphy”). Using the detailed core description logs, we only interpreted data measured on identifiable hemipelagic layers, as these appear the least disturbed and are more likely to provide information on the behavior of the geomagnetic field.

Results

NRM₀ (red) and NRM₂₀ (blue) intensities are shown for Holes U1398A and U1398B in Figures F10 and F11. NRM₀ intensity is relatively high at ~0.5–1 m/A. NRM₂₀ is <10% of NRM₀. This percentage reflects both the low coercivity of the sediment and the susceptibility of the material to acquire a strong drill string overprint. This overprint is typically removed by 20 mT when using nonmagnetic barrels, though significant overprinting and remagnetization of sediment when standard steel barrels are used is a persistent feature of Expedition 340 cores.

Hemipelagic sediment is highly discontinuous at Site U1398, punctuated by the significant thicknesses of turbidites, and comparison between the two holes is not as straightforward as for other sites (see “Lithostratigraphy” and “Physical properties”). Rifling of the core barrel is evident by scattered declination values in all cores recovered with the XCB; thus, inclination is the only indicator of polarity below 108 mbsf in Hole U1398A and 171 mbsf in Hole U1398B.

Between 0 and ~170 mbsf in Holes U1398A and U1398B, all SRM and discrete inclination data show scattered but positive inclination, clustering around the expected GAD inclination. Similarly, declination shows little variation, suggesting that all sediment above 170 mbsf was deposited under normal polarity conditions and within the Brunhes Chron (<780 ka). These ages agree with biostratigraphic datums from planktonic foraminifers and nannofossils, suggesting the bases of Holes U1398A and Hole U1398B are no older than 250 ka.

Below 170 mbsf the inclination record from hemipelagic sediment becomes increasingly discontinuous because of poor recovery. However, negative inclination is a persisting feature evident in both SRM data and discrete samples indicating material older than 780 ka. This is in conflict with biostratigraphic ages (see “Paleontology and biostratigraphy”); thus, additional rock magnetic tests were performed to assess the nature of the remanence carrying material. Three discrete samples, two from horizons with negative inclination and one from the upper part of the record with positive inclination, were demagnetized and given an ARM and IRMs (Fig. F12). NRM is ~4× weaker and more resistant to demagnetization in the deeper samples, but inclination appears strong and stable in all three samples, suggesting the negative inclination is not an overprinted and/or a viscous artifact. All three samples show >85% acquisition of the 1000 mT IRM (SIRM) in a field of 300 mT, suggesting ferrimagnetic minerals (probably magnetite and maghemite phases) are the main remanance carriers throughout the core. Magnetic grain size, estimated by ARM/SIRM, shows no significant difference between samples, suggesting that magnetic grain size and mineralogy do not vary greatly throughout the core.

Consistently negative inclination in the lower parts of Holes U1398A and U1398B span over ~80 m with a single period of positive inclination at ~220 mbsf. This sediment was exclusively recovered with the XCB, and the deepest sediment is from a mostly intact semilithified mudstone. Inclinations are generally shallower than would be expected by a GAD model, and without declination to reinforce the shallow inclination values, definitive interpretation of polarity is difficult. Some sections of core do show evidence for postdepositional deformation; however, this cannot account for all the negative inclination intervals (see “Lithostratigraphy”). If indeed these consistently negative values show true geomagnetic behavior, it would suggest that sediment deeper than ~170 mbsf was deposited at a time that experienced reversed polarity and is older than 780 ka. However, this is inconsistent with relatively young biostratigraphic ages, and further detailed measurement is needed to provide constraint on the paleomagnetic data below ~170 mbsf at Site U1398.

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