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doi:10.2204/iodp.proc.334.105.2012

Paleomagnetism

Remanent magnetization was measured on archive-half cores and on discrete samples taken from the working half recovered in Hole U1380A. All archive-half cores were demagnetized in an alternating field (AF) to 15 mT and measured with the pass-through superconducting rock magnetometer (SRM), whereas discrete samples were subjected to stepwise AF demagnetization and measured in both the SRM and JR6 magnetometer. Two discrete samples were subjected to stepwise thermal demagnetization.

Natural remanent magnetization

Behavior of natural remanent magnetization (NRM) during paleomagnetic measurements was relatively uniform at Site U1380 compared to Holes U1378A and U1379C investigated during this expedition. All cores were drilled using the RCB system. Apparently, the drilling-induced remagnetization still exists in the recovered cores (Fig. F16), as evidenced by inclinations steeper than the value expected from the latitude of the site. However, the characteristics of the remagnetization are different from those observed in extended core barrel (XCB) cores at previous sites. NRM has no preferred declinations and shows slightly shallower inclination (mean = ~45°) than those of XCB cores from other Expedition 334 sites. From 470 to 480 mbsf in Core 334-U1380-10R, however, inclinations are much steeper (~70°–80°), indicating perhaps a different type of barrel was used to core these sediments and/or the steep inclinations are affected by the fairly steep bedding dips (see “Structural geology”). The mean NRM intensity is ~10–3 A/m and exhibits no significant variations downhole.

Demagnetization behavior

For the pass-through measurements of the archive halves, AF demagnetization up to 15 mT was not effective to recover primary remanence magnetization. Inclination only shifted to slightly toward shallower values but still seems to be dominated by the near-vertical drilling induced remagnetization (Fig. F16). Declination values for each measurement interval are relatively stable during the demagnetization. NRM intensity decreased by approximately one order of magnitude after 15 mT AF demagnetization (Fig. F16B).

AF demagnetization experiments of discrete samples revealed rapid removal of a near-vertical overprint at low demagnetization steps. After this, many discrete samples showed complicated demagnetization paths that did not simply go toward the origin (Fig. F17). Although some part of those demagnetization paths may represent primary magnetizations, we cannot resolve them at this point. Several discrete samples displayed more straightforward demagnetization behavior (Fig. F17). The number of samples from recovered cores is too limited to confidently define the magnetic polarity for the cores from Site U1380.

To evaluate the thermal behavior and identify magnetic mineralogy, we also conducted thermal demagnetization on two selected discrete samples. Heating was conducted using a thermal demagnetizer (Schonstedt TSD-1), and magnetization was measured using the SRM (see “Paleomagnetism” in the “Methods” chapter [Expedition 334 Scientists, 2012a]). Samples 334-U1380A-7R-4, 23–25 cm, and 8R-1, 95–97 cm, were heated from 75° to 300°C at 25°C increments. As shown in Figure F18A and F18B, these two samples have very low unblocking temperatures. By 125°C demagnetization, >60% of NRM intensity has been lost, suggesting the possible presence of goethite. Thermal demagnetization on Sample 334-U1380A-7R-41, 23–25 cm, successfully removed the drilling-induced component (Fig. F18C), but it is not obvious for Sample 8R-1, 95–97 cm (Fig. F18D). After 150°C demagnetization, the characteristic remanent magnetization component is evident from the demagnetization paths, with a fair amount of scatter (Fig. F18). Judging by the unblocking temperature (150°–200°C), titanomagnetite or maghemite may be the magnetic carriers for these two samples. Compared with AF demagnetization results, it appears that thermal demagnetization is not effective in isolating the primary remanence for samples recovered in Hole U1380.

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