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doi:10.2204/iodp.proc.319.103.2010

Paleomagnetism

Note: This section was contributed by Hirokuni Oda (Institute of Geology and Geoinformation, National Institute of Advanced Industrial Science and Technology, Central 7, 1-1-1 Higashi, Tsukuba 305-8567, Japan) and Xixi Zhao (Earth and Planetary Sciences Department, University of California Santa Cruz, 1156 High Street, Santa Cruz, CA 95064, USA).

Shipboard paleomagnetic studies for Site C0009 consisted of continuous measurements and progressive demagnetization of archive-half core sections. The natural remanent magnetization (NRM) was measured at 5 cm intervals for each core section (51 total), followed by alternating-field (AF) demagnetization at 10 and 20 mT peak fields. Magnetic polarity has been assigned on the basis of the inclination of the stable remanent magnetization because of the lack of core orientation due to rotary drilling. As Site C0009 is situated at moderate latitude in the Northern Hemisphere, positive (downward directed) inclinations are taken to signify normal polarity and negative (upward directed) inclinations signify reversed polarity. As shown in Figure F117, remagnetization imparted by the coring process is commonly encountered at Site C0009: NRM inclinations are strongly biased toward vertical (mostly toward +90°) in a majority of cores except the uppermost 30 m where sediments appear to be dominated by negative inclinations (Fig. F117A). Upon AF demagnetization to 20 mT, a significant decrease in intensity (about an order of magnitude, see Fig. F117B) and a shift of inclination toward shallower or negative values were observed (Fig, F117A). The negative shift in inclination is also clearly observed in histograms of inclinations for NRM and that for 20 mT AF demagnetization and shows modes at 80°–85° and 75°–80°, respectively (Fig. F118). These values are significantly different from the expected time-averaged geomagnetic field inclination at Site C0009 (±52°). The absence of peaks in the negative part of the histogram may indicate the incomplete removal of coring-induced remanence and/or that perhaps the entire measured interval was deposited during a normal polarity period.

Because of the time constraints caused by the problems related to the ship's cryogenic magnetometer (see "Paleomagnetism" in the "Methods" chapter for more detailed explanation), we could not increase the level of AF demagnetization on the ship's cryogenic magnetometer to further remove the remagnetization or to collect discrete samples for stepwise thermal or AF demagnetization experiments to verify the polarity.

In summary, preliminary pass-through paleomagnetic data have revealed important magnetic signatures that await further verification in terms of age and origin. Further integrated work with shipboard micropaleontological data and structural measurements is required to constrain the timing and origin of the magnetization recorded by Site C0009 sediments.

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