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doi:10.2204/iodp.proc.338.107.2014

Paleomagnetism

Routine magnetic measurements were conducted with a superconducting rock magnetometer on Hole C0022B archive halves of cores. Because of time constraints, only two demagnetization steps and no discrete sample measurements were performed on samples on board the Chikyu. These procedures may not be sufficient to remove artificial magnetization components induced by drilling, so caution is needed when interpreting results, especially for the EPCS and the ESCS sections (below 76.5 mbsf). The initial paleomagnetic record can be totally concealed because of magnetization by the drilling slurry and cannot be adequately evaluated without discrete sample measurements (see the “Site C0002” chapter [Strasser et al., 2014b]).

Remanent magnetization of archive-half sections from Holes C0022B was measured at demagnetization levels of 0, 10, and 20 mT peak fields to identify characteristic remanent magnetization. Vector component diagrams of progressive alternating field (AF) demagnetization (Fig. F43) indicate that large downward-directed vertical components were successfully removed with 10 mT. Such a vertical component is generally considered to be imparted by the coring process, as noted in previous ocean drilling research (e.g., Gee et al., 1989).

Because Hole C0022B is located between Sites C0004 and C0008, which were cored during Expedition 316 (Expedition 316 Scientists, 2009a, 2009b), a similar paleomagnetic stratigraphic pattern is expected. Paleomagnetic data show the Brunhes/Matuyama boundary (0.78 Ma) in Hole C0004C above 20 mbsf. The Brunhes/Matuyama boundary is unclear in Holes C0008A and C0008C, probably due to truncation of surface sediment by submarine landslides (Strasser et al., 2011).

In Hole C0022B, the interval between ~70 and ~140 mbsf is likely stratigraphically and structurally disturbed by the splay fault (see “Background and objectives,” “Logging while drilling,” “Biostratigraphy,” and “Structural geology”). In such disturbed environments, magnetostratigraphic interpretation is not straightforward. The inclination profile shows indistinct inclination changes from normal to negative values in the upper several tens of meters (Fig. F44). The signature of the profile recognized is similar to that of Hole C0008A (Expedition 316 Scientists, 2009b), but biostratigraphy data for Hole C0022B indicate that the Brunhes/Matuyama boundary (0.78 Ma) should be located shallower than 48 mbsf and the Jaramillo Subchron (0.99–1.07 Ma) should be located between 48 and 57.5 mbsf (see “Biostratigraphy”). Therefore, the paleomagnetic stratigraphy interpretation remains nonunique. In contrast, the interval dominated by positive inclinations between 330 and 380 mbsf in Hole C0022B may be correlated to the interval between 210 and 270 mbsf in Hole C0008A, referring to seismic reflection layers in the cross-section images along Sites C0004, C0008, and C0022 (Fig. F2) (Strasser et al., 2011; Kimura et al., 2011). The interval assigned to the 1.77–1.95 Ma Olduvai Subchron in Hole C0008A (Expedition 316 Scientists, 2009b) is extrapolated to Site C0022.

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