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doi:10.2204/iodp.proc.329.109.2011

Paleomagnetism

At Site U1371, we measured natural remanent magnetization (NRM) of all archive-half sections for Holes U1371B–U1371H using the three-axis cryogenic magnetometer at 2.5 cm intervals before and after alternating-field (AF) demagnetization. The archive-half sections were demagnetized by alternating fields of 10 and 20 mT. The present-day normal field in this region, as expected from the geocentric axial dipole model at Site U1371, has a negative inclination (approximately –64.2°), so positive remanence inclination indicates reversed polarity. Data from Hole U1371E provides only a partial record because whole-round core samples were taken from these holes for geochemical and microbiological analyses. The primary objective of the shipboard measurements for Site U1371 was to provide chronostratigraphic constraint by determining magnetic polarity stratigraphy. During coring operations at Site U1371, nonmagnetic core barrels were used for all holes to ~100 mbsf and the Flexit core orientation tool was used for Hole U1371D (see “Operations”).

Results

Paleomagnetic data for Holes U1371B–U1371F and U1371H are presented in Figures F22, F23, F24, F25, F26, and F27, together with the whole-core susceptibility data measured on the WRMSL (see “Physical properties”). The lithology at Site U1371 changed from clay-bearing diatom ooze (Unit I) at ~105 mbsf to zeolitic and metalliferous clays (Unit II) (see “Lithostratigraphy”).

Using magnetic susceptibility data, it was possible to correlate between Holes U1371B–U1371F (Figs. F28, F29) in the 0–70 mbsf interval. This correlation was applied to the magnetic intensity data (Fig. F30). Postexpedition application of such correlations to declination and intensity will be used to test if changes in magnetic polarity (Figs. F30, F31) are consistent between the holes.

Assignment of polarity intervals to specific chrons must wait for postexpedition biostratigraphic work to provide general age control. The basement age at Site U1371 is ~73 Ma (Gradstein et al., 2004).

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