Previous   |    Next

doi:10.2204/iodp.proc.329.107.2011

Paleomagnetism

At Site U1369, we measured natural remanent magnetization of all archive-half sections for Holes U1369B, U1369C, and U1369E using the three-axis cryogenic magnetometer at 2.5 cm intervals before demagnetization. The archive-half sections were demagnetized at alternating fields (AF) of 10 and 20 mT. The present-day normal field in this region, as expected from the geocentric axial dipole model at Site U1369, has a negative inclination (approximately –58.6°), so positive remanence inclinations indicate reversed polarity. Data from Holes U1369C and U1369E provide only a partial record because whole-round core samples were taken from these holes for geochemical and microbiological analyses. From Hole U1369B, 11 discrete sediment samples (7 cm³ cubes) were taken and compatibility of magnetization between archive half and working half was analyzed. The primary objective of the shipboard measurements for Site U1369 was to provide chronostratigraphic constraint by determining magnetic polarity stratigraphy. During the coring operation at Site U1369, neither nonmagnetic core barrels nor the Flexit core orientation tool were used because of the shallow drilling depth of the sediment column (see “Operations”).

Results

Paleomagnetic data for Holes U1369B, U1369C, and U1369E are presented in Figures F22, F23, and F24, together with the whole-core susceptibility data measured on the WRMSL (see “Physical properties”). The lithology at Site U1369 changes from metalliferous zeolitic pelagic clay (lithologic Unit I) at the top to zeolitic metalliferous pelagic clay (Unit II) at the bottom (see “Lithostratigraphy”). The metalliferous zeolitic pelagic clay unit extends from 0 to ~6 mbsf in Hole U1369B. Using magnetic susceptibility data, it was possible to correlate between Holes U1369B and U1369C (Fig. F25). This correlation was applied to the magnetic intensity data and to the inclination and declination data (Fig. F26).

Magnetic directions at Site U1369 show both reversed and normal polarity. The records show steep normal polarity for most of the site, possibly because of a magnetic overprint acquired during coring (high negative inclination), viscous remanent magnetization, or diagenetic changes in the sediment.

Given the difficulty in determining the age of the sediment section by shipboard paleomagnetic studies, chronostratigraphy for Site U1369 must be determined by postexpedition studies, including use of other chronostratigraphic tools and further magnetic cleaning by increased magnetic field AF demagnetization.

Top of page   |    Previous   |    Next