Detailed paleomagnetic records during the Brunhes-Matuyama geomagnetic reversal, and a direct determination of depth lag for magnetization in marine sediments

Abstract

Detailed paleomagnetic records were studied from homoclinal bathyal siltstone deposited during the Brunhes-Matuyama geomagnetic reversal outcropping in the Boso Peninsula, central Japan. The records are based on three continuous sediment cores from three sections within a distance of 14 km. The stratigraphic horizons were correlated using a volcanic ash bed which occurs about 2 m above the Brunhes-Matuyama reversal. The sedimentation rate, according to the oxygen isotope stratigraphy for the three sections, decreases eastward and is 3.70, 2.20 and 1.83 m ka ^-1. The depth lag of magnetization in the sediments was calculated directly as 42 cm, based on the differences in the relative position of the key ash layer in the correlated virtual geomagnetic pole (VGP) paths for the three sections. VGP movement during the geomagnetic reversal is characterized by short intervals of very rapid motion between periods of relative stability. The last swing of the VGP from the southern hemisphere to the northern hemisphere in the Brunhes-Matuyama geomagnetic reversal took only 38 yr. Intensities of the remanent magnetization normalized by anhysteretic remanent magnetization indicate that the intensity of the geomagnetic field decreased to one-fifth for about 5.4 ka including the geomagnetic reversal. The movement of the VGP longitude is 0.36° yr ^-1 westward and this westward drift can be related to the rotation of the non-dipole field.