THE MATUYAMA-BRUNHES AND UPPER JARAMILLO TRANSITIONS RECORDED IN A LOESS SECTION AT WEINAN, NORTH-CENTRAL CHINA

The paleomagnetic records of the Matuyama-Brunhes (MB) and Upper Jaramillo (UJ) polarity reversals have been obtained from a loess section at Weinan, Shaanxi province, in China. An investigation of the rock magnetic properties using stepwise IRM acquisition, hysteresis loops, thermomagnetic analysis and X-ray diffraction on magnetic extracts, identifies pseudo-single domain magnetite as the carrier of the main fraction of the remanence, associated with maghemite and hematite. The total duration of the directional change (based upon independent loess accumulation rates), is about 5000 yr for the MB reversal and about 3200 yr for the UJ reversal. These values are consistent with previous estimates obtained from marine or lacustrine sediments. Apart from a few intermediate points observed on North America (MB) and in the Pacific (UJ), the transitional Virtual Geomagnetic Pole (VGP) paths are situated in two longitudinal bands over eastern Asia and Australia (MB) and the Americas (UJ). The bulk of the UJ record and the part of the MB record over the Americas are not consistent with the hypothesis of a record arising from smoothing of pre- and post-transitional directions [1]. A reappraisal of the statistical approach of McFadden et al. [2] shows that the longitudinal distribution of the records of reversals is consistent with the hypothesis of genuine, geographically preferred sectors for the paths, independent of site longitude, with a probability higher than 0.45 when the two Weinan records are considered. Thus, these results, obtained from wind-blown loess, sustain the hypothesis of two preferential longitudinal bands for reversing transitional VGPs suggested from the study of marine and lacustrine sediments.