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 Jaram illo (UJ) polarity reversals have been obtained from a loess section a t Weinan, Shaanxi province, in China. An investigation of the rock mag netic properties using stepwise IRM acquisition, hysteresis loops, the rmomagnetic analysis and X-ray diffraction on magnetic extracts, ident ifies pseudo-single domain magnetite as the carrier of the main fracti on of the remanence, associated with maghemite and hematite. The total duration of the directional change (based upon independent loess accu mulation rates), is about 5000 yr for the MB reversal and about 3200 y r for the UJ reversal. These values are consistent with previous estim ates obtained from marine or lacustrine sediments. Apart from a few in termediate points observed on North America (MB) and in the Pacific (U J), the transitional Virtual Geomagnetic Pole (VGP) paths are situated in two longitudinal bands over eastern Asia and Australia (MB) and th e Americas (UJ). The bulk of the UJ record and the part of the MB reco rd over the Americas are not consistent with the hypothesis of a recor d arising from smoothing of pre- and post-transitional directions [1]. A reappraisal of the statistical approach of McFadden et al. [2] show s that the longitudinal distribution of the records of reversals is co nsistent with the hypothesis of genuine, geographically preferred sect ors for the paths, independent of site longitude, with a probability h igher than 0.45 when the two Weinan records are considered. Thus, thes e results, obtained from wind-blown loess, sustain the hypothesis of t wo preferential longitudinal bands for reversing transitional VGPs sug gested from the study of marine and lacustrine sediments.