MATUYAMA BRUNHES (M/B) TRANSITION RECORDED IN CHINESE LOESS/

Here we present a detailed record of the Matuyama/Brunhes (M/B) polari ty transition recorded in a loess section at Xifeng (36-degrees-N, 108 -degrees-E), in the central part of the Chinese loess plateau. The tra nsition zone, which occurs in loess unit L8, has a thickness of 3.8 m and was sampled in full. At this point in the section the loess accumu lation rate has been estimated at 10 cm/ka, and with the size of sub-s ample used provides a potential time resolution of 240 years. Magnetit e has been identified as the main magnetic mineral with a small contri bution from maghaemite or cation deficient magnetite. The remanence of all samples was investigated using thermal demagnetisation from room temperature up to 650-degrees-C. The results indicate a duration of 32 ,000 years for the decay and recovery of the main dipole intensity, an d 20,000 years for the period of major directional change. During the transition the field intensity falls to an average of 30% of its value before and after the transition, although there are strong variations about this weak value which accompany a sequence of 4 reversed/normal /reversed fluctuations of the local field vector. These fluctuations i mmediately preceed the final reversed-normal transition, after which t he field intensity recovers to its pre-transition value. Calculated VG P paths of most stages are continuous and confined to preferred longit ude bands which include America, the Atlantic Ocean or Africa. Field b ehaviour during the transition period suggests that non-dipolar terms are dominant while the dipole field is weak, with the dipole decay and recovery associated with the increased influence of low order zonal t erms of the geomagnetic field.