The Matuyama/Brunhes geomagnetic polarity transition at Lingtai and Baoji,Chinese Loess Plateau

Two sections on the Chinese Loess Plateau (CLP) at Lingtai and Baoji have b een investigated to obtain detailed information about the exact stratigraph ic position and the recording quality of the Matuyarna/Brunhes geomagnetic polarity boundary (MBB). The continuously sampled MBB occurs in the glacial period loess layer L8 in both sections at a similar profile depth indicati ng comparable average sedimentation rates of about 8 cm/kyr. This stratigra phic position has been recognized in many previous Chinese loess studies an d contrasts with observations in marine sediments where this polarity trans ition is found at the beginning of interglacial oxygen isotope stage 19. Th e directional patterns of the characteristic remanent magnetization (ChRM) of the two transitional loess records are inconsistent and even depend on t he demagnetization technique used to isolate the ChRM. The MBB records also differ significantly from those obtained from the loess section at Weinan and at ODP site 792A near Japan. The VGP paths recorded in loess are not we ll defined, switch polarity several times and do not prefer specific longit udinal bands. It is suggested that the loess MBB transitional records do no t reflect geomagnetic field variations directly, but are the result of a co mplicated magnetization lock-in mechanism. By comparison with the astronomi cal ly-tuned prediction, this process has been delayed in the loess sedimen t column by 1.5 to 2 m corresponding to about 25000 yr after deposition. (C ) 2001 Elsevier Science Ltd. All rights reserved.