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

Abstract

Two sections on the Chinese Loess Plateau (CLP) at Lingtai and Baoji have been investigated to obtain detailed information about the exact stratigraphic position and the recording quality of the Matuyama/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 indicating comparable average sedimentation rates of about 8 cm/kyr. This stratigraphic position has been recognized in many previous Chinese loess studies and contrasts with observations in marine sediments where this polarity transition is found at the beginning of interglacial oxygen isotope stage 19. The directional patterns of the characteristic remanent magnetization (ChRM) of the two transitional loess records are inconsistent and even depend on the 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 well defined, switch polarity several times and do not prefer specific longitudinal bands. It is suggested that the loess MBB transitional records do not reflect geomagnetic field variations directly, but are the result of a complicated magnetization lock-in mechanism. By comparison with the astronomically-tuned prediction, this process has been delayed in the loess sediment column by 1.5 to 2 m corresponding to about 25'000 yr after deposition.