Misleading positions of geomagnetic reversal boundaries in Eurasian loess and implications for correlation between continental and marine sedimentarysequences

The Matuyama-Brunhes boundary (MBB) where the Earth's magnetic field change d to its present orientation is one of the most frequently used time marker s in continental Quaternary stratigraphy. Although magnetic signals have be en successfully recovered from loess deposits and an apparently coherent ma gnetostratigraphy seems to be in place, a close examination of the publishe d loess records from northern mid-latitudes reveals significant uncertainty and variability for the position of the MBB. In Asia and part of Central E urope, the MBB has been found in loess, a glacial deposit. This is in contr ast to the well-established palaeomagnetic records in deep-sea sediments wh ere the MBB is located in the interglacial Oxygen Isotope Stage 19. This di screpancy has caused serious confusion in stratigraphic correlation between loess and marine sequences. Here we examine the causes of the discrepancy between the two sedimentary environments in their records of the MBB. Our a pproach includes a comparison of the MBB in loess and other continental rec ords, and an evaluation of the stratigraphic significance of the microtekti tes found around the MBB. Based on the occurrence of the MBB in an intergla cial stage in lake and continental margin sequences and the correlative str atigraphic positions of the microtektites in marine sediments and loess, we demonstrate that the measured positions of the MBB in loess are erroneous and misleading, and its true occurrence was during the time of formation of the palaeosol corresponding to Oxygen Isotope Stage 19. The most plausible cause for the apparent offset, i.e. the MBB being found in the underlying loess instead, is that the acquisition of remanent magnetisation in loess o ccurs at a certain depth substantially below the land surface. The degree o f such displacement is dependent on the lithology and structure of the depo sits formed prior to the reversal, and the time delay is estimated to be in the order of 10(3)-10(4) years. We show that other geomagnetic boundaries, such as those of Jaramillo and Cobb Mountain, in loess have been similarly displaced though to a varying extent. We suggest that the remanence acquis ition in loess is closely related to the post-depositional evolution in the structure of loessic materials which is in part influenced by regional cli mate conditions. Our conclusions on the displaced remanence acquisition hav e implications for loess chronology, magnetostratigraphy as well as the geo magnetic interpretation of detailed magnetic signatures from loess. (C) 199 9 Elsevier Science B.V. All rights reserved.