Paleomonsoon route reconstruction along a W–E transect in the Chinese Loess Plateau using the anisotropy of magnetic susceptibility: Summer monsoon model

Abstract

The anisotropy of magnetic susceptibility (AMS) was investigated in three Chinese Loess Plateau sedimentary sections along a W–E transect (400 km). The loess–paleosol sequences in our study represent the most recent 130 kyr interval. The measured AMS was compared to expected theoretically derived magnetic fabrics occurring in strong and weak current airflow conditions. The major and minor AMS ellipsoid axis orientations were used to evaluate the paleowind direction along the transect. Previously published models assumed that cold and dry winter monsoons bringing dust from northwestern desert areas were responsible for the magnetic fabric formation of loess sequences. In our new interpretation, the stronger summer monsoons from the southeast played the major role in magnetic fabric orientation in the studied west and central parts of the Chinese Loess Plateau. Although the material was brought to the area by the winter monsoon, the AMS was generated during the rainy summer monsoon when the sedimentary particles including magnetite were rearranged, settled, and fixed. We reconstruct the summer paleomonsoon routes for the last 130 kyr. These winds prevail from SE to NW but appear to be affected by regional topographic factors. In the western section, the corridor between the north and south Liupan Mountains disturbed the summer monsoon route, shifting it from SE to SEE.