Grain Size Abrupt Shift around 0.9 Ma in Loess and Its Environmental Effect in Fenwei Basin,China

The field investigations and the analyses of paleomagnetic polarity reversals,magnetic susceptibility and grain size on Laozhuang section which is on the fifth terrace of the Yellow River in Fenwei basin show that this section contains most of the aeolian stratigraphic units over the past 1.2 Ma.This section contains abundant sand(63 μm) and the curve of the percentage of the grain size exhibits an abrupt shift around 0.9 Ma.And since then there are several aeolian sand units in loess.Because of far from the desert,the abundant sand(63 μm) and the aeolian sand units are caused by the sand from near source.The sand beach of the Yellow River is the main source of sand in loess in Laozhuang section.The abrupt shift recorded in the section around 0.9 Ma possibly responded to the world-wide middle Pleistocene climatic transition event.This transition event may be related to the aggravation of drought in interior Asian triggered by the accelerated uplift of the Tibetan plateau.