Supported by the National Natural Science Foundation of China 41172308 and by the Open Project of the State Key Laboratory of Loess and Quaternary Geology SKLLQG1309 and by the Knowledge Innovation Project of The Chinese Academy of Sciences KZZD-EW-04-02;国家自然科学基金41172308;中国科学院黄土与第四纪地质国家重点实验室开放课题SKLLQG1309;中国科学院知识创新工程重大项目KZZD-EW-04-02