Pedostratigraphy and paleomagnetism of a similar to 7.0 Ma eolian loess-red clay sequence at Lingtai, Loess Plateau, north-central China and the implications for paleomonsoon evolution

A 305-m-thick loess-red clay sequence was discovered recently at Lingtai, w hich is located in the middle part of the Chinese Loess Plateau. It consist s of a complete Pleistocene loess-soil sequence with a thickness of about 1 75 m and 130 m of Tertiary red clay deposits. The red clay sequence at Ling tai is the thickest one presently known in the Loess Plateau. Field observa tions show that the Lingtai red clay contains over 110 couplers of pedogeni c B horizons and horizontal carbonate nodule horizons, and that it can be r egarded as an extremely thick soil complex. Paleomagnetic studies of 680 sa mples suggest that the basal age of the Lingtai loess-red clay sequence is about 7.05 Ma. Grain size analysis of samples taken at 3.3 cm intervals ind icates that the red clay has the same sedimentological characteristics as t hose observed in the loess, thus suggesting a wind-blown origin of the Tert iary red clay and continuous atmospheric dust deposition in the Loess Plate au during the last 7.05 Ma. The pedogenic characteristics of the paleosols within the Pleistocene loess and the B horizons in the red clay suggest tha t the East-Asia summer monsoon in the latest Miocene may have already been slightly stronger than that during the Holocene. This implies that at about 7.5 Ma BP, the Tibetan Plateau could have been uplifted to a critical heig ht in maintaining the East-Asia summer monsoon system. Observations of the loess-red clay sequence also suggest that the long-term changes in the East -Asia summer monsoon strength have been nonlinear since the latest Miocene. (C) 1999 Elsevier Science B.V. All rights reserved.