Micromorphology of the Beiyuan loess-paleosol sequence in Gansu Province, China: geomorphological and paleoenvironmental significance

Abstract

The 35.5 m thick loess section at Beiyuan in Gansu Province, China provides the most detailed record on the Loess Plateau of the periodicities in earth orbital changes during and since the last interglacial. It contains ten paleosols, one of which is in the Holocene loess, four in the Malan loess and five in the upper Lishi loess and associated alluvium. Micromorphological study of the S1 paleosol series indicates that all of the component soils were subjected to moderate carbonate leaching and biological activity. Paleosols S1-c, S1-d and S1-e developed in alluvium over a time interval equivalent to stage 5e of the deep-sea core V 28–238. Alluvial deposition was episodic as evidenced by the separation of the paleosols. Since the development of S1-c, the terrace has not been affected by floods and has received only wind-blow dust additions. The S BC index, a ratio of the content of channels and excrements to the content of primary calcite grains, appears to be a sensitive indicator of environmental change between the arid and sub-humid environments represented by the loess and paleosols, respectively. The trends, periodicities and amplitudes of environmental change revealed by the S BC ratio are in good agreement with those derived from other indices through the Beiyuan section and with records from other parts of the world.