SPATIAL AND TEMPORAL PATTERNS OF QUATERNARY DUNE ACTIVITY IN THE ARID AND SEMIARID CHINA

Episodic dune formations during the Quaternary were found in many deserts of China. Generally, desert evolutions on the orbital time scales were the response to the astronomical forcing, characterized by expanded deserts during glacial maxima. However, due to the close link between Chinese deserts and the adjacent high mountains (High Asia), the high mountain processes (e.g., glacial grinding, frost weathering, rock denudation) have also played an important role in controlling desert evolution through its effects on sand and silt production. The most striking tectonic control of desert expansion occurred at 1.1 and 0.9 Ma ago, and these tectonic control desert evolutions had durations longer than the Milankovitch cyclicity thus imposed on the glacial-interglacial climatic changes induced desert evolutions. Spatial scale studies on desert evolution indicate that the last two extreme scenarios of a glacial maximum and a Holocene climatic optimum marked extreme ecosystems in China. The deserts margin changed mainly in its longitudinal range due to changes in east Asia monsoonal circulations, and it shifted from 125^oE of the last glacial maximum to 105^oE of the climatic optimum. Historical desertification in the semiarid China is not a response to climate drought but largely associated with the human impacts (mainly over-cultivation) since about 2300 years ago. Over-cultivation combined with the high wind energy leads to the quick deflation of the 'protection layer' of the Holocene sandy loam soil and ultimately results in the reworking of the underlying LGM sands. Our view of the importance of land use-use practice suggests that, in the fragile ecological system, where a great quantity of sand is available for reworking and high wind energy is concentrated, any unreasonable human activities will greatly accelerate the sand reworking processes. Episodic conversion of nomadic livestock systems to cropping during historical time is the main reason for the reworking processes of relict dunes in the Mu Us Desert and other semiarid regions in China.