Loess record of the aerodynamic environment in the east Asia monsoon area since 60,000 years before present

Abstract

The multimodal distribution of grains in loess and dust storm deposits has been widely reported in recent years. Our analysis demonstrates that the interaction of wind, atmospheric turbulence, and dust grain gravity along the dust transportation path results in a multimodal grain size distribution for suspended dust. Changes in the median sizes of the coarse and medium modes are related to variation in aerodynamic forcing (lift force related to vertical wind and turbulence) during dust entrainment in the source area and turbulence intensity in the depositional area. On the basis of the numeric characteristics of the three modes of suspended particles, a tentative model has been constructed, which is used to study fluctuations of paleoaerodynamic environments. From a time series of paleoatmospheric environmental changes of the loess plateau for the past 60,000 years, we find that there are three patterns of aerodynamic environments in the dust source area (stronger, strong, weak) and 13 strong turbulence events since 60,000 years B.P. Climatic variability in the dust depositional area is stronger than in the source area. During the last glacial period the aerodynamic environment changed synchronously in the dust source and depositional areas over the entire trend. However, after 8000 years B.P. the aerodynamic force is strong in the dust source area, while the turbulence intensity is weak in the dust depositional area, differing from that of the last glacial period. The strong turbulence periods correspond to the cooling events such as the Heinrich events and the Younger Dryas cold event and may not be related to factors forcing the alternation between glacial and interglacial periods. Since 60,000 years B.P. the three patterns of aerodynamic environments are mainly controlled by the environmental changes in the dust source area and may be related to fluctuations of the winter monsoon. In each pattern, fluctuations of the turbulence intensity in the dust depositional area and the distance from dust source to dust depositional area are the major changes of the environment. The summer monsoon, recorded by magnetic susceptibility, may vary prior to the winter monsoon since 50,000 years B.P. Changes in the intensity of the summer monsoon may force changes in the advance‐retreat cycles of dust source areas in each winter monsoon pattern.