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abstract

Atmospheric dust deposition is a common phenomenon in arid and semi-arid regions. Bimodal grain size distribution (BGSD) (including the fine component and coarse component) of aeolian deposits has been widely reported. But the origin of this pattern is still debated. Here, we focused on the sedimentary process of modern dust deposition, and analyzed the grain size distribution of modern dust deposition, foliar dust, and aggregation of the aeolian dust collected in Cele Oasis, southern margin of Tarim Basin. The results show that BGSD also appear in a dust deposition. The content of fine components (<20 lm size fraction) change with temporal and spatial variation. Fine component from dust storm is significant less than that from subsequent floating dust. Fine component also varies with altitude. These indicate that modern dust deposition have experienced changing aerodynamic environment and be reworked during transportation and deposition, which is likely the main cause for BGSD. The dusts from different sources once being well-mixed in airflow are hard to form multiple peaks respectively corresponding with different sources. In addition, the dust deposition would appear BGSD whether aggregation or not. Modern dust deposition is the continuation of ancient dust deposition. They both may have the same cause of formation. Therefore, the origin of BGSD should provide a theoretical thinking for reconstructing the palaeo-environmental changes with the indicator of grain size.

5. Conclusions

Modern dust deposition in Cele Oasis provides an excellent media to understand the origin of BGSD of aeolian deposits. Based on the analysis of BGSD of Cele’s dust deposition, we found that BGSD also appears in a dust deposition. The content of fine components (<20 lm) change with temporal and spatial variation. The fine component from dust storm is significant less than the one from subsequent floating dust in a dust deposition. Fine component also changes with altitude. These indicate modern dust deposition with BGSD has experienced changing aerodynamic environment, and been reworked during transportation and deposition, which is likely the main cause for BGSD. The particles from different sources once being well-mixed in airflow are hard to form multiple peaks corresponding with different sources. In addition, aeolian deposits would appear BGSD whether aggregation or not. Consequently, aeolian deposits with BGSD cannot be simply regarded as multi-sources origin, or even that fine component in the loess is an intensity indicator of the westerlies. Modern dust deposition is the continuation of dust deposition in geological history. BGSD not only appear in modern dust deposition but also in ancient aeolian deposits. They both probably have the same origin. Therefore, the origin of BGSD of aeolian deposits in this study may provide a theoretic thinking for reconstructing the paleo-environmental changes with the indicator of grain size.