Grain-size and Sr-Nd isotopic compositions of dry- and wet-deposited dusts during the same dust-storm event in Harbin, China: implications for source, transport-deposition modes, dynamic mechanism and formation of eolian loess

摘要

A strong dust-storm event occurred in Harbin, China on May 11, 2011. The dry and wet dust deposits from this dust-storm event were examined in terms of grain-size and Sr-Nd isotopic compositions, along with scanning electron microscope. The dry-deposited dusts are characterized by bimodal grain-size distributions with fine mode of 3.6 mu m and coarse mode of 28 mu m, while the wet-deposited dusts are indicative of unimodal grain-size modes with a fine mode of 6 mu m. These dust-storm depositions are derived from distal sources, for instance > 1000 km away from dust sink spot, regardless of the occurrence of relatively coarse component with coarse mode at 28 mu m for dry-deposited dusts. The Sr-Nd isotopic compositions of the dust-storm depositions are suggestive of the derivation from Horqin Sandy Land and, to a certain extent, Hunsandake Sandy Land, for both dry-deposited dust and wet-deposited dust. The fine mode (< 10 mu m) constituting fine tail and bimodality of grain-size distributions reflect the presence of fine particles as aggregates and/or adhering to larger grains, rather than fine grains having the derivation different from coarse grains. The fine particles may be transported and deposited by forming aggregates and/or by adhering to larger grains in low level. The fine grains are transported along with coarse grains during dust storms in spring, indicating dust storms being an important atmospheric circulation mode for the transportation of eolian dust. The fine and coarse particles in the CLP loess have the identical dust sources.