Do fine-grained components of loess indicate westerlies: Insights from observations of dust storm deposits at Lenghu (Qaidam Basin, China)

The hyper-arid Qaidam Basin is a major dust source area in China. Mineral materials that settled during 11 dust storms from November 2003 to April 2005 in the northern Qaidam Basin were comprised of an amount of fine particles. The particles <20 μm varied between 11.4 and 19.4 vol.-%, while the mean volume percentages of <2, 2-5, 5-10 and 10-20 μm particles were 2.9, 3.6, 4.0 and 5.3 vol.-%, respectively. The deposits were characterised by very similar multi-modal grain size distributions. Stronger winds (>14 m s−1 at a height of 10 m above ground) during these dust storms were unfavourable for the gravitational settling of single fine mineral grains. Fine particles might instead settle either by forming aggregates or by adhering to larger grains. As such, the great amount of clay and fine silt in Chinese loess deposits can largely be correlated with dust storm processes and are likely derived from proximal source areas. Fine dust records from the Chinese Loess Plateau (CLP) therefore have to be interpreted cautiously as climatic proxies for changes in the westerlies.