Size-dependent geochemical signatures of Holocene loess deposits from the Hexi Corridor (China)

Well dated Holocene loess sequences from uplifted river terraces at the northern margin of the Tibetan Plateau are characterised by their chemical, grain size, Sr isotope, and their oxygen and carbon isotope composition of the carbonate fractions. Prevailing wind directions suggest the source areas of the loess to be largely restricted to dry lands in Northwest China and the deserts north to the Hexi Corridor (e.g. Badain Jaran and Tengger Desert) which receive fluvial input by rivers draining the Qilian Shan. The dry lands of Northwest China are one of the most important dust source areas globally. Local and conditions make the investigated loess sequences insensitive to element leaching. The loess is characterised by a relatively narrow variation of Sr-87/Sr-86 (0.7139-0.7164). The Sr-87/Sr-86 ratios of surface- and carbonate-bound Sr are similar (0.7114 and 0.7111) and lower than those of silicate-bound Sr (63-2 mu m fractions: similar to 0.7196, <2 mu m fractions: similar to 0.7247). Since the clay fractions have higher CaCO3 contents than the silt fractions, the isotopic contrast between the silicate debris of clay and silt size is camouflaged in the bulk compositions of these grain size fractions. Oxygen isotope and chemical compositions of clay- and silt-sized carbonate particles differ significantly. Silt-sized loess carbonates formed authigenically and they are largely present as coatings of silicate grains, whereas clay-sized carbonate particles are of remote provenance and precipitated under evaporative conditions. In line with further implications from the multi-proxy study, upward trends in the composition of the pedogenic carbonate document climate change to drier conditions after the mid Holocene. (C) 2009 Elsevier Ltd. All rights reserved.