LEAD ISOTOPIC SIGNATURES OF CENOZOIC EOLIAN DEPOSITS AND IMPLICATIONS FOR ATMOSPHERIC CIRCULATION OVER CENTRAL ASIA

Isotopic studies are a powerful tool for identification of the provenance of sediments. Only a few studies, however, have investigated the isotopic signatures of Chinese loess or the underlying Red Clay formation. In this study, lead isotopes are used to investigate the provenance of the three major sedimentary deposits in the Chinese Loess Plateau (CLP). Pb isotopic ratios of samples from the loess – paleosol sequence, the Red Clay formation and the fluvial siltstones at Lingtai suggest that the Red Clay and loess share at least one source area, which is distinctly different from the provenance of the siltstones.

A small but significant increase in the lead isotopic ratios of the Red Clay samples as compared to the loess – paleosol ratios suggests an important difference between the two eolian deposits. The difference between the ratios can be explained by two different mechanisms. Grain-size experiments show that the smallest particles of loess have the highest Pb isotopic ratios. Therefore, the decrease in average grain size of the Red Clay may explain the increase of Pb isotopic ratios. Alternatively, the change in ratios may represent a different mixture or mass balance of sources from changes in atmospheric circulation patterns. The Red Clay ratios are also similar to those of loess deposits in Tajikistan and eolian sediments in the North Pacific. This suggests that the Red Clay has a significant component from westerly sources that were cut off by the uplift of the Tibetan Plateau prior to loess deposition. Therefore, the change in Pb isotopic ratios between the Red Clay and the loess – paleosol sequences of the CLP appears to represent the modification of the mixture of mineral aerosol sources and thus could record atmospheric circulation changes over Central Asia from the uplift of the Tibetan Plateau.