THE ULTRAFINE COMPONENT RECORD FROM THE LATE CENOZOIC SEQUENCE IN THE CENTRAL TARIM BASIN AND ITS PALAEOCLIMATIC IMPLICATIONS

Grain size data from the Hongbaishan section reveal that the late Cenozoic deposits in the central Tarim basin generally contains an ultrafine component, which has an consistent modal size of 0.87μm and a variable proportion of 0.3%~10%. For the same lithological unit, proportions of the ultrafine component are quite distinctive along with the alternation of lithology. Aeolian sand owns the lowest value, loess has slightly higher value but lower than that of the fluvial deposits, whereas lacustrine clay maintains the highest value. Nevertheless, the long-term variations of the ultrafine component in different sediments show a similar trend, implying that same factor contributed to the long-term changes. Intensities of the chemical weathering and/or pedogenisis probably play the leading role in driving the changes. The abrupt decreasing of the ultrafine component at 2.8 Ma indicates a dramatic weakening of the chemical weathering and/or pedogenisis, which in turn suggest a significant strengthening of aridity in the Tarim basin.