A ?6Ma chemical weathering history, the grain size dependence of chemical weathering intensity, and its implications for provenance change of the Chinese loess-red clay deposit

摘要

Major element chemical analyses of the Baishui loess-red clay sequence (35° 24' 10″N, 106° 54' 43″E, Gansu Province, China) reveal that, the chemical weathering intensities inferred from CIA (chemical index of alteration) values have progressively weakened and increased in amplitude with decreasing age. The general trend of the CIA can be correlated well with the oxygen isotope variations of the deep sea sediments. Analyses of multiple size fractions (<5μm, 5-20μm and 20-63μm) indicate that the CIA and the major element abundances are strongly grain size dependent. The chemical alternation of all three size fractions is relatively weak, and the long-term variation in chemical composition of the loess-red clay is mainly controlled by the mixing of fresh detritus from different source areas. The CIA variability of the coarse fractions (5-20μm and 20-63μm) within the section is apparently different from that of the fine fraction (<5μm), implying a different erosion-transportation-deposition-weathering history. From 1Ma onward, CIA values for all size fractions decrease, indicating an increasing input of fresh or poorly weathered detritus in the desert source regions of loess deposits. Sequential variations in the K_2O/Al_2O_3, Na_2O/Al_2O_3, CaO/Al_2O_3 and MgO/Al_2O_3 ratios from 4.5Ma to about 0.6Ma in bulk and size fractioned samples may reflect provenance change as well as changing chemical weathering processes.