A ∼6 Ma chemical weathering history, the grain size dependence of chemical weathering intensity, and its implications for provenance change of the Chinese loess–red clay deposit
A ∼6 Ma chemical weathering history, the grain size dependence of chemical weathering intensity, and its implications for provenance change of the Chinese loess–red clay deposit
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DOI：
10.1016/j.quascirev.2010.04.009
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发表时间：
2010-07
影响因子：
4
通讯作者：
S. Xiong;Z. Ding;Yuanjian Zhu;Rufang Zhou;Haijian Lu
中科院分区：
文献类型：
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作者：
S. Xiong;Z. Ding;Yuanjian Zhu;Rufang Zhou;Haijian Lu
甘肃白水黄土-红粘土序列（35° 24′ 10″N，106° 54′ 43″E）的常量元素化学分析表明，化学风化强度随年代的减小而减弱，但幅度增大。CIA的总体趋势与深海沉积物的氧同位素变化有很好的相关性。多粒级（<5μm、5-20μm和20-63μm）分析表明，CIA和常量元素丰度与粒度有很强的相关性。三个粒级的化学变化相对较弱，黄土-红粘土化学成分的长期变化主要受不同源区新鲜碎屑混合的控制。剖面中粗粒级（5-20μm和20-63μm）和细粒级（<5μm）的CIA变异性明显不同，反映了不同的侵蚀-搬运-沉积-风化历史。从1 Ma起，CIA值的所有尺寸级减少，表明在黄土沉积物的沙漠源区的新鲜或风化不良碎屑的输入增加。块状和粒度分级样品中K2 O/Al 2 O3、Na 2 O/Al 2 O3、CaO/Al 2 O3和MgO/Al 2 O3比值在4.5 Ma至约0.6 Ma之间的顺序变化可能反映了物源的变化以及化学风化过程的变化。
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 K2O/Al2O3, Na2O/Al2O3, CaO/Al2O3and MgO/Al2O3ratios from 4.5Ma to about 0.6Ma in bulk and size fractioned samples may reflect provenance change as well as changing chemical weathering processes.