摘要
白水剖面黄土-红粘土全样化学分析表明,以CIA指数为代表的化学风化强度从剖面下部往上逐步变弱,波动增大,总体趋势与深海氧同位素代表的全球冰量变化可以很好对比。分粒级组分化学分析则显示,黄土-红粘土主要元素迁移及化学风化强度与粒度强烈相关,CIA指数表征的化学风化强度变化在相当程度上受到粒度变化的控制。总体上,黄土-红粘土各粒级颗粒的风化程度都不高,而且沉积之后的成土风化作用对其主要元素含量的影响有限,黄土-红粘土化学成分的长期变化趋势主要受到物源的新加入和混合过程的影响。从剖面变化看,粗粒级部分(5～20μm和20～63μm)的风化指数的变化特征与细粒级部分(<5μm)不完全相同,表明不同粒级组分接受化学风化以及在物源区停留的历史不尽相同。1Ma以来,各粒级CIA指数都整体降低,显示物源区新鲜、弱风化碎屑影响到各粒级颗粒的混合过程。进一步对细粒级部分(<5μm)化学成分演化进行的分析显示,其元素含量呈现不同组分递次变化的"序列模式",从红粘土下部(约4.5Ma)到黄土上部(约0.6Ma),K2O/Al2O3,Na2O/Al2O3,CaO/Al2O3和MgO/Al2O3比率的变化依次出现转折,这很可能与晚新生代黄土-红粘土物源区构造抬升、冰川剥蚀能力不断加大,使得新鲜基岩不断被剥蚀、去顶,由此造成主要造岩矿物顺序剥蚀、接受风化,成为粉尘新物源有关。
Chemical （ major element） 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） value have undergone a progressively upward weakening with amplifying magnitudes of fluctuation,and the general trend of CIA can be correlated well with oxygen isotope variations of deep sea sediments. Analyses on size-fractions （〈5μm,5 -20μm,and 20- 63μm） of the samples indicate that CIA and the major element abundances are of strongly grain-size dependence. It is estimated that the chemical alternations on all the three size fractions are relatively weak, and the long-term variation in chemical composition of the loess-red clay is mainly controlled by the input of fresh detritus and mixing process with different sources of deposits. Variation patterns of CIA for the coarse fractions（5 ~ 20μm and 20- 63μm） along the section is apparently different from that for the fine fraction （〈5μm）,implying different histories through erosion-transportation-deposition-weathering processes. From 1Ma onward, CIA values for each size fraction of the samples have all decreased,indicating an increasing input of fresh or poorly weathered detritus into the source regions of loess deposits. Further analyses on the fine fraction（ 〈 5μm） display a pattern of sequential variations in KEO/Al2O3 , Na20/Al2O3 , CaO/Al2O3 , and MgO/Al2O3 ratios from 4.5Ma to about 0.6Ma, probably reflecting a progressively strengthened erosion with new crustal addition, due to progressive unproofing in the uplifted source regions and increasing glaciations associated with the Late Cenozoic global cooling.
出处
《第四纪研究》
CAS
CSCD
北大核心
2008年第5期812-821,共10页
Quaternary Sciences
基金
国家自然科学基金项目(批准号:40672117)
中国科学院知识创新工程重要方向项目(批准号:KZCX2-YW-130)资助
