镇江下蜀土剖面的化学风化强度与元素迁移特征

对镇江下蜀土剖面的化学风化强度及元素迁移特征进行了研究, 结合与黄土高原第四纪黄土、古土壤、晚第三纪红粘土以及安徽宣城风成红土等典型风成堆积剖面的对比分析, 得出如下结论: (1) 镇江下蜀土剖面经历了中等强度的化学风化, 明显强于洛川黄土以及古土壤, 显著弱于宣城风成红土, 而与西峰晚第三纪红粘土非常接近。下蜀土的化学风化过程及 其与其他风成堆积剖面风化强度的差异主要受气候条件的控制, 气候条件通过年平均温度和年降水量对化学风化的地球化学环境起重要的影响, 其中降水因素在化学风化过程中可能起着更为重要的制约作用。(2) 镇江下蜀土剖面风化过程中, 绝大部分常量元素的地球化学行为 表现为迁移淋失, 仅Fe 和Ti 轻微富集, 元素的活动性由强至弱依次为: P > Na > Ca > Mg > K > Fe²⁺ > Si > Mn > Al > Ti > Fe³⁺； 常量元素的迁移特征揭示下蜀土的化学风化已经完成初级阶段的去Ca、Na 过程, 并初步进入到去K 风化的中级阶段； 微量元素除Sr、Ga 迁移淋 失外, Th、Ba、Cu、Zn、Co、Ni、Cr、V 等均表现出富集特征, 这可能与下蜀土风化成壤过 程中的生物地球化学过程以及粘粒、有机质对微量元素的吸附作用有关。(3) 元素迁移在剖面 中的变化特征揭示, 在0.24 Ma 之前的中更新世早、中期, 该区气候较为暖湿, 兼有干湿交替的特征； 中更新世晚期气候偏干凉, 风化淋溶最弱； 至晚更新世早期则又出现一段明显暖湿的成壤时期, 形成下蜀土剖面中的S1 古土壤层。总之, 中更新世以来本区气候整体上向干凉的方向发展。

Chemical Weathering Intensity and Element Migration Features of the Xiashu Loess Profile in Zhenjiang

The chemical weathering intensity and element migration features of the Xiashu loess profile in Zhenjiang are studied in this paper.Based on the comparison with other aeolian-dust deposits,i.e.Quaternary loess-palesol and late-tertiary red-clay profiles in the Loess Plateau,and aeolian dust red earth in Xuancheng of Anhui province,we draw the following conclusions:(1)The Xiashu loess profile underwent moderate chemical weathering,which was stronger than that of the loess and palesol in Luochuan,much weaker than that of the aeolian dust red earth in Xuancheng,and similar to that of late-tertiary red-clay in Xifeng.The chemical weathering differences among the studied aeolian-dust deposits were mainly induced by the distinction of the climate condition,which imposed important influence on the geochemical environment through the mean annual temperature and annual precipitation.It seems that the annual precipitation has more important influence on the chemical weathering process.(2)The major elements such as Si,K,Na,Ca,Mg,Mn and P were migrated and leached,while the elements Fe and Ti were slightly enriched during the chemical weathering process.According to the migration ability,the major elements are ranked in the following order:P > Na > Ca > Mg > K > Fe2 > Si > Mn > Al > Ti > Fe3 .The migration features of the major elements reveal that the Xiashu loess finished the primary process of chemical weathering characterized by leaching of Ca and Na,and almost reached the secondary process characterized by leaching of K.Except the elements Sr and Ga,other trace elements such as Th,Ba,Cu,Zn,Co,Ni,Cr and V were enriched during the chemical weathering process.And this enrichment might be caused by both the biogeochemical process and the adsorption of trace elements by clay mineral and organic materials during the chemical weathering.(3)The difference of element migration down the Xiashu loess profile reveals that the climate was warm and wet at the early-middle stage of the Middle Pleistocene(before 0.24 Ma),however,bearing the feature of an alternate dry and wet fluctuation.At the end of the Middle Pleistocene,it became dry and cool,which resulted in the weakest weathering and element migration.At the early stage of the Late Pleistocene,the paleoclimate became warm and wet again,which led to rather strong pedogenesis and the formation of the paleosol S1 in the Xiashu loess profile.As a whole,the paleoclimate generally became drier and cooler in this region from the beginning of the Middle Pleistocene.