戈壁表层沉积物地球化学元素组成及其沉积意义

作为干旱、半干旱区的一类重要荒漠景观,戈壁在中国分布面积较广,其独特的物质组成和沉积特征对解释它的物源和发生过程有重要的意义.本文通过筛分处理对戈壁表层不同粒径沉积物地球化学元素组成进行了分析.结果表明:各类戈壁表层沉积物的主要元素组成特征基本相似,但不同粒级沉积物主要元素的含量略有差异.与UCC的平均化学组成相比,戈壁表层沉积物中主要元素除Si基本与UCC保持一致外,其余各主要元素均在不同程度上表现出了富集和淋失特征,微量元素在两大类型戈壁中随沉积物粒径的变化与主要元素基本一致.在极端干旱的气候条件下戈壁表层沉积物缺少有效的化学风化过程,基本处在未受或较弱的大陆化学风化的初、中级阶段.各个粒级的沉积物在经历侵蚀和风化过程中较好的继承关系表明,戈壁砾石层下伏的细粒沉积层不是外源搬运沉淀,而是局地岩屑通过就地风化和成壤作用形成.

Geochemical Compositions of Surface Sediment from Gobi Desert in Northern China and Its Sedimentary Significance

Gobi (desert pavement) occupies a large area of the China's mainland as one of the important desert landscapes in arid and semi-arid regions. Its unique composition and sedimentary characteristics have important significance to explain its provenances and generate process. But, there still exist some lack of systematic investigation of the geochemical features of different types of gobi sediments in China. In this paper, we analyzed the geochemical element compositions of different particle size sediments through sieving treatment. The results show that the characteristics of major element composition of surface sediment are similar from different gobi surfaces, but it shows slightly different between different grain size sediments. Compared with the average chemical composition of UCC, major elements except Si is consistent with the UCC in surface sediments from gobi, the rest of the main elements are in varying degrees showed enrichment and leaching characteristics. Trace elements variation between different grain sizes in two mainly kinds of gobi shows basically the same trends with the main elements. Surface sediment from gobi lack of effectively chemical weathering processes in the extreme arid climate conditions, basically at no or weak initial, intermediate stages of the continental chemical weathering process. Each grain size sediment have better inheritance relationship after erosion and weathering processes shows that gobi gravels underlying fine-grained sedimentary layers are not exogenous handling precipitation, but formed through in situ weathering and pedogenesis of local rock fragments.