氧化还原敏感微量元素对古海洋沉积环境的指示意义

Redox Sensitive Trace Elements as Paleoenvironments Proxies

海洋的氧化还原条件控制着U、V和Mo等氧化还原敏感微量元素在沉积物或沉积岩中的富集程度,所以可以利用沉积物或沉积岩中这些微量元素的含量来重建古海洋的氧化还原状态和沉积环境。海洋沉积物或沉积岩中U、V和Mo的来源少,除了海水提供外,还有陆源碎屑,不过陆源碎屑提供的那部分含量能够被估计。沉积物埋藏之后这些元素(几乎)不发生迁移,保存了沉积时的组成和含量,能较好地反映沉积环境的特征,是古海洋环境的理想指示。在氧化—次氧化的海水环境中U、V和Mo不会富集,缺氧的条件下U和V富集,而在硫化(含溶解的硫化氢)的沉积环境中U、V和Mo在沉积物中都会强烈地富集。利用这种差异,可以根据沉积物或沉积岩中各种微量元素的含量变化来判别沉积时海水的氧化还原状态。但是,必须注意沉积环境的开放与否、成岩作用及重新氧化作用等对微量元素含量的影响。在利用氧化还原敏感元素进行古环境重建时,应首先评估沉积物或沉积岩中特定元素的来源和贡献,剔除非自生部分的影响,然后利用多元素指标综合判别古环境的氧化还原条件,才能获得可靠的结论。

Redox conditions in marine settings controlled the concentrations of redox sensitive trace elements in sediments and sedimentary rocks. Thus, the concentrations of them could be used to reconstruct the paleoredox. Redox sensitive trace elements, U, V and Mo, have fewer origins and （mostly） do not move after deposition and burial, consequently, the sediments and sedimentary rocks retained the primary concentrations of these elements and could reflect the characteristics of the depositional conditions. These confer to U, V and Mo a good value as proxies for paleoredox. In oxie to suboxic conditions, U, V and Mo do not enrich; however, U and V enrich in anoxic and euxinic conditions but Mo just enriches in euxinic settings. Therefore, the combined use of U, V and Mo concentrations may allow us to distinguish the redox conditions. However, we should pay attention to the influences of the degree of depositional environments restriction, diagenesis and postdepositional reoxygenation on the concentrations of these elements. When using redox sensitive trace elements to reconstruct the paleoredox, we must assess sources of trace elements concentrations in the sediments or sedimentary rocks, and eliminate the influence of non-hydrogenous sources of trace elements, then use a suite of trace elements to reconstruct paleordox may give credible conclusions.