塔里木盆地阿克苏地区早寒武世海洋氧化还原状态及时空演化：来自玉尔吐斯组铁组分变化的约束

Oceanic redox conditions play an important role in biological evolution and element fractionation. Recent studies on the Lower Cambrian successions, particularly on the Yangtze Platform, showed that oceanic redox conditions were dynamically evolved with complex tempo-spatial variability, which may have exerted significant influence on biogeochemical cycling and element enrichment, and biological evolution. However, little is known about the oceanic redox conditions in the Early Cambrian in Tarim Basin. What was the nature of redox conditions of oceanic water masses (oxic, suboxic, and anoxic) there? Whether the ocean was ferruginous- or euxinic-dominant? How did the oceanic redox states change spatially and temporally? Which factors (such as seawater sulfate concentration, primary productivity, current upwelling, submarine hydrothermal activity, sea-level change etc.) had played a role in controlling their variation? How did the oceanic redox changes temporally link to the evolution of metazoan and the enrichment of organic matter? To better understand the oceanic redox changes in the Early Cambrian in Tarim Basin, in this study, we are going to use high-resolution iron speciation data from the Lower Cambrian Yurtus Formation in Aksu area to decipher the natures of ocean redox conditions and to reconstruct their tempo-spatial variability. Meanwhile, this study will further explore the relationships between the variation of oceanic redox conditions and evolution of metazoans or organic matter accumulation in the Early Cambrian.

海洋氧化还原条件是理解生物演化及元素分布的关键要素。最近基于下寒武统地层（尤其是扬子台地）的研究结果显示，早寒武世海洋氧化还原状态具有动态演化的时空结构，并显著影响着生物地球化学循环与元素富集，以及生物演化。然而目前，我们对于塔里木盆地早寒武世古海洋的认识，却知之甚少。它那时海水处于何种氧化还原状态（氧化、贫氧、缺氧）？是否发育缺氧铁化或硫化水体？各种不同性质的水体在时间和空间上如何演化？这种演化受控于何种因素（如海水硫酸盐浓度、初级生产力、上升洋流、海底热液活动、海平面变化等）？海洋氧化还原状态的时空演化与生物演化和有机质富集有何联系？对此，本项目以阿克苏地区下寒武统玉尔吐斯组为研究对象，采用可靠的铁组分方法，对早寒武世海洋氧化还原性质及其时空演化进行精细刻画。在此基础上，本项目将进一步探讨氧化还原状态演化的控制因素，以及它们与生物演化和有机质富集的关系。

海洋氧化还原状态是理解早寒武世生物演化和有机质富集的关键要素，具有重要的研究意义。塔里木盆地阿克苏地区下寒武统玉尔吐斯组下段以含磷质结核的层状硅质岩为主，中段以薄层黑色页岩-层状硅质岩为主，上段以薄层灰岩夹（钙质）泥岩为主，沉积于潮下-内陆架-外陆架环境。沉积学研究揭示，玉尔吐斯组硅质岩可见非晶质硅至微晶石英基质、软沉积塑性变形、保存较好的硅质微体化石、压实度较低且被玉髓和微晶石英胶结的硅质球粒和磷质内碎屑等沉积特征，表明源于沉积物-水界面附近原生硅质沉淀及早成岩阶段快速硅化成岩作用。因此，下寒武统玉尔吐斯组为早寒武世海水富硅观点提供了直接的沉积学证据，对于理解该时期大量的硅质岩沉积、广泛的硅质生物矿化和石化作用，以及硅相关的地球化学循环有很好的启示。地球化学研究揭示，玉尔吐斯组硅质岩具有明显的贫Al、富Fe、Ce负异常特征，轻微的Eu正异常特征。研究区玉尔吐斯组下部硅质岩和中部黑色页岩FeHR/FeT > 0.38，FePY/FeHR<0.6，并显著富集多种微量元素（如U、V、Ni、Cu、Zn、Ba等），指示沉积水体为缺氧铁化环境，并在时空上未见显著变化，也未见“硫化楔”结构。. 同时，本项目为厘清玉尔吐斯组钡富集成因，进一步对沉积地层重晶石成因类型进行了综述；为充分掌握并应用沉积地球化学方法，对沉积地球化学研究进行了现状分析与未来展望；为更全面认识黑色页岩沉积环境及有机质富集规律，对塔里木盆地萨尔干组黑色页岩沉积背景进行了对比分析。此项目研究成果将为理解塔里木盆地古海洋环境演化提供重要支持，并促进硅质岩、重晶石等沉积成因学研究，拓展对沉积地球化学研究现状的认识，具有重要学术意义。