寒武系第二统至芙蓉统浅水沉积记录中的稀土组成特征及演化规律研究

There are increasing studies focusing on seawater rare earth element (REE) composition and evolution by using reliable sedimentary recorders, such as ooid, microbialite, and early marine cement. These proxies can provide valuable insights into marine oxidation state, REE signature and their fluctuation trend at that time. For these reasons, this study intends to adopt oolite and microbialite as targets that were extensively distributed in Cambrian (Series 2 to Furongian) to recognize their characteristics of original microstructures and temporal distributions from pure carbonate and mixed carbonate-siliciclastic depositional regions, respectively. On basis of these natures from ooid and microbialite, the manners both from solution- and from in situ laser ablation-inductively coupled plasma-mass spectrometry would be performed to explorer shallow seawater REE compositions and their secular fluctuations in Cambrian. Additionally, the possible REE signatures from pore-water condition and from siliciclastic-influenced region would also be carefully evaluated as a function of similarly diagnostic signals from modern early diagenesis and mixed carbonate-siliciclastic circumstances. Therefore, the work from petrology and element geochemistry on Cambrian ooid and microbialite (Series 2 to Furongian) would provide insights on original seawater REE signals and on the coevolution of early life and temporal environment.

地质历史时期海水稀土组成和演化趋势是沉积学和古海洋化学领域研究的一个新热点，利用可靠沉积记录（例如鲕粒、微生物岩和早期胶结物等）能为恢复当时水体的氧化程度、稀土元素特征及变化规律等提供重要参考。本课题拟选择寒武系第二统至芙蓉统广泛发育的鲕粒灰岩和微生物岩为研究对象，系统研究这两类沉积物在华北、华南不同相区（清水碳酸盐与混积岩沉积区）发育特征和分布层位。在此基础上，利用溶液（区分碳酸盐组分和陆源碎屑组分）和原位激光剥蚀（点分析和半定量微区成像）两种微量稀土元素测试方法对当时的海水氧化还原条件、稀土组成及演化、早期成岩作用以及陆源碎屑污染物示踪等方面特征展开探索。通过系统研究寒武系第二统—芙蓉统鲕粒与微生物岩在岩石学和元素地球化学等方面的特征，能为进一步了解当时海水的稀土组成特点，以及特殊的生物与环境协同演化关系提供参考。

本项目旨在通过对寒武系第二统—芙蓉统浅水碳酸盐鲕粒和微生物岩中原始海水微量和稀土元素信号的提取，分析水体氧化还原条件及稀土组成特点，探讨海水稀土组成长期演化规律及对寒武纪特殊生物和环境事件的响应。具体研究内容主要包括三个方面：（1）年代地层框架，鲕粒与微生物岩分布和发育特征研究；（2）鲕粒与微生物岩微量和稀土元素组成特征研究；以及（3）寒武系浅水沉积记录与古海洋化学演化规律研究。目前，三方面研究内容已基本完成，预期成果已满足设定要求并超额完成，预算执行率在95%以上，圆满完成了项目设定目标。项目取得的成果主要包括：（1）创建一整套用于碳酸盐岩原位微区微量和稀土元素分析的技术方法、实施方案及标样专利技术；（2）在复杂、古老碳酸盐岩中识别出海水、陆源和成岩三种不同性质的稀土元素信号；（3）综合我国埃迪卡拉纪—寒武纪之交微生物岩特征后认为寒武纪早期“蓝细菌钙化作用幕”的发生与古海水性质转变有关；（4）研究区汉南—米仓山寒武纪早期碳酸盐工厂发育主要受气候控制，类型变化与营养水平等一系列因素有关；（5）形成基于基体匹配的碳酸盐岩原位微区全元素面扫描成像技术研发和应用技术。依托项目执行，发表SCI论文8篇，CSCD论文5篇，培养博士毕业生1名，硕士毕业生2名，完成了项目预期目标。