华北地台中元古代凝块石的成因,生物矿化机制及古环境分布

Thrombolites, as an important type of microbialites, constitute frequently major component in shallow marine carbonate platform in geologic history. They were often associated with reef ecosystems and formed industrial reservoirs for oil-gas resource. Thrombolites were especially well developed in the periods of Late Neoproterozoic to Early Ordovician, but were not known from Mesoproterozoic interval. As to the geneses of thrombolites, opinions are divergent; the biomineralization and predominant microbial-geochemical processes in their morphogenesis are not clear. In recent years, abundant thrombolites were recognized in the Mesoproterozoic Wumishan Formation of the North China Platform. The proposal intends to reveal the genesis, paleoecological distribution pattern and organomineralization of the thrombolites through the study of their internal micro- to ultra-fabrics, organomineral components and predominant microbial groups, thus to better understand the interactions of microbe-environments and their impacts on the paleoceanographic conditions and the early evolution of eukaryotes in early Earth history. The proposed study would select 2-4 potential sections in the North China platform, with specific emphases on silicified samples of early diagenesis. We will use the methodology of FESEM and TEM for ultra-fabric study, ICP-MS, EDS and XRD for elemental mapping and composition analyses, as well as Mass Spectrum Instruments for the Fe, C and S isotope composition detection. The study will mainly focus on the roles of microbial-geochemical processes played in the biomineralization of thrombolites, and the co-evolution of microbes and environments in the particular setting of atmospheric and paleoceanographic conditions during Mesoproterozoic. Furthermore, the proposed program will try to provide insights into the evolution of Earth surface systems, and the relative importance of microbial communities in changing environments and in the formation of some mineral resource.

凝块石是一种重要的微生物岩类型，并构成浅海碳酸盐岩台地的重要组分。它们常与礁体共生，形成重要的油气储层；在新元古代晚期-早奥陶世尤其丰富，中元古界尚无报道。有关凝块石的成因机制的认识存在分歧，生物矿化作用与微生物化学过程还不清楚。申请人在华北中元古代雾迷山组发现了丰富的凝块石。立项试图通过对内部微-超微组构、有机矿物组成和主导微生物类群的研究，查明凝块石的成因、生态分布和生物矿化过程，从而更好地深刻理解早期微生物-环境相互作用及其对海洋环境和真核生物演化的影响机理。立项拟选择2-4条重点剖面，以早期成岩硅化标本为主要研究对象，运用电镜超微观察、ICP-MS和EDS成分分析以及同位素质谱测试Fe, C, S同位素组成。揭示微生物化学作用在凝块石矿化中的作用和中元古代特定海洋与大气环境条件下的微生物与环境协同进化关系。从而深刻理解地球表层系统演变以及微生物群落在环境改造和矿产形成中的重要作用。

凝块石是一类重要的微生物岩,多见于海相碳酸盐沉积地层.能够为深刻理解地球早期海洋环境演变及生物与环境相互作用过程提供重要信息,而且也是一种潜在的烃源岩. 过去对前寒武纪凝块石的成因、生态分布及古环境条件缺乏了解。. 研究表明，华北中元古代白云岩富含凝块石. 尽管其外形多变，但均由不规则凝块、微晶基质和亮晶充填三种主要组分构成。凝块由富有机质微晶核和贫有机质纤维状文石组成的微亮晶外环构成. 微晶核含丰富的石化球-丝状细菌残余和粘液-膜状胞外聚合物(EPS)及密切共生的纳米球和亚微米多形晶等有机矿物。凝块外环含大量微孔，代表细菌外模和丝状菌化石. 凝块基质主要由20-30 um的微球粒和微量陆源碎屑组成. 凝块具锯齿状边缘，基质环绕凝块的微纹发育生长. 这些特征表明凝块为原生的微生物-沉积构造，其核心源于细菌体和EPS的原位矿化, 且矿化时间早于基质. 凝块矿化经历了2个阶段: 1)微晶核受增强的BSR作用促进通过细菌和EPS厌氧降解产生的纳米微粒置换有机质而矿化；2) 其外环由微生物-环境相互作用导致的高碱度条件诱发无机沉淀而形成. 厌氧条件是凝块形成和保存的重要条件,雾迷山组凝块石是由微生物-环境复杂的相互作用形成的生物-沉积构造，代表地史上最早的微生物成因凝块石。. 对华北中元古代凝块石相时空分布、沉积环境及共生的特殊矿物及沉积地化研究表明，雾迷山组凝块石主要分布于相对低能的浅海环境。从潮下带下部至潮下带上部，随着可容纳空间的减小和水动力条件增强，其外形组合显示出从丘状到层状为主的显著变化。不同形态的凝块趋于集中在特定类型的凝块石中，且其纤维文石外环的比例也随水深变浅而减少，表明环境条件不仅对其成形过程，而且对内部组构也有明显的控制作用。基质中发育的文石扇内保存有良好的丝状菌群和凝块外环中发育的多层纤维状微结构，都要求有过饱和的海水条件和快速的多相自生碳酸盐沉淀; 它们对环绕凝块的文石纤维层外环形成至关重要。而亚氧化-缺氧、高碱度海水及活跃的BSR作用和相对低能条件则是促进中元古代浅海环境凝块石发育和保存的最有利因素。因此，华北中元古代凝块石的广泛发育指示了大气低氧、海洋缺氧、海水硫酸盐浓度较低并具间歇性硫化环境条件。