塔里木盆地下古生界微生物白云岩成因及同期古海水对白云石形成的影响和制约

The knowledge on the mechanism of microbially mediated dolomite is of great importance towards a better understanding of the origin of dolomite. However, it is hard to find an analogue from the geological record because of strong recrystallization and diagenesis transformation. This study provides an analogue for microbial mediated dolomite precipitation in ancient microbial mats and biofilms from Lower Paleozoic in the Tarim Basin. On the basis of sedimentary microfacies, petrography, cathodoluminescence, preservation of organic matter, trace and rare earth elements, the samples with well original crystallization are only selected. Sequently, compared with the results of microbial culture experiments, both nanoglobules texture as a biosignature and crystals series from the initial nucleation to the euhedral dolomite are observed by means of high-resolution scanning electron microscopy and atomic force-laser Raman system, while compositions of minerals are identified in situ in nanometer scale. Therefore, these lines of evidence will confirm the origin of microbial dolomites from the Lower Paleozoic in the Tarim Basin. In addition, boron isotopes from microbial dolomites are used to calculate palaeo-pH. Oxygen and strontium isotopes reveal palaeo-temperature and palaeo-alkalinity of the ancient seawater. Sequently, program PHREEQC will be used to calculate the supersaturations with respect to dolomite and to analyze the main controlling factor in different early Paleozoic seawater properties. This research will test the mechanism of microbially mediated dolomites from the geological record. To understand constraints on dolomite precipitation by palaeoeseawater properties will sparkle to decipher the "Dolomite Problem ".

微生物介导原生白云石的机理是当前白云石成因研究的关键问题，但因重结晶、成岩作用等改造，很少能在古老地层中找到实例。本项目拟以塔里木盆地下古生界微生物白云岩为地质实例，综合沉积微相，岩相学，阴极发光，有机质特征，微量和稀土元素等结果，选取保存原始结晶信息的样品。将今论古的对比微生物培养实验的结果，在高分辨率扫描电镜和原子力—激光拉曼系统下，识别与微生物介导的白云石直接相关的纳米球状结构，及从最初成核到完整自形晶的矿物形态演化序列，并分析其微区的矿物成分，研究下古生界微生物白云岩的成因机制。同时利用微生物白云岩硼同位素计算海水古pH值，氧、锶同位素推测古温度和古碱度，应用PHREEQC软件模拟不同属性早古生代海水对白云石饱和度的影响和主要控制因素。研究结果有望校验古老地层中微生物成因白云石的机理，同时定量模拟同期海水对白云石沉淀的影响和制约，或可成为解决“白云岩问题”的另一种新的思路和方法。

微生物介导原生白云石的机理是当前白云石成因研究的关键问题，但因重结晶、成岩作用等改造，很少能在古老地层中找到实例。.本项目选取塔里木盆地下古生界保存有原始海水信息的微生物白云岩，通过详细的沉积微相的划分，将今论古的对比微生物培养实验的结果和波斯湾地区的阿布扎比地区现代的萨布哈环境中微生物成因的白云岩模式，以微生物地质学为基础，在中上寒武统的叠层石白云岩的暗层中发现了直径为50-100 nm 的球状小颗粒集合组成的球形白云石；和直径是50-170 nm, 相互集中呈哑铃状或链状排列的球形白云石；以及硅化后的丝状体及成哑铃状或链状相连的显微结构。这些纳米级的显微结构可能分别是白云石成核的最初阶段在微生物的作用下形成的纳米球粒；被胞外聚合物(EPS)或黏液等包裹而矿化的丝状物, 及因硅化而保存良好的球形细菌(可能是蓝细菌)。胞外聚合物能够结合如硅、镁、钙等不同的元素, 这些组分适宜球形细菌的矿化和有机底物上白云石的结晶。塔里木盆地的下古生界古萨布哈中有微生物介导形成的原生白云石，与现代的微生物培养实验和萨布哈环境中微生物促进沉淀的白云石的机理类似，同样是EPS对白云石的沉淀起到至关重要的作用。.塔里木盆地微生物白云岩的硼同位素计算寒武纪海水的古pH值， 结果给出“最佳的”古pH值约为8.1~8.6，比现代海洋的pH略偏碱性。.运用 PHREEQC模拟了寒武纪古海水不同的属性参数，结果显示不同pH值及钙元素的浓度条件会影响古海水相对于各种碳酸盐矿物的饱和指数。其中pH值对不同元素组成的碳酸盐矿物之间饱和指数的变化（如方解石和白云石间）影响更显著，而钙浓度则更多的影响同种元素组成的碳酸盐类质同相矿物（如方解石和文石）。.此研究结果有望校验古老地层中微生物成因白云石的机理，同时定量模拟同期海水对白云石沉淀的影响和制约，或可成为解决“白云岩问题”的另一种新的思路和方法。