Research On The Characteristics And Genesis Mechanism Of Thrombolite In Zhangxia Formation Of The Cambrian In Yishui,Shandong Province

Carbonate rocks are widely distributed in the nature and are one of the main reservoir rocks for oil and gas.Microbes play an important role in the formation of carbonate rocks and microbial mat can capture or bind carbonate minerals and other mineral particles in water.Especially,microbialites have also become a research hot spot after the microbial fossils were found in the carbonate rocks.Previous studies have found that the minerals in microbialites have a unique morphology and crystal structure compared with general limestone due to the influence of microbial metabolic activities.There are still some confusing problems.It has been found that the biotic calcite minerals have special shapes such as spherical or dumbbell shapes in the laboratory studies,but current experimental results show that these special morphologies are also found under conditions rich in organic matter.Therefore,the judgment of microbialites should be carried out with extreme caution,and it cannot be judged only by the morphologies of minerals.In addition,for the formation mechanism of microbialites,the formation mechanism of biomineralization is also of great significance besides of the microbial trapping or adhesion,and autogenic calcification of microorganisms.Extracellular polymers(EPS)of cells not only provide a place for the nucleation of carbonate minerals,but also change the environments of mineral precipitation through bacterial unique metabolic activities,and promote the formation of carbonate mineral.Therefore,in order to further explore the typical characteristics of microbialites,microbially induced calcite precipitation indoors is used to further enrich the formation mechanism of microbialite.In this paper,the thrombolite of microbialites in Zhangxia formation of Cambrian in Yishui area,Shandong Province,is taken as the research object,and the micrite of non-microbialites is selected as the contrast.X-ray diffraction(XRD)was used to determine the mineral composition of thrombolite and micrite,and differential thermal analysis was used to explore the thermal stability of the cloted part and cemented part in the same thrombolite.By analyzing the difference of stable carbon isotopes between the cloted part and the cemented part,the formation environment of thrombolite can be understood,and the above research could be used to analyze the characteristics of microbialites.Aiming at the formation mechanism of calcite in the microbialites,Sulfate Reducing Bacteria-Bacillus cereus MRR2 was used to induce calcite precipitation in the laboratory,while organogenic and inorganogenic calcite were investigated in parallel.Based on the study of simulated microbial biomineralization,it was found that in addition to the high content of glutamic acid and aspartic acid in EPS with high mineralization capacity,the content of glycine was also high.Finally,glycine was selected as an organic template to explore the biomimetic mechanism of calcium carbonate at different glycine concentrations and pH values.The conclusion of this paper are as follows:The results of X-ray diffraction(XRD)show that the mineral components of the cloted part in the thrombolite and micrite are calcites,but their crystallinity is different.The crystallinity of the cloted part in the thrombolite is higher than that of the micrite.Thermal stability analysis results show that the activation energy and the thermal stability of the cloted part in the thrombolite are relatively higher.In addition,the crystallinity and thermal stability of the cloted part with more microorganisms in the same thrombolite is also higher than that of the cemented part with less microorganisms.The stable carbon isotope results show that,compared with the chemical precipitated sparry calcite in the cemented part,the cyanobacteria in the cloted part absorb more 12C through photosynthesis,which results in more 13C in the cloted part,and carbon isotope ?13C value is higher.Differences in the crystallographic and thermodynamic characteristics can help to identify microbialites in the nature.A facultative anaerobic microorganism,Bacillus cereus MRR2,was selected to induce calcite precipitation.The results showed that microorganism could promote calcite precipitation by producing ammonia to provide alkaline environment.Glutamic acid was the most abundant amino acid in the extracellular polymeric substances of the bacteria,and its free carboxyl groups with a large number of negative charges in the alkaline environment facilitated the adsorption of Ca2+.In addition,the biotic calcite showed a unique shape of mineral aggregate,such as elongated calcite,abundant organic functional groups,higher thermal stability and crystallinity compared with organogenic and inorganogenic calcite.Bacillus cereus MRR2 consumes organic substances(beef extract and tryptone)to produce carbon dioxide through metabolic activities.The hydration of carbon dioxide results in the combination of carbonate ion and calcium ion to form calcite.The stable carbon isotope results showed that the carbon source of calcite induced by Bacillus cereus MRR2 came from the organic substances(beef extract and tryptone)in the culture medium,resulting in the carbon isotope ?13C value was more negative.The typical characteristics of biotic minerals were different from those of abiogenic minerals,which provided some reference for distinguishing the biogenic minerals and microbialites in the nature.The experimental results of biomimetic mineralization of calcium carbonate minerals with glycine as template showed that the chelation time,glycine concentration and pH value would affect the phase.The phase of calcium carbonate minerals were calcites under the condition of shorter chelating time,lower glycine concentration and pH value.With increasing chelation time,glycine concentration and pH value,glycine molecules could be adsorbed on the surface of minerals or enter into the interior of minerals,which made glycine interact with the formed calcite to dissolve calcite,and spherical-or dumbbell-shaped vaterite appeared,but this unstable vaterite gradually disappeared with the extension of reaction time.In addition,calcite dissolved in solution under the action of glycine with the extension of reaction time,and corrosion pores appeared on the surface of calcite.The dissolution of calcite by glycine resulted in the decrease of crystallinity and thermal stability of calcite.The biomimetic mineralization of calcium carbonate minerals with glycine as a template can provide a theoretical basis for exploring the formation mechanism of secondary dissolution pores of microbial rocks in the process of diagenesis.Through the above research,it is expected to provide a theoretical reference for the identification of microbialites and ordinary limestone in the nature,and at the same time,it will help to deeply understand the biomineralization mechanism induced by microorganisms and the biomimetic mechanism with some organic molecules as the template.