| The study that cave deposit isotope used for restoring and reconstructing the related palaeoenvironment, and discussing the past, present and future trend about the earth’s climate change is a hot in the Karst cave research. But information of the cave deposit isotope is so influenced by many factors that it is often abnormal, which is difficult to explain.Microbes that play an important role in calcium carbonate deposition, but whose details are not well known, were studied. In this paper, samples of microorganisms were collected from the cave sediment surface of four Karst caves in the Midwestern of Guizhou province and traditional biological methods were combined with molecular biological technology. Samples of microorganisms were separated by a B-4culture medium, which is a common oligotrophic medium for carbonate depositing bacteria separation. The mineral composition and morphology of the microbially induced CaCO3crystals were analyzed using an X-ray diffraction analyzer (XRD) and a scanning electron microscope (SEM), respectively. It showed that the microorganisms induced CaCO3deposits are widespread and the microbially induced CaCO3precipitation is a common phenomenon.On that basis, microbially induced calcium carbonate precipitation how influence environment for the microorganisms survival of acid, alkaline, ion concentration, precipitation amount were further studied. In order to expound the effect of microbe on CaCO3precipitation in the Karst caves combining with the monitoring data and geological background, samples of microorganisms collected from the surface of cave sediment in Shijiangjun cave which has been monitored for years once again. Samples of microorganisms collected from the cave sediment surface were separated by a B-4culture medium, which is a common oligotrophic medium for carbonate depositing bacteria separation. B-4C medium (added Mg2+) was used to analyze the effect of Mg in the system. Additionally, simulation the dynamic process of cave dripping water was to study microbially induced CaCO3precipitation which will how influence the primitive cave deposits. The feature and carbon isotope information of the primitive cave deposits’crystals were investigated for that need. With the research the details conclusion can be as follows:1. Samples of microorganisms were collected from the cave sediment surface of four Karst caves in the Midwestern of Guizhou province for twice, which were to separate the microorganisms induced CaCO3deposits. We chose samples of microorganisms from the cave sediment surface of four Karst caves to screen microbially that could induce CaCO3precipitation at first. A total of31strains screened out single bacteria in which12strains’colonies surface produced a lot of white particles or solid crystals. In four caves, the microorganisms which could induce CaCO3precipitation were found. Samples of microorganisms once again collected from the cave sediment surface of the Shijiangjun cave which has been monitored for years for dripping water hydrogeochemical parameters. It screened out18strains single bacteria induced CaCO3precipitation in which the strains named SJJ-8-7was separated from almost every sample sites of microorganisms. The SJJ-8-7strains was identified as Nocardia which always lived in soil. It indicated that the soil microbe can be taken by fissure water and pipe water into the cave, and widely exist in the cave, and influence the process of deposition in cave.2. Calcium carbonate crystal morphologies, elements, and crystal shapes of microbial deposition were analyzed after strain screening and simulation experiment. The XRD results showed that three carbonate compositions were found in the B-4medium liquid system (without Mg2+): calcite, calcite-vaterite mixture, and vaterite. Moreover, the B-4C medium mixing Ca2+and Mg2+only generated calcite. In addition, the aragonite and calcite crystals had been detected in the primitive cave deposits. But the aragonite crystals and any other types had not been detected in this study. The elements analysis showed that microbial depositions were mainly composed of C,O and Ca, without Mg. This suggested that the presence of Mg2+may be more conducive to the stability of microbial sedimentary environment and form the stable crystal calcite of calcium carbonate. Microbial did not deposit aragonite crystals with low Mg/Ca (such as1:2) on this study. Thus the mechanism of microbial deposited aragonite needs further research.3. The simulation experiment showed that microbes promoted the happening of the deposition by changing its own survival environment such as Ca2+concentration. pH and so on. With the growth of bacteria, the pH value of culture medium increased gradually and tended to be stable. The greater increase of the pH value, the relatively more deposition quantity is. At the same time, the Ca2+concentration of culture system decreased significantly. This kind of high range change of pH in experiment system were related that microbial metabolic activity significantly improve environment pH to Ca2+saturation in order to eliminate pressure of bacterial physiological toxicity.4. The microbial strains of depositing calcium carbonate in the cave may disturbe the stability of the experiment system differently, which reflected by various physical and chemical properties and eventually affected the formation of crystal structure. There were various morphology of carbonate crystals, including the plume, shoe shape layer, rods, cylindrical objects, irregular hexagonal polytypes, square layers, oblique six-party body, hemispherical types and geometry combined hemisphere forms in this study, which are rarely reported in the chemical system. Moreover, bacteria-shaped holes were observed on the crystal surface, implying the involvement of microbes in the deposition process. The special morphological characteristics of microbial deposition crystals can become distinguishable between abiotic carbonate minerals and microbially deposited calcium carbonate in geological research.5. Carbon isotope analysis showed that the δ13C of microbially deposited calcium carbonate might lighter than the δ13C of deposition in cave. The δ13CpDB of the simulation system microbial deposits were lighter than the cave Secondary sediment, whose mean difference was16.814%o. Furthermore, the δ13CPDB of microbially deposited calcium carbonate in still equipment was more enriched than it was in simulation experiment. It may be closely related that microbial species itself differed with the disturbance of experiment system. That is to say, the more unstable the experiment system were, the lighter the Carbon isotope of microbially deposited calcium carbonate would be, and vice versa.The intercoupling between microbially deposited calcium carbonate and other influencing factors of caves deposition process makes the environmental significance of cave microbially deposited calcium carbonate more complicated. Therefore, the study of cave microbially deposited calcium carbonate should not only pay attention to the comparison and analysis of all kinds of physical and chemical conditions, but also more to consider the comparison microbially deposition and primary precipitation process. Thereby it would gain the specific impact of microbially deposition to the Karst cave primary precipitation. |
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