The Formation Of Authigenic Minerals During Methane Seeping In Seafloor: Insight From Laboratory Test

The formation of Authigenic carbonate and sulfide minerals in the methane seeping zone, which stores the sulfur of seawater and methane in the marine sediment, plays an important role in global carbon and sulfur balance. These minerals can also indicate the occurrence of methane hydrates in the marine sediment. It is therefore of critical importance to understand how the carbonate and sulfide are created, especially with the influences of microorganism.In this study, we use the sewage sludge contaminated by the oil leakage sampled from Jilin Province to cultivate the microorganism, typically sulfate-reducing bacteria and methanotroph, which is an analog to the microorganism in the methane seeping zone in seafloor. The microorganisms are then injected into the kettle, where the temperature and pressure is changeable, to investigate the influence of temperature, pressure, the types of microorganism on the water chemistry and the formation of authigenic minerals. As a consequence, the formation conditions of authigenic minerals are determined, and the results offer a relevance to further understand the formation mechanism of authigenic carbonate and sulfide.Based on this study, the following major conclusions are drawn:(1) Microorganism have been sent to Shanghai Sangon Biological Engineering CO., LTD, as a result, Desulfovibrio, Pseudomonas, Syntrophomonas, Porphyromonas and Proteiniclasticum are found in these microorganism by 16 s DNA Genomic Sequencing, among them, Desulfovibrio is one of the Sulfate-Reducing Bacteria(SRB), the metabolism of which produces H2 S and HCO3-. Microorganism appears to be red after gram stain, and thus belongs to Gram negative bacteria. The shape of them changes from initial aureus to bacillus at the end of the experiment. Three phases for microbial growth are found in the temperatures of 10oC and 35oC, respectively. In 10oC, the microbial growth follows the lag phase, exponential phase and stable phase, and in 35oC, it follows the lag phase, exponential phase and senescent phase.(2) The experiments under temperatures of 5oC,8oC and 10oC and constant pressure of 5.0 MPa suggests that 1 Microbial concentration varies nonlinearly in different temperatures, which increases initially and then decreases. The peak growth rate is observed in 10oC. 2 With the increase of temperature, S2- concentration increases at an increasing rate to the maximum concentration, the total iron concentration varies greatly as well, Oxidation- Reduction Potential(ORP) decreases to the smallest value quickly, but p H does not changes significantly. 3 The authigenic minerals mainly includes calcium carbonate, iron carbonate and magnesium carbonate, Fe S and Fe S2. The mineral components are sensitive to the temperature. The calcium carbonate takes a dominant part in the minerals in the temperature of 5oC, while 10oC tends to create the siderite which has a complete mineral morphology. Moreover, the iron sulfide dominates in the minerals when the temperature is set at 8oC.(3) The experiments under different pressures, 2.5 MPa,5.0 MPa and 7.5 MPa, reveal that the concentration of S2- increases with the pressure, and more HCO3- generated by metabolism of microorganisms. On the contrary, more carbonate is generated under low pressure.(4) The activity of microorganism significantly affects the formation of authigenic minerals, as both HCO3- and S2- generated by microbial metabolism are the necessary ion for the formation of authigenic carbonate. Authigenic carbonate are favorably formed under the temperature of 10oC, pressure of 7.5 MPa, the change of HCO3- concentration of 347.2 to 946.2 mg/L, and p H between 6.00 and 7.60.Overall, the laboratory experience provide effective tool to understand the influence of microbial components, temperature and pressure on the formation of authigenic minerals. The results can help explore the submarine natural gas hydrate. The main part of the thesis has been adopted in the total project report which has been approved by the experts in the project examination and assessment.