Numerical study on biogeochemical processes induced by methane leakage and its application in estimating the time of cold seeps formation

摘要

In shallow marine sediments, cold seeps of methane fluid is a widely observed phenomenon, which will lead to a series of biogeochemical reactions. These processes will change the pore water environment and trigger the formation of authigenic minerals. In this study, based on the data from the site GC10 in the NE South China Sea, numerical simulation studies were conducted to investigate the relative biogeochemical processes induced by methane leakage. The results of simulations show that anaerobic methane oxidation is the dominant sulfate consuming process in methane leakage environment, and the typical sulfate-methane interface is indicative of methane leakage. The pore water chemistry conditions are closely related to methane flux. Methane flux was calibrated from GC10 site data, when methane leaks upwards at flux of 2.42 x 10(-3) mmol cm(-2) a(-1), the simulation results fit well with measured data. Some kinetic parameters of related reaction were calibrated during the fitting process. The microbially-mediated reactions in methane leakage environment trigger the precipitation of authigenic minerals, such as calcite and pyrite, which can sometimes be used as indicators of the occurrence of hydrate reservoirs. Through simulating the change of sulfate concentrations along the vertical column, the model was also used to estimate duration of active cold seeps at the study site. The results showed that the methane leakage in GC10 has been going on at least 5600 years. This study provided a new method to the study of cold seeps activities, and was significant for future investigations on the biogeochemical processes above the hydrate reservoirs.