Study On Carbon-Sulfur Cycle Process And Numerical Simulation For The Leakage Of Methane Cold Seep In Marine Sediments

In the marine sedimentary system,the tectonics movement caused by the earth’s internal force and the exploration and development activities under the action of human external force are likely to lead to the leakage of deep oil and gas resources and the decomposition of natural gas hydrates,which will release the methane containing fluid migrating along the dominant channel to form a cold seep.The cold seep leakage process carries methane and some reducing sulfides formed under the action of biogeochemical reaction into the sea water,which is an important link of carbon-sulfur cycle between the sea water and the marine sediment.At present,a large number of records of seafloor cold seeps have been found,and cold seep fluids typically contain multi-gases such as methane and hydrogen sulfide,however,under the background of modern active cold seep leakage,especially the seepage process and microbial geochemical mechanism involving multi-gases bearing fluids and the process of carbon-sulfur cycle in seafloor sediments are relatively insufficient.At the same time,the problem of carbon-sulfur cycle carried by gases still needs to be deepened.Therefore,it is necessary to deeply study the mechanism and mode of carbon-sulfur cycle in the active cold seep area and quantitatively calculate the carbon-sulfur cycle flux,which has important theoretical significance for marine ecology and climate change impact assessment.The"Site F"active cold seep area in the southwest of the continental slope in the northern South China Sea was selected to study the mechanism of carbon-sulfur cycle:through analyzing the chemical characteristics of seawater and sediment samples,the mechanism of seawater-sediment action and the"source"of carbon-sulfur cycle was determined;based on the physical characteristics of sediment samples,the mechanism of gas liquid seepage of cold seep fluid was clarified,and the"path"of carbon-sulfur cycle was determined;the gas-water-sediment-microorganism interaction experiment was carried out to reveal the biogeochemical mechanism and determine the"sink"of the carbon-sulfur cycle.Based on the mechanism of carbon-sulfur cycle,a“gas-liquid-solid three phases”carbon-sulfur cycle mode was built;the multi-component,multi-phase carbon-sulfur cycle mathematical and numerical models were established;a quantitative simulation program TOUGH+HR＿Multigas for carbon-sulfur cycle of seabed sediments was developed,which is applied to quantitative study the numerical simulation of carbon-sulfur cycle in the study area.The main results are as follows:（1）The“carbon source”and“sulfur source”were determined;the characteristics of fluid flow were clarified;the microbial mechanisms were revealed.The field sampling and testing indicate that the“carbon source”comes from methane cold seep leakage,and the“sulfur source”comes from seawater sulfate infiltration;the multi-component,multi-phase flow of cold seep meets the generalized Darcy’s law,and the formation of authigenic minerals affects the seepage parameters.Gas-water-sediment-microorganism experiment shows that anaerobic microorganisms drive carbon-sulfur cycle in gas,liquid and solid phases,revealing the microbial geochemical mechanism of gas-liquid-solid system.（2）According to the“source-path-sink”mechanism of carbon-sulfur cycle,“gas-liquid-solid three phases”carbon-sulfur cycle mode was constructed.Under this mode,carbon and sulfur not only cycle in the liquid phase and solid phase,but also release carbon dioxide and hydrogen sulfide gas into the gas phase.The theory of carbon-sulfur cycle is supplemented and perfected,laying the theoretical foundation for quantitative study.（3）The quantitative simulation code for carbon-sulfur cycle was developed and verified,and microbial reaction kinetics model was improved.Based on the physical and chemical process of the carbon-sulfur cycle,the multi-component,multi-phase flow and biogeochemistry mathematical and numerical models were constructed.Based on the TOUGH+HR program,a quantitative simulation program TOUGH+HR＿Multigas for the carbon-sulfur cycle of the cold seep sediment was developed.Based on the experiments,the reliability of TOUGH+HR＿Multigas was verified for quantitative calculation of carbon-sulfur cycle of cold seep.The kinetic parameters of microbial reaction in the study area were obtained through the reactor experiment:the maximum reaction rate constant q_m=2.8×10^-5mol/g·cells/s,microbial attenuation constant b=5.0×10^-71/s,microbial yield coefficient y=0.085g·cells/mol CH₄.（4）The conceptual model of carbon-sulfur cycle of sedimentary column in the"Site F"cold seep area was established,and the numerical simulation results indicate that the activity of cold seeps can be divided into a“leakage period”from 200 years ago to the present（only leak in liquid phase）and a“eruption period”in recent years（leak in gas and liquid phases）;the methane leakage flux during the“leakage period”is 0.11～0.14 mmol/day/m²;when the flux during the“eruption period”exceeds 0.54mmol/day/m²and reaches 18.9 mmol/day/m²,the gas erupts out of the seabed;the carbon in the“leakage period”mainly cycles into solid minerals,accounting for56-64%of the“carbon source”,while the carbon in the“eruption period”mainly enters seawater,accounting for 66-90%of the“carbon source”;the sulfur in the“leakage period”mainly cycles in the liquid phase,accounting for over 80%of the“sulfur source”;the sulfur in the“eruption period”mainly enters seawater,accounting for about 50%of the“sulfur source”.In recent years,the cold seep activity at the study site has changed from the“leakage period”of low methane flux to the“eruption period”of high flux,which has greatly increased the risk of greenhouse carbon and reducing sulfur with certain biological toxicity discharged into seawater.Although its impact on climate change and marine ecology cannot be accurately evaluated,this study further obtains the quantitative calculation method of carbon-sulfur cycle in the cold seep sediment,providing theoretical support for relevant research.