南海北部深海浅层沉积物中甲烷生物地球化学过程数值模拟研究

SIMULATION OF THE BIOGEOCHEMICAL PROCESSES IN METHANE-BEARING SURFACE SEDIMENTS OF HAIYANG 4 AREA,NORTHERN SLOPE OF SOUTH CHINA SEA

富甲烷浅层海相沉积物中的生物地球化学过程已引起了国内外学者的广泛关注。研究采用数值模拟的方法对"海洋四号区"浅层沉积物中甲烷生物地球化学过程进行定量研究。依据研究区域实际地质资料,使用Mathematica建立起一维反应运移稳态模型。模拟结果认为研究区深层沉积物内赋存有甲烷源,释放的甲烷气以气泡的形式运移至沉积物表层,并造成气泡淋滤现象。气泡淋滤使孔隙水中SO_4^(2-)等溶质浓度在海底以下0~2.8m的范围内保持不变。甲烷气泡在浓度梯度作用下向孔隙水中溶解,溶解通量为160mmol·m^(-2)·a^(-1),溶解甲烷在微生物作用下被SO_4^(2-)氧化,氧化速率为140mmol·m^(-2)·a^(-1)。甲烷通量与氧化速率均远小于水合物脊等甲烷渗漏活跃地区,SMTZ埋藏也相对较深,故推测甲烷源埋藏较深或规模较小,也有可能是良好的圈闭条件阻止了甲烷逸出。作为AOM过程的重要自生矿物,本地区碳酸盐和硫化物矿物沉淀速率都比较低(分别为35mmol·m^(-2)·a^(-1)和70mmol·m^(-2)·a^(-1)),且碳酸盐的沉淀受到了硫化物矿物的影响。

The biogeochemical processes in the methane-bearing surface sediments have been an interesting research field worldwide.Numerical simulation method is used in this study to quantify the biogeochemical processes in methane-bearing surface sediments of the Haiyang 4Area,northern slope of the South China Sea.According to the actual geological data in the study area,a numerical transport-reaction model has been developed with Mathematica.According to the simulation results,it is inferred that there are methane sources in the study area.Methane could be rapidly transported to the surface sediments as gas bubbles from the methane reservoir in deep sediments.Gas irrigation could drive the exchange of solutes which led to the similarity of sulfate concentrations in the upper 0~2.8 mto those in the bottom water.The small inflow of methane from below(160 mmol·m^(-2)·a^(-1))induces the low reactive rate of anaerobic methane oxidation(AOM)of 140mmol·m^(-2)·a^(-1) and a low carbonate precipitation rate of 35mmol·m^(-2)·a^(-1).This may be caused by a small or deep methane reservoir in the sediments.The well trap conditions can also prevent the escaping of methane.The precipitation rates of carbonate and sulfide turned out to be small in this area.The simulation also revealed that when the rate of sulfide mineral precipitation is high,the rate of carbonate precipitation will be diminished due to the alkalinity reduction.