In Situ Observation Of Methane Seepage In The South China Sea

The relation of gas hydrates and global warming is one of the most pressing issues inthe current global change research. The underwater cold seeps accompany withmethane seepage, which is as the sign of gas hydrate storage and an important way ofdecompose of submarine hydrates, not only causes a complex marine biogeochemicalcycle and ecological process, but also plays an important role in the global carboncycle and climate change. The south China sea is an important prospect area ofhydrate resources, but the research about methane seepage associated with cold seepactivity is still in its infancy, which is limited by deep-sea exploration technology fora long time, particularly, the research of cold seeps based on the long term in situobservation technique and the time-series variation of methane seepage is still notcomprehensive.An new in situ environmental monitoring system for deep-sea benthic boundarylayer（BBL） has been developed, base on it, in situ observation of cold seeps of thesouth China sea was carried out in the BBL, the long time series change data, such asthe CH₄, CO₂, turbulence, temperature, salinity, pressure, turbidity are obtained, andthe related research is conducted. Research shows that:（1） There are high methane concentrations in the study area, the methaneconcentrations range between3.8μmol/L and4.2μmol/L in the BBL, which is300¹000times of the seawater above and the background values, this is the directevidence of submarine cold seeps.（2） In the diurnal cycle, seafloor pressure changes could lead to changes in seafloormethane seeps strength directly, may also control the currents, and thereby controlmethane concentration in bottom water by affecting the bottom water transport. Thetemperature has little effect on the strength of methane seeps under the seabed.（3） In the study area, the seepage methane migrates in horizontally translational currents transport mainly, and the vertical conveyor pan only about12%.（4） The seepage methane also effect by turbulent diffusion and chemical processes inthe translational transport, culminating in the role of turbulent diffusion orbiochemical, the methane concentration characteristics become blurred.