Changes Of Dissoved Methane At Near Sea Bottom And Its Implication To Active Seep Area At Southeast Baiyun Sag

Submarine cold seep with methane leaking is a hotspot of current global changeresearches. As an indicate of gas hydrate and important way to release methane, itbrings complex biogeochemical processes at water-sediment interface, and hassignificant impact to the global carbon cycle and climate change. It is urgent to assessand quantify methane flux at the South China Sea, one of the world’s mostconsiderable methane reservoirs. Carrying on deep research in this area is valuable forunderstanding the response of gas hydrate to global warming.Based on data collected by "In situ environmental monitoring system (Benvir)", atBaiyun sag, Shenhu area, middle of the northern South China Sea, in September,2012,series of study has been conducted, and the results is showed as follow.1. Refererring dissolved methane concentration of near bottom water at typical coldseep areas, sea bottoms has been divided into4kinds of methane leakageenvironments: background area, low methane leakage area, middle methane leakagearea and high methane leakage area.2. Changes of dissolved methane in near the bottom of Baiyun Sag have beendeeply studied. It shows that the area belongs to the middle methane leakage area.3. The result reveals the way that temperature, pressure and salinity affect dissolvedmethane concentration:(1) periodic pressure variation (tidal) is one of the factors thatcontrols seep activity, but the impact is small, because of the depth.(2) Temperatureaffects changes of dissolved methane concentration by controlling the activity ofmethanogens and anaerobic methane-oxidizing bacteria, and equilibrium of methaneoxidation. Compared with pressure, temperature has little effect on the stability ofhydrates in sediments, but controls the dissolved methane concentration more directly.(3) The environment influences ion concentration in seawater. It also causes changes of salinity, which is the reason that changes of salinity and methane are similar. Andsalinity would influence dissolved methane concentration by affecting gas hydratestability. However, none of temperature, salinity and pressure is main factor thataffects changes of dissolved methane concentration in near bottom water.4. The way that dissolved methane concentration responses to biogeochemicalenvironment has been discussed. The results show that changes of carbon dioxidedissolved oxygen concentration and pH value might be caused by ocean currents orwater masses. And it might be the major factor of controlling dissolved methaneconcentration in near-bottom water. Because exogenous carbon dioxide and oxygenmight affect methane oxidation equilibrium. Otherwise, changes of pH will influenceactivity of methanogen bacteria, thereby controlling dissolved methane concentration.These studies are helpful to understand responding of environmental andbiogeochemical processes to methane leakage flux at seabed, and changes ofsubmarine methane leakage flux and its impacts and constraints.