Quantification of Methane Fluxes from Hydrocarbon Seeps to the Ocean and Atmosphere: Development of an in situ and Online Gas Flux Measuring System

摘要

Natural hydrocarbon seeps in the marine environment are important contributors to greenhouse gases in the atmosphere.Such gases include methane,which plays a significant role in global carbon cycling and climate change.To accurately quantify the methane flux from hydrocarbon seeps on the seafloor,a specialized in situ and online gas flux measuring (GFM) device was designed to obtain high-resolution time course gas fluxes using the process of equal volume exchange.The device consists of a 1.0-m diameter,0.9-m tall,inverted conical tent and a GFM instrument that contains a solenoid valve,level transducer,and gas collection chamber.Rising gas bubbles from seeps were measured by laboratory-calibrated GFM instruments attached to the top of the tent.According to the experimental data,the optimal anti-shake time interval was 5 s.The measurement range of the device was 0-15 L min-1,and the relative error was ±1.0％.The device was initially deployed at an active seep site in the Lingtou Promontory seep field in South China Sea.The amount of gas released from a single gas vent was 30.5 m3 during the measurement period,and the gas flow rate ranged from 22 to 72 Lh-1,depending on tidal period,and was strongly negatively correlated with water depth.The measurement results strongly suggest that oceanic tides and swells had a significant forcing effect on gas flux.Low flow rates were associated with high tides and vice versa.The changes in gas volume escaping from the seafloor seeps could be attributed to the hydrostatic pressure induced by water depth.Our findings suggest that in the marine environment,especially in the shallow shelf area,sea level variation may play an important role in controlling methane release into the ocean.Such releases probably also affect atmospheric methane levels.