Deep generated methane in the global methane budget

Abstract: Methane is a significant part of the global carbon cycle. The distribution of methane above and below the Earth’s surface suggests that atmospheric methane might be related to methane originating from the deep mantle.The purpose of the present study is to identify this relationship between methane emissions to the atmosphere and methane, which can be abiogenically generated within the Earth’s interior. Methane hydrates within the Earth’s surface sediments might be among the possible hosts of migrated deep methane.In this thesis, experimental work is presented, which aimed to reveal the depth at which methane and other hydrocarbons in the upper mantle are abiogenically generated, considering pT and redox conditions of the surrounding environment. High-pressure, high-temperature experiments were conducted using a large reactive volume device with a toroid-type chamber in specially prepared sample containers.The present study evaluates the formation of methane and other hydrocarbons at temperatures higher than 300 °C at pressures of 2.5-6.5 GPa despite the redox conditions of the surroundings. These conditions correspond to a depth below 70 km on the surface of a cold subducting slab.The proposed hypothesis claims that the deep-mantle-generated methane can contribute to the formation of methane hydrates and accumulation of free gas below hydrates. 

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