The abnormally light carbon isotope compositions have been recognized in kerogenous and graphitic samples from the 2.7 to 2.1 Ga sedimentary rocks. Such light carbon isotope compositions suggest that methanotrophic micro-organism flourished in the contemporary oceans, also postulating methane accumulation in the stratified oceans. Periodic occurrence of the methanotrophic micro-organism is coincident with the periodicity of global volcanisms represented by the mantle-plume activity. Methane emission from magma, thermal decomposition of pre-existing organic matter and serpentinization of oceanic crusts are considerable factors for the methanogenesis in the early Precambrian oceans. Large-scale activity of methanotrophic micro-organism has not been recorded in marine sediments after 2.1 Ga. This is probably related to the decrease of global volcanism and complete transition of the marine primary producer to photo-synthesizing micro-organism.
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