Methanogenic microbes may be one of the most primitive organisms1, although it is uncertain when methanogens first appeared on Earth. During the Archaean era (before 2.5 Gyr ago), methanogens may have been important in regulating climate, because they could have provided sufficient amounts of the greenhouse gas methane to mitigate a severely frozen condition that could have resulted from lower solar luminosity2 during these times. Nevertheless, no direct geological evidence has hitherto been available in support of the existence of methanogens in the Archaean period, although circumstantial evidence is available in the form of ∼2.8-Gyr-old carbon-isotope-depleted kerogen3. Here we report crushing extraction and carbon isotope analysis of methane-bearing fluid inclusions in ∼3.5-Gyr-old hydrothermal precipitates from Pilbara craton, Australia. Our results indicate that the extracted fluids contain microbial methane with carbon isotopic compositions of less than -56‰ included within original precipitates. This provides the oldest evidence of methanogen (> 3.46 Gyr ago), pre-dating previous geochemical evidence by about 700 million years.
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We thank M. Terabayashi, Y. Kato, K. Okamoto, T. Ota, T. Kabashima, K. Kitajima and K. Shimizu for assistance in field work, A. Thorne, K. J. McNamara and A. H. Hickman for field collaboration, H. Nara, Y. Matsui and M. Nishizawa for assisting in the construction of the vacuum line, and R. Buick and J. F. Kasting for comments on early versions of this manuscript. This research was supported by the 21st Century COE Program ‘How to build habitable planets,’ Tokyo Institute of Technology, sponsored by the Ministry of Education, Culture, Sports, Technology and Science, Japan. Y.U. thanks the Research Fellowships of the Japan Society for the Promotion of Science for Young Scientists.
This file contains Supplementary Methods, Supplementary Figures 3 and 4 with legends, and Supplementary Tables 1 and 2, which describe methods and results of the laser Raman and carbon isotope analyses. Supplementary Data includes the list of modern hydrothermal vent sites compiled in Fig. 2c of the main text with data source used for the compilation.
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FTIR microspectroscopy of carbonaceous matter in ~ 3.5 Ga seafloor hydrothermal deposits in the North Pole area, Western Australia
Progress in Earth and Planetary Science (2018)