Abstract
The emerging interest in using stable bedrock formations for industrial purposes, e.g., nuclear waste disposal, has increased the need for understanding microbiological and geochemical processes in deep crystalline rock environments, including the carbon cycle. Considering the origin and evolution of life on Earth, these environments may also serve as windows to the past. Various geological, chemical, and biological processes can influence the deep carbon cycle. Conditions of CH4 formation, available substrates and time scales can be drastically different from surface environments. This paper reviews the origin, source, and cycling of methane in deep terrestrial crystalline bedrock with an emphasis on microbiology. In addition to potential formation pathways of CH4, microbial consumption of CH4 is also discussed. Recent studies on the origin of CH4 in continental bedrock environments have shown that the traditional separation of biotic and abiotic CH4 by the isotopic composition can be misleading in substrate-limited environments, such as the deep crystalline bedrock. Despite of similarities between Precambrian continental sites in Fennoscandia, South Africa and North America, where deep methane cycling has been studied, common physicochemical properties which could explain the variation in the amount of CH4 and presence or absence of CH4 cycling microbes were not found. However, based on their preferred carbon metabolism, methanogenic microbes appeared to have similar spatial distribution among the different sites.
Highlights
Introduction and Historical PerspectiveMethane is a key compound in the global carbon cycle
The possibility of abiotic CH4 synthesis at low temperatures, together with findings of methanogenic microbes indicates that the formation of CH4 is an on-going process in deep Precambrian continental bedrock
The identified carbon sources of microbes range from inorganic CO2 to CH4 and other small organic carbon molecules, but there are intriguing hypotheses on microbial utilization of refractory organic carbon of minerals
Summary
Introduction and Historical PerspectiveMethane is a key compound in the global carbon cycle. Deeper in the geological strata CH4 is found in large quantities within sedimentary formations and unconventional resources, such as shale gas, have proven to be important reserves of CH4 (Arthur and Cole, 2014). CH4 is a dominant gas in many Precambrian continental bedrock formations (Fritz et al, 1987; Karus et al, 1987; Nurmi et al, 1988; Sherwood Lollar et al, 1993a,b; Ward et al, 2004; Sherwood Lollar et al, 2006; Pitkänen and Partamies, 2007; Hallbeck and Pedersen, 2008a; Stotler et al, 2010; Kietäväinen et al, 2013, 2014).
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