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The groundwaters bathing crystalline bedrock, shaped by high temperatures and extreme pressures, can have surprisingly high concentrations of methane, even in places where ancient organic carbon has transformed into graphite or other inorganic forms. Researchers are exploring the sources of this methane to figure out how it is produced and its role in the deep carbon cycle.
Riikka Kietäväinen, a member of DCO’s Deep Energy and Deep Life Communities, along with Lasse Ahonen, also a member of the Deep Life Community, (both at the Geological Survey of Finland, in Espoo), and colleagues investigated factors that control methane formation at two sites in Finland: the Outokumpu Deep Drill Hole and the Pyhäsalmi copper-zinc mine, which both extend about 2.5 kilometers into Precambrian bedrock.
In a new paper in Geochimica et Cosmochimica Acta, the researchers combine isotopic measurements from water, gas, and minerals sampled from the two sites with existing chemical and microbiological analyses. They find that methane levels depend on the surrounding rock type, with metasedimentary rocks yielding more methane than volcanic or igneous ones. Their results also suggest that methane at deeper levels likely came from chemical processes such as the interactions between hydrogen and inorganic intermediates like graphite, and that methane generated by microbes mainly occurs up to 1.5 kilometers deep.
Read more here.