Abstract
Fractured rocks of impact craters may be suitable hosts for deep microbial communities on Earth and potentially other terrestrial planets, yet direct evidence remains elusive. Here, we present a study of the largest crater of Europe, the Devonian Siljan structure, showing that impact structures can be important unexplored hosts for long-term deep microbial activity. Secondary carbonate minerals dated to 80 ± 5 to 22 ± 3 million years, and thus postdating the impact by more than 300 million years, have isotopic signatures revealing both microbial methanogenesis and anaerobic oxidation of methane in the bedrock. Hydrocarbons mobilized from matured shale source rocks were utilized by subsurface microorganisms, leading to accumulation of microbial methane mixed with a thermogenic and possibly a minor abiotic gas fraction beneath a sedimentary cap rock at the crater rim. These new insights into crater hosted gas accumulation and microbial activity have implications for understanding the astrobiological consequences of impacts.
Highlights
Fractured rocks of impact craters may be suitable hosts for deep microbial communities on Earth and potentially other terrestrial planets, yet direct evidence remains elusive
This strong greenhouse gas can form via three main mechanisms in the deep subsurface: abiotic, that is, during inorganic reactions of compounds, e.g. H2 and CO2; thermogenic, that is, by organic matter breakdown at high temperatures; and microbial activity[10,11], which has been largely overlooked at Siljan and other impact structures
The highly variable δ13Ccalcite values between different fractures and within single crystals point to spatiotemporal variation of the processes that lead to calcite precipitation
Summary
Fractured rocks of impact craters may be suitable hosts for deep microbial communities on Earth and potentially other terrestrial planets, yet direct evidence remains elusive. In the largest impact structure in Europe, the Late Devonian (380.9 ± 4.6 Ma9) Siljan crater in Sweden, the past and present deep biosphere communities remain unexplored, but the structure has been thoroughly studied for potential methane accumulation. This strong greenhouse gas can form via three main mechanisms in the deep subsurface: abiotic, that is, during inorganic reactions of compounds, e.g. H2 and CO2; thermogenic, that is, by organic matter breakdown at high temperatures; and microbial activity[10,11], which has been largely overlooked at Siljan and other impact structures. Methane accumulations have been detected during the drilling campaigns, both in the sedimentary rock (proposed cap rocks) and deep within the granite fracture system, but no qualified estimate of total gas volumes has yet been made public
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