Episodic records of deep microbial methanogenesis throughout the Caledonian mountain range exhumation

Abstract The deep continental biosphere hosts microbial communities employing ancient metabolisms such as methanogenesis. Yet, it is one of Earth’s least explored ecosystems with its long-term history remaining poorly constrained. This study integrated mineral, groundwater, and dissolved gas stable isotope analyses along with geochronological and previously published metagenomic data from the central Sweden 2276 m-deep COSC-2 borehole that intersects Silurian Caledonian thrust sheets and Precambrian basement. Extremely ¹³C-enriched fracture-filling calcite (δ¹³C up to + 28.9‰ V-PDB ) indicated ancient microbial methanogenesis down to 1907 m depth. U–Pb dating revealed multiple episodes of ¹³C-rich carbonate precipitation over hundreds of millions of years, including ages of 186 ± 20, 124 ± 20, 100 ± 22, 109 ± 17, 59 ± 4, and 1.8 ± 0.4 Ma along with a less certain age of 378 ± 65 Ma. Clumped-isotope temperatures decreased from ~70 to ~43 °C that reflected progressive exhumation. These results extend the temporal range of deep methanogenic biosignatures and demonstrate the resilience of subsurface life during long-term uplift and mountain range erosion.