Abstract
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.