Carbon cycling in the Pliocene Velenje Coal Basin, Slovenia, inferred from stable carbon isotopes

► In this study we examine carbon cycling in Coal Basin. ► We study stable carbon isotopes in different geological media. ► Majority of methane was generated via microbial methanogenesis. ► Calcite precipitation was driven by microbial methanogenesis via CO2 reduction. ► Highly alkaline waters in the basin provide further evidence of microbial methane.