Mountain uplift and erosion have regulated the balance of carbon between Earth’s interior and atmosphere, where prior focus has been placed on the role of silicate mineral weathering in CO 2 drawdown and its contribution to the stability of Earth’s climate in a habitable state 1–5 . However, weathering can also release CO 2 as rock organic carbon (OC petro ) is oxidized at the near surface 6,7 ; this important geological CO 2 flux has remained poorly constrained 3,8 . We use the trace element rhenium in combination with a spatial extrapolation model to quantify this flux across global river catchments 3,9 . We find a CO 2 release of ${68}_{-6}^{+18}$ 68 − 6 + 18 megatons of carbon annually from weathering of OC petro in near-surface rocks, rivalling or even exceeding the CO 2 drawdown by silicate weathering at the global scale 10 . Hotspots of CO 2 release are found in mountain ranges with high uplift rates exposing fine-grained sedimentary rock, such as the eastern Himalayas, the Rocky Mountains and the Andes. Our results demonstrate that OC petro is far from inert and causes weathering in regions to be net sources or sinks of CO 2 . This raises questions, not yet fully studied, as to how erosion and weathering drive the long-term carbon cycle and contribute to the fine balance of carbon fluxes between the atmosphere, biosphere and lithosphere 2,11 .