Old carbon routed from land to the atmosphere by global river systems

Rivers and streams are an important pathway in the global carbon cycle, releasing carbon dioxide (CO 2 ) and methane (CH 4 ) from their water surfaces to the atmosphere 1,2 . Until now, CO 2 and CH 4 emitted from rivers were thought to be predominantly derived from recent (sub-decadal) biomass production and, thus, part of ecosystem respiration 3–6 . Here we combine new and published measurements to create a global database of the radiocarbon content of river dissolved inorganic carbon (DIC), CO 2 and CH 4 . Isotopic mass balance of our database suggests that 59 ± 17% of global river CO 2 emissions are derived from old carbon (millennial or older), the release of which is linked to river catchment lithology and biome. This previously unrecognized release of old, pre-industrial-aged carbon to the atmosphere from long-term soil, sediment and geologic carbon stores through lateral hydrological routing equates to 1.2 ± 0.3 Pg C year −1 , similar in magnitude to terrestrial net ecosystem exchange. A consequence of this flux is a greater than expected net loss of carbon from aged organic matter stores on land. This requires a reassessment of the fate of anthropogenic carbon in terrestrial systems and in global carbon cycle budgets and models.