The oxidation of organic carbon contained within sedimentary rocks (“petrogenic” carbon, or hereafter OC petro ) emits nearly as much CO 2 as is released by volcanism, thereby playing a key role in the long-term global C budget. High erosion rates in mountains have been shown to increase OC petro oxidation. However, these settings also export unweathered material that may continue to react in downstream floodplains. The relative importance of OC petro oxidation in mountains versus floodplains remains difficult to assess as disparate methods have been used in the different environments. Here, we investigate the sources and fluxes of rhenium (Re) in the Rio Madre de Dios to quantify OC petro oxidation from the Andes to the Amazon floodplain using a common approach. Dissolved rhenium concentrations (n = 131) range from 0.01 to 63 pmol L −1 and vary depending on lithology and geomorphic setting. We find that >75% of the dissolved Re derives from OC petro oxidation and that this proportion increases downstream. We estimate that in the Andes, OC petro oxidation releases 11.2 +4.5 / −2.8 tC km −2 y −1 of CO 2 , which corresponds to ~41% of the total OC petro denudation (sum of oxidized and solid OC petro ). A Re mass balance across the Rio Madre de Dios shows that 46% of OC petro oxidation takes place in the Andes, 14% in the foreland-lowlands, and 40% in the Andean-fed floodplains. This doubling of OC petro oxidation flux downstream of the Andes demonstrates that, when present, floodplains can greatly increase OC petro oxidation and CO 2 release.