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The oxidation of organic carbon contained within sedimentary rocks (“petrogenic” carbon, or hereafter OC_petro) emits nearly as much CO₂as 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_petrooxidation. However, these settings also export unweathered material that may continue to react in downstream floodplains. The relative importance of OC_petrooxidation 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_petrooxidation from the Andes to the Amazon floodplain using a common approach. Dissolved rhenium concentrations (n = 131) range from 0.01 to 63 pmol L⁻¹and vary depending on lithology and geomorphic setting. We find that >75% of the dissolved Re derives from OC_petrooxidation and that this proportion increases downstream. We estimate that in the Andes, OC_petrooxidation releases 11.2^+4.5/_−2.8tC km⁻²y⁻¹of CO₂, which corresponds to ~41% of the total OC_petrodenudation (sum of oxidized and solid OC_petro). A Re mass balance across the Rio Madre de Dios shows that 46% of OC_petrooxidation takes place in the Andes, 14% in the foreland-lowlands, and 40% in the Andean-fed floodplains. This doubling of OC_petrooxidation flux downstream of the Andes demonstrates that, when present, floodplains can greatly increase OC_petrooxidation and CO₂release.