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Rivers are generally supersaturated with respect to carbon dioxide,resulting in large gas evasion fluxes that can be a significantcomponent of regional net carbon budgets1,2. Amazonian riverswere recently shown to outgas more than ten times the amount ofcarbon exported to the ocean in the form of total organic carbonor dissolved inorganic carbon1. High carbon dioxide concentrationsin rivers originate largely from in situ respiration oforganic carbon1-3, but little agreement exists about the sources orturnover times of this carbon2,4,5. Here we present results of anextensive survey of the carbon isotope composition (13C and 14C)of dissolved inorganic carbon and three size-fractions of organiccarbon across the Amazonian river system. We find that respirationof contemporary organic matter (less than five years old)originating on land and near rivers is the dominant source ofexcess carbon dioxide that drives outgassing in medium to largerivers, although we find that bulk organic carbon fractionstransported by these rivers range from tens to thousands ofyears in age. We therefore suggest that a small, rapidly cyclingpool of organic carbon is responsible for the large carbon fluxesfrom land to water to atmosphere in the humid tropics.

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