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Knowing the rate at which carbon is cycled is crucial to understanding thedynamics of carbon transfer pathways. Recent technical developments nowsupport measurement of the 14C age of evaded CO2 from fluvialsystems, which provides an important "fingerprint" of the source of C. Herewe report the first direct measurements of the 14C age of effluxedCO2 from two small streams and two rivers within the western AmazonianBasin. The rate of degassing and hydrochemical controls on degassing are alsoconsidered. We observe that CO2 efflux from all systems except for theseasonal small stream was 14C-depleted relative to the contemporaryatmosphere, indicating a contribution from "old" carbon fixed before~ 1955 AD. Further, "old" CO2 was effluxed from the perennialstream in the rainforest; this was unexpected as here connectivity with thecontemporary C cycle is likely greatest. The effluxed gas represents allsources of CO2 in the aquatic system and thus we used end-memberanalysis to identify the relative inputs of fossil, modern andintermediately aged C. The most likely solutions indicated a contributionfrom fossil carbon sources of between 3 and 9% which we interpret asbeing derived from carbonate weathering. This is significant as the currentlyobserved intensification of weather has the potential to increase the futurerelease of old carbon, which can be subsequently degassed to the atmosphere,and so renders older, slower C cycles faster. Thus 14C fingerprinting ofevaded CO2 provides understanding which is essential to more accurately model thecarbon cycle in the Amazon Basin

Old carbon contributes to aquatic emissions of carbon dioxide in the Amazon