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

Abstract. Knowing the rate at which carbon is cycled is crucial to understanding the dynamics of carbon transfer pathways. Recent technical developments now support measurement of the ¹⁴C age of evaded CO₂ from fluvial systems, which provides an important "fingerprint" of the source of C. Here we report the first direct measurements of the ¹⁴C age of effluxed CO₂ from two small streams and two rivers within the western Amazonian Basin. The rate of degassing and hydrochemical controls on degassing are also considered. We observe that CO₂ efflux from all systems except for the seasonal small stream was ¹⁴C-depleted relative to the contemporary atmosphere, indicating a contribution from "old" carbon fixed before ~ 1955 AD. Further, "old" CO₂ was effluxed from the perennial stream in the rainforest; this was unexpected as here connectivity with the contemporary C cycle is likely greatest. The effluxed gas represents all sources of CO₂ in the aquatic system and thus we used end-member analysis to identify the relative inputs of fossil, modern and intermediately aged C. The most likely solutions indicated a contribution from fossil carbon sources of between 3 and 9% which we interpret as being derived from carbonate weathering. This is significant as the currently observed intensification of weather has the potential to increase the future release of old carbon, which can be subsequently degassed to the atmosphere, and so renders older, slower C cycles faster. Thus ¹⁴C fingerprinting of evaded CO₂ provides understanding which is essential to more accurately model the carbon cycle in the Amazon Basin.