Sustained old carbon dioxide release from river surfaces across an Andes to Amazon floodplain transect

Globally, river surfaces release ~1.8 ± 0.3 PgC yr^-1 of carbon dioxide (CO₂). This is larger than the net removal of anthropogenic CO₂ to the land surface of 1.6 ± 0.5 PgC yr^-1, meaning that river CO₂ could act as a leak of carbon back to the atmosphere over the coming decades. To better understand the impact of this large flux on the carbon cycle, we must seek to connect the geomorphic, hydrological and ecological controls on the export of carbon from the terrestrial biosphere, soils and rocks to river networks. Despite the recognition that the release of CO₂ from river surfaces is substantial, we still lack insight on the source, delivery and/or production of CO₂ along rivers. Here we assess the source of riverine CO₂ along a ~250 km transect from the high Andes to the lowland Amazon floodplain, across the upper Madre de Dios basin in the wet season of March 2019. Using floating chamber methods, we quantify CO₂ release from river surfaces. To explore the competition of CO₂ sources from weathering (rock-derived C) and from the biosphere, we use a headspace method to trap CO₂ on zeolite sieves for isotopic analysis (stable carbon isotopes and radiocarbon). The major and trace element dissolved chemistry was also assessed to quantify the dominant weathering reactions. We find downstream variability in CO₂ release from river surfaces (ranging from ~650 to 2900 gC m^-2 yr^-1), with the mainstem of the Madre de Dios at our most downstream location having the highest flux. In contrast, the radiocarbon activity (reported as Fraction Modern, F¹⁴C) of the CO₂ varied much less, with the two major tributaries the Rio Manu and Rio Alto Madre de Dios having F¹⁴C values of CO₂ of 0.818 and 0.824, respectively, while ~150 km downstream the mainstem F¹⁴C of CO₂ was 0.809. The F¹⁴C of a lowland river, not sourced from the Andes, had a F¹⁴C of CO₂ of 0.954, suggesting old organic matter degradation may be underway upstream. Together with the stable C isotope composition and dissolved chemistry, these findings suggest a sustained release of old CO₂ from carbonate weathering sources across this tropical floodplain transect, but that the overall flux is dominated by CO₂ from the terrestrial biosphere that must be efficiently delivered to the river channel.