Amazonian river system. We find that respiration of contemporary organic 2 matter (less than 5 years old) originating on land and near rivers is the dominant source of excess carbon dioxide that drives outgassing in mid-size to large rivers, although we find that bulk organic carbon fractions transported by these rivers range from tens to thousands of years in age. We therefore suggest that a small, rapidly cycling pool of organic carbon is responsible for the large carbon fluxes from land to water to atmosphere in the humid tropics.Riverine CO 2 concentrations in Amazonian lowlands are 5-30 times supersaturated with respect to atmospheric equilibrium 1 ; such conditions may be prevalent throughout the humid tropics. In situ respiration is the primary source of CO 2 sustaining supersaturation in rivers, although inputs from groundwater supersaturated by soil respiration can be important in small systems and from submerged riparian root respiration in floodplain influenced systems [1][2][3][6][7][8] . While air-water gas exchange is a bidirectional process, atmospheric CO 2 invasion has a negligible role compared to the large CO 2 evasion fluxes, except at low supersaturation 2,3,6,7 . 13 C and 14 C isotopes can provide constraints on sources and turnover times of organic carbon (OC) fuelling river respiration, yet no previous tropical study has used a dual-isotope approach to address these questions. Studies in temperate eastern USA provide contrasting findings. In the Hudson River, up to 70% of the centuries-old terrestrial OC entering the river is respired in transit, and the average age of riverine OC decreases downstream 2 .However, the youngest components of dissolved OC (DOC) are preferentially respired in the York River 5 , and modern dissolved inorganic carbon (DIC) in the Parker River may be explained by respiration of young DOC produced within the estuary 4 .Documenting key patterns and controls on CO 2 sources in diverse ecosystems is critical to advance our understanding of CO 2 outgassing from rivers and its contribution to regional net carbon budgets.
3To identify dominant sources and turnover times of riverine carbon throughout the Amazon basin, we analysed 14 C and 13 C of DIC, DOC, and suspended fine and coarse particulate OC fractions (FPOC and CPOC), grouping sites topographically (Fig. 1).This survey represents the most extensive dual carbon isotope inventory to date in a large, diverse basin, and the first 14 C analysis of DIC in Amazonian rivers. It complements but greatly exceeds previous carbon isotope surveys 5,7,9 , enabling an integrated assessment of carbon cycling.DIC is composed of dissolved CO 2 and bicarbonate and carbonate ions in pHdependent chemical and isotopic equilibrium 10 . In studies of marine and homogeneous river systems, where pH is nearly uniform, it has been the convention to report the isotopic composition of total DIC, which is directly measured. However, when the turnover of DIC by CO 2 fluxes is as rapid as in many of these tropical rivers, a quasisteady-state co...
