Extended Data Figure 5: Modelling how far dissolved CO₂ is transported before being outgassed.

a, We assessed the potential for lateral CO₂ transport in rivers and floodplains using a simple one-dimensional model that simultaneously calculates the CO₂ lost by outgassing and the CO₂ that remains dissolved in water and is transported downstream by the currents. The model starts from a point source in the wetland (set here at 12,000 p.p.m.v., which is a typical value observed in the vicinity of a flooded forest; Fig. 2b). The iteration time was 1 min. In the model, F(CO₂) is calculated from , using representative values of k₆₀₀. The quantity of CO₂ lost to the atmosphere during one iteration is subtracted from the initial CO₂ quantity present in a column of water of a determined depth H. Note that this procedure is adequate only for acidic, non-buffered waters, such as those in the Amazon. b, c, When integrated (Supplementary Information), the equation gives a one-phase exponential decay function of versus the distance x, the water current velocity w, the normalized gas transfer velocity k₆₀₀, and the water depth H. The curves give the potential extent of saturation that can be maintained without the necessity of aquatic respiration (Fig. 2b). D½ is the half-evasion distance, which is the theoretical distance the water mass travels before outgassing half of its initial excess CO₂. T½ is the associated half-evasion time. d, Typical half-evasion distances of wetland CO₂ in river–floodplain systems vary from less than 1 km in a shallow, stagnant, wind- and heat-protected lake to more than 300 km in a deep and fast-flowing river. This illustrates, on the one hand, how far wetland CO₂ can be exported downstream, and on the other hand, the large heterogeneity of the transport and outgassing processes in the river–floodplain complex.