Extended Data Fig. 4: Drought-induced perturbation of the terrestrial inorganic carbon cycle.

Prolonged periods of drought promote secondary carbonate formation within the landscape, leading to increased CO₂ evasion compared to long term, ambient conditions. Combined with lower water flow, this process impedes lateral carbon flux by rivers. These shifts in carbon fluxes are signaled by an attendant stability in calcium concentrations while sodium and other major ions increase due to evapo-concentration. These increased concentrations counterbalance reduced flow rates to maintain a high mass flux. High-frequency hydrochemical recording by the RiverLab installation in the agricultural Upper Sangamon River catchment provides evidence of these shifts, manifesting as a systematic decrease in the Ca/Na molar ratio of the river.