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Naturally occurring CO2 reservoirs across the USA provide critical insights into the fate of CO2 in the subsurface over geological timescales. Previous measurements of CO2 to 3He ratios within gas samples obtained from six natural CO2 reservoirs show that the CO2 originated from magmatic degassing. Variation in CO2/3He across each reservoir suggests that significant amounts of CO2, equivalent to hundreds of megatonnes, have been stored by solubility trapping. Does this dissolution occur during emplacement, or by diffusion through geological time? Here we present the results of integrating the geochemical measurements with reservoir modelling to quantify both the mass of CO2 emplaced and the proportion dissolved within each of the CO2 reservoirs. Given the magmatic origin of the CO2, we use the known age dates of associated igneous rocks to estimate the timing of CO2 emplacement in each …