The injection of carbon dioxide (CO 2 ) captured at large point sources into deep saline aquifers can significantly reduce anthropogenic CO 2 emissions from fossil fuels. Dissolution of the injected CO 2 into the formation brine is a trapping mechanism that helps to ensure the long-term security of geological CO 2 storage. We use thermochronology to estimate the timing of CO 2 emplacement at Bravo Dome, a large natural CO 2 field at a depth of 700 m in New Mexico. Together with estimates of the total mass loss from the field we present, to our knowledge, the first constraints on the magnitude, mechanisms, and rates of CO 2 dissolution on millennial timescales. Apatite (U-Th)/He thermochronology records heating of the Bravo Dome reservoir due to the emplacement of hot volcanic gases 1.2-1.5 Ma. The CO 2 accumulation is therefore significantly older than previous estimates of 10 ka, which demonstrates that safe long-term geological CO 2 storage is possible. Integrating geophysical and geochemical data, we estimate that 1.3 Gt CO 2 are currently stored at Bravo Dome, but that only 22% of the emplaced CO 2 has dissolved into the brine over 1.2 My. Roughly 40% of the dissolution occurred during the emplacement. The CO 2 dissolved after emplacement exceeds the amount expected from diffusion and provides field evidence for convective dissolution with a rate of 0.1 g/(m 2 y). The similarity between Bravo Dome and major US saline aquifers suggests that significant amounts of CO 2 are likely to dissolve during injection at US storage sites, but that convective dissolution is unlikely to trap all injected CO 2 on the 10-ky timescale typically considered for storage projects.geological carbon storage | thermochronology | noble gases | porous media convection | carbon sequestration C arbon capture and storage has been identified as a potential technology for reductions in carbon dioxide (CO 2 ) emissions from coal-and natural gas-fired power plants. CO 2 that would otherwise be released into the atmosphere is captured at power plants and injected into porous geological formations for permanent storage. Carbon capture and storage has the potential for significant reductions of anthropogenic CO 2 emissions, because deep saline aquifers provide large storage volumes (1, 2) and existing operations have demonstrated that CO 2 injection and monitoring in saline aquifers are feasible (3).The leakage of CO 2 from the storage formation into potable aquifers or back into the atmosphere is an inherent risk of largescale geological CO 2 storage (4-6). Long-term storage security is therefore enhanced by physical and chemical processes that increasingly trap the injected CO 2 in the subsurface over time. Injected CO 2 can be trapped by capillary forces through the formation of disconnected ganglia or by precipitation as solid phases (7-9). Dissolution of CO 2 into the brine not only is a required first step for the subsequent permanent trapping in stable minerals, but also is considered a trapping mechanism itself. The density of the brin...

The injection of carbon dioxide (CO 2 ) captured at large point sources into deep saline aquifers can significantly reduce anthropogenic CO 2 emissions from fossil fuels. Dissolution of the injected CO 2 into the formation brine is a trapping mechanism that helps to ensure the long-term security of geological CO 2 storage. We use thermochronology to estimate the timing of CO 2 emplacement at Bravo Dome, a large natural CO 2 field at a depth of 700 m in New Mexico. Together with estimates of the total mass loss from the field we present, to our knowledge, the first constraints on the magnitude, mechanisms, and rates of CO 2 dissolution on millennial timescales. Apatite (U-Th)/He thermochronology records heating of the Bravo Dome reservoir due to the emplacement of hot volcanic gases 1.2-1.5 Ma. The CO 2 accumulation is therefore significantly older than previous estimates of 10 ka, which demonstrates that safe long-term geological CO 2 storage is possible. Integrating geophysical and geochemical data, we estimate that 1.3 Gt CO 2 are currently stored at Bravo Dome, but that only 22% of the emplaced CO 2 has dissolved into the brine over 1.2 My. Roughly 40% of the dissolution occurred during the emplacement. The CO 2 dissolved after emplacement exceeds the amount expected from diffusion and provides field evidence for convective dissolution with a rate of 0.1 g/(m 2 y). The similarity between Bravo Dome and major US saline aquifers suggests that significant amounts of CO 2 are likely to dissolve during injection at US storage sites, but that convective dissolution is unlikely to trap all injected CO 2 on the 10-ky timescale typically considered for storage projects.geological carbon storage | thermochronology | noble gases | porous media convection | carbon sequestration C arbon capture and storage has been identified as a potential technology for reductions in carbon dioxide (CO 2 ) emissions from coal-and natural gas-fired power plants. CO 2 that would otherwise be released into the atmosphere is captured at power plants and injected into porous geological formations for permanent storage. Carbon capture and storage has the potential for significant reductions of anthropogenic CO 2 emissions, because deep saline aquifers provide large storage volumes (1, 2) and existing operations have demonstrated that CO 2 injection and monitoring in saline aquifers are feasible (3).The leakage of CO 2 from the storage formation into potable aquifers or back into the atmosphere is an inherent risk of largescale geological CO 2 storage (4-6). Long-term storage security is therefore enhanced by physical and chemical processes that increasingly trap the injected CO 2 in the subsurface over time. Injected CO 2 can be trapped by capillary forces through the formation of disconnected ganglia or by precipitation as solid phases (7-9). Dissolution of CO 2 into the brine not only is a required first step for the subsequent permanent trapping in stable minerals, but also is considered a trapping mechanism itself. The density of the brin...

Environmental monitoring of shale gas production and geological carbon dioxide (CO 2) storage requires identification of subsurface gas sources. Noble gases provide a powerful tool to distinguish different sources if the modifications of the gas composition during transport can be accounted for. Despite

Budgets of 4 He and 40 Ar provide constraints on the chemical evolution of the solid Earth and atmosphere. Although continental crust accounts for the majority of 4 He and 40 Ar degassed from the Earth, degassing mechanisms are subject to scholarly debate. Here we provide a constraint on crustal degassing by comparing the noble gases accumulated in the Bravo Dome natural CO 2 reservoir, New Mexico USA, with the radiogenic production in the underlying crust. A detailed geological model of the reservoir is used to provide absolute abundances and geostatistical uncertainty of 4 He, 40 Ar, 21 Ne, 20 Ne, 36 Ar, and 84 Kr. The present-day production rate of crustal radiogenic 4 He and 40 Ar, henceforth referred to as 4 He * and 40 Ar * , is estimated using the basement composition, surface and mantle heat flow, and seismic estimates of crustal density. After subtracting mantle and atmospheric contributions, the reservoir contains less than 0.02% of the of the radiogenic production in the underlying crust. This shows unequivocally that radiogenic noble gases are effectively retained in cratonic continental crust over millennial timescales.

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