Abstract Sub-seabed geological CO2 storage is discussed as a climate mitigation strategy, but the impact of any leakage of stored CO2 into the marine environment is not well known. In this study, leakage from a CO2 storage reservoir through near-surface sediments was mimicked for low leakage rates in the North Sea. Field data were combined with laboratory experiments and transport-reaction modelling to estimate CO2 and mineral dissolution rates, and to assess the mobilization of metals in contact with CO2-rich fluids and their potential impact on the environment. We found that carbonate and silicate minerals reacted quickly with the dissolved CO2, increasing porewater alkalinity and neutralizing about 5% of the injected CO2. The release of Ca, Sr, Ba and Mn was mainly controlled by carbonate dissolution, while Fe, Li, B, Mg, and Si were released from silicate minerals, mainly from deeper sediment layers. No toxic metals were released from the sediments and overall the injected CO2 was only detected up to 1 m away from seabed CO2 bubble streams. Our results suggest that low leakage rates of CO2 over short timescales have minimal impact on the benthic environment. However, porewater composition and temperature are effective indicators for leakage detection, even at low CO2 leakage rates.