420,000 year assessment of fault leakage rates shows geological carbon storage is secure.

Carbon capture and storage (CCS) technology is routinely cited as a cost effective tool for climate change mitigation. CCS can directly reduce industrial CO emissions and is essential for the retention of CO extracted from the atmosphere. To be effective as a climate change mitigation tool, CO must be securely retained for 10,000 years (10 ka) with a leakage rate of below 0.01% per year of the total amount of CO injected. Migration of CO back to the atmosphere via leakage through geological faults is a potential high impact risk to CO storage integrity. Here, we calculate for the first time natural leakage rates from a 420 ka paleo-record of CO leakage above a naturally occurring, faulted, CO reservoir in Arizona, USA. Surface travertine (CaCO) deposits provide evidence of vertical CO leakage linked to known faults. U-Th dating of travertine deposits shows leakage varies along a single fault and that individual seeps have lifespans of up to 200 ka. Whilst the total volumes of CO required to form the travertine deposits are high, time-averaged leakage equates to a linear rate of less than 0.01%/yr. Hence, even this natural geological storage site, which would be deemed to be of too high risk to be selected for engineered geologic storage, is adequate to store CO for climate mitigation purposes.