Ground air: A first approximation of the Earth's second largest reservoir of carbon dioxide gas

• Published in: The Science of the total environment 2018-03, Vol.616-617, p.1007-1013
• Main Authors: Baldini, James U.L., Bertram, Rachel A., Ridley, Harriet E.
• Format: Article
• Language: English
• Subjects: Carbon dioxide; Carbon reservoirs; Caves; Ground air; Vadose zone
• ISSN: 0048-9697; 1879-1026
• DOI: 10.1016/j.scitotenv.2017.10.218

It is becoming increasingly clear that a substantial reservoir of carbon exists in the unsaturated zone of aquifers, though the total size of this reservoir on a global scale remains unquantified. Here we provide the first broad estimate of the amount of carbon dioxide gas found in this terrestrial reservoir. We calculate that between 2 and 53 PgC exists as gaseous CO2 in aquifers worldwide, generated by the slow microbial oxidation of organic particles transported into aquifers by percolating groundwater. Importantly, this carbon reservoir is in the form of CO2 gas, and is therefore transferable to the Earth's atmosphere without any phase change. On a coarse scale, water table depths are partially controlled by local sea level; sea level lowering therefore allows slow carbon sequestration into the reservoir and sea level increases force rapid CO2 outgassing from this reservoir. High-resolution cave air pCO2 data demonstrate that sea level variability does affect CO2 outgassing rates from the unsaturated zone, and that the CO2 outgassing due to sea level rise currently occurs on daily (tidal) timescales. We suggest that global mean water table depth must modulate the global unsaturated zone volume and the size of this carbon reservoir, potentially affecting atmospheric CO2 on geological timescales. [Display omitted] •The research discusses evidence for a large carbon dioxide gas reservoir in the unsaturated zone of aquifers worldwide.•A first estimate of this reservoir's size suggests that it contains between 2 and 53 PgC as carbon dioxide gas.•Field data suggest that rising tides force carbon dioxide out of the reservoir.•CO2 exchange between this reservoir and the Earth’s atmosphere may contribute to climate change on geological timescales.