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

Baldini, James U.L. and Bertram, Rachel A. and Ridley, Harriet E. (2018) 'Ground air : a first approximation of the Earth's second largest reservoir of carbon dioxide gas.', Science of the total environment., 616-617 . pp. 1007-1013.


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.

Item Type:Article
Full text:(AM) Accepted Manuscript
Available under License - Creative Commons Attribution Non-commercial No Derivatives.
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Publisher statement:© 2017 This manuscript version is made available under the CC-BY-NC-ND 4.0 license
Date accepted:20 October 2017
Date deposited:10 November 2017
Date of first online publication:31 October 2017
Date first made open access:31 October 2018

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