The fate of carbon dioxide in water-rich fluids under extreme conditions.
basic_science · Level V
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- Record sourced from PubMed, PMID 27757424.
- Also identified by PMC identifier 5061492.
- Licence recorded as CC BY-NC.
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Abstract
Investigating the fate of dissolved carbon dioxide under extreme conditions is critical to understanding the deep carbon cycle in Earth, a process that ultimately influences global climate change. We used first-principles molecular dynamics simulations to study carbonates and carbon dioxide dissolved in water at pressures (<i>P</i>) and temperatures (<i>T</i>) approximating the conditions of Earth's upper mantle. Contrary to popular geochemical models assuming that molecular CO<sub>2</sub>(aq) is the major carbon species present in water under deep Earth conditions, we found that at 11 GPa and 1000 K, carbon exists almost entirely in the forms of solvated carbonate ([Formula: see text]) and bicarbonate ([Formula: see text]) ions and that even carbonic acid [H<sub>2</sub>CO<sub>3</sub>(aq)] is more abundant than CO<sub>2</sub>(aq). Furthermore, our simulations revealed that ion pairing between Na<sup>+</sup> and [Formula: see text]/[Formula: see text] is greatly affected by <i>P</i>-<i>T</i> conditions, decreasing with increasing pressure at 800 to 1000 K. Our results suggest that in Earth's upper mantle, water-rich geofluids transport a majority of carbon in the form of rapidly interconverting [Formula: see text] and [Formula: see text] ions, not solvated CO<sub>2</sub>(aq) molecules.