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Abstract

Carbonic acid (H₂CO₃) forms in small amounts when CO₂ dissolves in H₂O, yet decomposes rapidly under ambient conditions of temperature and pressure. Despite its fleeting existence, H₂CO₃ plays an important role in the global carbon cycle and in biological carbonate-containing systems. The short lifetime in water and presumed low concentration under all terrestrial conditions has stifled study of this fundamental species. Here, we have examined CO₂/ H₂O mixtures under conditions of high pressure and high temperature to explore the potential for reaction to H₂CO₃ inside celestial bodies. We present a novel method to prepare solid H₂CO₃ by heating CO₂/H₂O mixtures at high pressure with a CO₂ laser. Furthermore, we found that, contrary to present understanding, neutral H₂CO₃ is a significant component in aqueous CO₂ solutions above 2.4 GPa and 110 degrees C as identified by IR-absorption and Raman spectroscopy. This is highly significant for speciation of deep C-O-H fluids with potential consequences for fluid-carbonate-bearing rock interactions. As conditions inside subduction zones on Earth appear to be most favorable for production of aqueous H₂CO₃, a role in subduction related phenomena is inferred.