Large Presence of Carbonic Acid in CO₂-Rich Aqueous Fluids under Earth’s Mantle Conditions

Abstract

The chemistry of carbon in aqueous fluids at extreme pressure and temperature conditions is of great importance to Earth's deep carbon cycle, which substantially affects the carbon budget at Earth's surface and global climate change. At ambient conditions, the concentration of carbonic acid in water is negligible, so aqueous carbonic acid was simply ignored in previous geochemical models. However, by applying extensive ab initio molecular dynamics simulations at pressure and temperature conditions similar to those in Earth's upper mantle, we found that carbonic acid can be the most abundant carbon species in aqueous CO 2 solutions at ∼10 GPa and 1000 K. The mole percent of carbonic acid in total dissolved carbon species increases with increasing pressure along an isotherm, while its mole percent decreases with increasing temperature along an isobar.In CO 2 -rich solutions, we found significant proton transfer between carbonic acid molecules and bicarbonate ions, which may enhance the conductivity of the solutions. The effects of pH buffering by carbonic acid may play an important role in water-rock interactions in Earth's interior. Our findings suggest that carbonic acid is an important carbon carrier in the deep carbon cycle.

Carbon dioxide, bicarbonate and carbonate ions in aqueous solutions under deep Earth conditions

“…[5,59] Our simulations provide direct evidence that, as opposed to what is customarily assumed in geochemical models, CO 2 (aq) is not the major carbon species present in water-rich geological fluids in the Earth's deep crust and upper mantle. [6,7,[47][48][49] Nevertheless, we find that the equilibrium composition of the solutions depends critically on the initial conditions, which, in turn determine the equilibrium content of H 3 O + and OH − ions. This result highlights the importance of considering acidity, at the same level as temperature and pressure, to predict the composition of geological fluids at deep Earth conditions.…”

Water-Gas Shift Reaction Produces Formate at Extreme Pressures and Temperatures in Deep Earth Fluids

“…4 Some studies suggest that formic acid (HCOOH) or formate (HCOO − ) may be a possible reaction intermediate in the catalyzed 16 and uncatalyzed 17 8,[20][21][22][23][24][25] and many of them focus on fully oxidized carbon forms in supercritical water. [26][27][28][29] With increasing depth, Earth's interior becomes more reducing, 30 so reduced carbon is of great importance in deep Earth, but due to the lack of reliable data at extreme P-T conditions, the water-gas shift reaction was simply assumed to adjust the fluid composition in reducing environments. 18,31 Here, by performing extensive ab initio molecular dynamics (AIMD) simulations, we studied mixtures of CO and H 2 O and reaction products at extreme P-T conditions found in Earth's upper mantle.…”

Stabilizing the Exotic Carbonic Acid by Bisulfate Ion

“…Carbonic acid, H 2 CO 3 , is a diprotic oxyacid and has long been considered as nonexisting in isolated state [1]. Its importance in a range of fields, including astrophysics, astrobiology, astrochemistry, geography, and biochemistry, has been well-recognized [2][3][4][5][6][7]. It is of great significance in regulating blood pH, in adjusting ocean acidification, and in the dissolution of carbonate minerals [8].…”