Carbon Dioxide, Bicarbonate and Carbonate Ions in Aqueous Solutions at Deep Earth Conditions

Investigating the fate of dissolved carbon dioxide under extreme conditions is critical to understanding the deep carbon cycle in the 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 (P) and temperatures (T) approximating the conditions of the Earth's upper mantle. Contrary to popular geochemical models assuming that molecular CO$_2$...

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 ...

The distribution, recycling and storage of carbon in the Earth are of fundamental importance to understand the global carbon cycle between the deep Earth and near surface reservoirs. Degassing of CO2 at mid-ocean ridges may give information on the source region but the very low solubility of CO2 in tholeitic basalts has for consequence that near all Mid-Ocean Ridge Basalts glasses exsolve their CO2 rich vapor at shallow depth as they approach the ocean floor. Hence their CO2 ...

Ab initio random structure searching (AIRSS) and density functional theory methods are used to predict structures of calcium and magnesium carbonate (CaCO$_3$ and MgCO$_3$) at high pressures. We find a previously unknown CaCO$_3$ structure which is more stable than the aragonite and "post aragonite" phases in the range 32--48 GPa. At pressures from 67 GPa to well over 100 GPa the most stable phase is a previously unknown CaCO$_3$ structure of the pyroxene type with fourfold c...

We present a theoretical investigation, based on ab initio calculations and the quasi-harmonic approximation, on the stability properties of magnesium (MgCO$_3$) and calcium (CaCO$_3$) carbonates at high temperatures and pressures. The results indicate that those carbonates should be stable in Earth's lower mantle, instead of dissociating into other minerals, in chemical environments with excesses of SiO$_2$, MgO, or MgSiO$_3$. Therefore, considering the lower mantle chemical...

Hydrocarbons are of great importance in carbon-bearing fluids in deep Earth and in ice giant planets at extreme pressure (P)-temperature (T) conditions. Raman spectroscopy is a powerful tool to study the chemical speciation of hydrocarbons; however, it is challenging to interpret Raman data at extreme conditions. Here, we performed ab initio molecular dynamics simulations coupled with the modern theory of polarization to calculate Raman spectra of methane, ethane, and propane...

Although molten carbonates only represent, at most, a very minor phase in the Earth's mantle, they are thought to be implied in anomalous high-conductivity zones in its upper part (70-350 km). Besides the high electrical conductivity of these molten salts is also exploitable in fuel cells. Here we report quantitative calculations of their properties, over a large range of thermodynamic conditions and chemical compositions, that are a requisite to develop technological devices...

Carbonate precipitation in oceans is essential for the carbonate-silicate cycle (inorganic carbon cycle) to maintain temperate climates. By considering the thermodynamics of carbonate chemistry, we demonstrate that the ocean pH decreases by approximately 0.5 for a factor of 10 increase in the atmospheric carbon dioxide content. The upper and lower limits of ocean pH are within 1-4 of each other, where the upper limit is buffered by carbonate precipitation and defines the ocea...

The reactions of CO$_2$ in water under extreme pressure-temperature conditions are of great importance to the carbon storage and transport below Earth's surface, which substantially affect the carbon budget in the atmosphere. We applied ab initio molecular dynamics simulations to study aqueous carbon solutions nanoconfined by graphene and stishovite (SiO$_2$) at 10 GPa and 1000$\sim$1400 K. We found that CO$_2$(aq) reacts more in nanoconfinement than in bulk. The stishovite-w...

Atomistic simulations provide a meaningful way to determine the physico-chemical properties of liquids in a consistent theoretical framework. This approach takes on particular usefulness for the study of molten carbonates, in a context where thermodynamic and transport data are crucially needed over a large domain of temperatures and pressures (to ascertain the role of these melts in geochemical processes) but are very scarce in the literature, especially for the calco-magnes...