# Large presence of carbonic acid in CO$_2$-rich aqueous fluids under Earth's mantle conditions

## TL;DR

This study reveals that carbonic acid is a major carbon species in Earth's deep mantle conditions, significantly impacting the deep carbon cycle and water-rock interactions, contrary to previous assumptions at ambient conditions.

## Contribution

The paper demonstrates through ab initio simulations that carbonic acid is abundant in high-pressure, high-temperature aqueous fluids, highlighting its role in Earth's deep carbon cycle.

## Key findings

- Carbonic acid becomes the dominant carbon species at ~10 GPa and 1000 K.
- Proton transfer between carbonic acid and bicarbonate ions enhances solution conductivity.
- Carbonic acid's buffering capacity influences water-rock interactions in Earth's interior.

## 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.