Natural atmospheric CO2 mineralisation in unexpected places

Atmospheric CO2 naturally reacts with some rock types, particularly ultramafic igneous rocks. Ultramafic igneous rocks are composed of silicate minerals such as olivine which are rich in divalent metal cations like Mg and Ca (e.g. Oelkers et al., 2008).

These minerals are relatively unstable at Earth surface conditions and weather easily, releasing Ca and Mg into solution in water, while consuming atmospheric CO2 (e.g. Clark et al., 1992; Falk et al., 2016). In ultramafic rocks, high pH in the natural waters results in the carbon hosted by carbonate ions (or bicarbonate depending on the pH) (e.g.

Millero et al., 2006) which can react with dissolved Ca and Mg from the weathered ultramafic rocks to rapidly precipitate carbonate minerals such as calcite (e.g. Kelemen et al, 2011). The CO2 is therefore converted from a gas to a solid.

The generally held view is that only ultramafic igneous rocks will react with atmospheric CO2 this rapidly, and act as a rapid carbon capture and storage pathway. However, the supervisors have observed natural examples of CO2 mineralisation at Earth-surface conditions on other (i.e. not ultramafic) crystalline rock types with a wide range of component minerals.

There are no biogenic or lithogenic CO2 sources at the locations of these natural examples, and pilot carbon isotope analysis indicates an atmospheric CO2 source. This challenges the received wisdom that only ultramafic rocks can rapidly mineralise atmospheric CO2 and stabilise the C in a mineral form at Earth-surface conditions. This project seeks to understand the factors which allow atmospheric CO2 to be mineralised through reaction with a wider-than-previously-known range of rock types.

In a time of rising atmospheric CO2 levels, we are faced with an urgent need to extend our knowledge of the mechanism of CO2 mineralisation. While we know that ultramafic igneous rocks will mineralise CO2, these rocks are very rare at the Earth's surface (Amiotte Suchet et al., 2003). This study will determine the factors that lead to natural examples of rapid CO2 mineralisation by other crystalline rock types - hitherto undocumented - which cover at least 30% of the Earth's land surface area.

This advance in our understanding of the fundamentals of the CO2 mineralisation reaction could have implications for future carbon capture and storage measures as an approach to mitigating anthropogenically-induced climate change.