Natural atmospheric CO2 mineralisation in unexpected places

大气中的二氧化碳与某些岩石类型发生自然反应，尤其是超镁质火成岩。超镁铁质火成岩由橄榄石等硅酸盐矿物组成，富含镁和钙等二价金属阳离子（如Oelkers等，2008）。这些矿物在地球表面条件和天气条件下相对不稳定，容易将Ca和Mg释放到水中的溶液中，同时消耗大气中的CO2（例如Clark et al., 1992; Falk et al., 2016）。在超镁铁质岩石中，天然水体的高pH值导致碳酸盐离子（或碳酸氢盐，取决于pH值）所携带的碳（如Millero等人，2006年）可以与风化超镁铁质岩石中溶解的Ca和Mg反应，迅速沉淀出方解石等碳酸盐矿物（如Kelemen等人，2011年）。二氧化碳因此从气体转化为固体。人们普遍认为，只有超镁质火成岩才能如此迅速地与大气中的二氧化碳发生反应，并作为快速的碳捕获和储存途径。然而，监督者已经在地球表面条件下观察到其他（即非超镁铁质）晶体岩石类型的二氧化碳矿化的自然例子，这些岩石类型具有广泛的成分矿物。在这些自然样例的位置没有生物或岩石成因的CO2源，碳同位素中试分析表明大气CO2源。这挑战了公认的观点，即只有超镁铁质岩石才能快速矿化大气中的二氧化碳，并在地球表面条件下以矿物形式稳定碳。该项目旨在了解使大气中的二氧化碳通过与比以前已知的岩石类型范围更广的反应而矿化的因素。在大气中二氧化碳含量不断上升的时代，我们迫切需要扩展我们对二氧化碳矿化机制的了解。虽然我们知道超镁质火成岩会使二氧化碳矿化，但这些岩石在地球表面是非常罕见的（Amiotte Suchet et al., 2003）。这项研究将确定导致其他晶体岩石类型（迄今未记载）快速二氧化碳矿化的自然例子的因素，这些岩石类型至少覆盖了地球陆地面积的30%。我们对二氧化碳矿化反应基本原理的理解取得了进展，这可能会对未来的碳捕获和储存措施产生影响，作为缓解人为引起的气候变化的一种方法。

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.