Natural atmospheric CO2 mineralisation in unexpected places

大气中的二氧化碳自然会与某些岩石类型发生反应，特别是超镁铁质火成岩。超镁铁质火成岩由硅酸盐矿物如橄榄石组成，其富含二价金属阳离子如Mg和Ca（例如Oelkers等人，2008年）。这些矿物质在地球表面条件下相对不稳定，并且容易风化，将Ca和Mg释放到水中的溶液中，同时消耗大气CO2（例如Clark等人，1992; Falk等人，2016）。在超镁铁岩中，天然沃茨中的高pH导致碳由碳酸根离子（或碳酸氢根，取决于pH）承载（例如Millero等人，2006），其可与来自风化超镁铁岩的溶解的Ca和Mg反应以快速沉淀碳酸盐矿物，例如方解石（例如Kelemen等人，2011）。一般认为只有超镁铁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，监督员在地球表面条件下观察到其他（即非超镁铁质）结晶岩石类型的天然CO2矿化实例，其成分矿物范围广泛。在这些天然例子的位置没有生物或岩石成因的CO2来源，试验性碳同位素分析表明大气CO2来源。这挑战了公认的智慧，即只有超镁铁质岩石可以快速矿化大气中的CO2，并在地球表面条件下以矿物形式稳定C。该项目旨在了解使大气中的CO2通过与比以前已知的更广泛的岩石类型反应而矿化的因素。在大气中CO2水平上升的时候，我们迫切需要扩展我们对CO2矿化机制的认识。虽然我们知道超镁铁质火成岩会矿化CO2，但这些岩石在地球表面非常罕见（Amiotte Suchet等人，2003年）。这项研究将确定导致其他结晶岩石类型（迄今未记录）快速二氧化碳矿化的自然例子的因素，这些岩石覆盖了至少30%的地球陆地表面积。我们对CO2矿化反应基本原理的理解的这一进展可能会对未来的碳捕获和储存措施产生影响，这些措施是减轻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.