Muddying the waters: cation exchange processes as a major control on weathering fluxes?

化学风化是指岩石在天然酸性的雨水中溶解的过程。这是因为大气中的二氧化碳溶解在雨中形成碳酸，雨水与岩石相互作用使岩石溶解。溶解的二氧化碳以碳酸氢盐的形式被困在河流和海水中（存在于所有自然水域，例如矿泉水中），在那里稳定地存在数千年或数万年，然后以碳酸钙（如水垢）的矿物形式永久储存起来，并以石灰石的形式沉积在海洋中。岩石溶蚀或化学风化是全球碳循环的一个主要过程，人们认为，这种陆地岩石的化学风化，以及随后以碳酸钙形式埋藏的碳，在地球历史上起着控制碳循环和气候的反馈作用。与化学风化有关的碳通量通常是根据河流化学来估计的，假设河流成分可以与岩石溶解的类型相匹配。这是一种简化，因为化学反应意味着河流的化学成分与溶解的岩石不同。一组化学反应被称为阳离子交换反应。当带电的矿物表面和水之间形成化学平衡时，它们迅速发生。具有带电表面的最重要的矿物群之一是粘土。这些快速反应在土壤和含水层中得到了很好的研究，但研究河流化学的科学界在很大程度上忽视了这些反应。我们已经生成了一套初步数据，这些数据表明，一旦考虑到阳离子交换过程，它就会显著改变天然水的化学成分和通过化学风化消耗的碳总量。我们开发了一个新的工具箱，可以解决阳离子交换的重要性。我们的工具是：1)同位素地球化学，可以追踪快速的化学反应；2)核磁共振，可以表征正在发生交换的矿物表面；3)x射线衍射，对矿物中可交换部位的特定成分很敏感。我们已经计划了一系列的实验研究，在良好的约束控制的例子中量化这一过程，再加上对世界上最大的河流（对样本的存档收集）的研究，以确定这个问题的全球重要性。

Chemical weathering is the process by which rocks dissolve in rainwater, which is naturally acidic. This is because atmospheric carbon dioxide dissolves in rain to form carbonic acid, and the rainwater interacts with rocks making them dissolve. The dissolved carbon dioxide becomes trapped in river and seawater, as bicarbonate (present in all natural waters such as mineral water for example), where it resides stably for thousands, or tens of thousands of years, and is then stored permanently in a mineral form as calcium carbonate (like limescale) and deposited as limestone in the oceans. Rock dissolution or chemical weathering is a major process in the global carbon cycle and it is thought that this terrestrial chemical weathering of rocks, and subsequent burial of carbon as calcium carbonate, acts as the feedback which has controlled the carbon cycle and thus climate over Earth history.The carbon fluxes associated with chemical weathering are commonly estimated from river chemistry, assuming that the river composition can be matched to the type of rock dissolving. This is a simplification because chemical reactions mean that a river doesn't simply have the same chemical composition as a rock which dissolves. One suite of chemical reactions are referred to as cation exchange reactions. They occur rapidly as a chemical equilibrium develops between charged mineral surfaces and a water. One of the most important mineral groups which have charged surfaces are clays. These rapid reactions are well studied in soils and aquifers, but the scientific community working on river chemistry has largely neglected these reactions. We have generated a suite of preliminary data that shows that once the cation exchange process is taken into account it changes significantly the chemistry of natural waters and the total amount of carbon consumption through chemical weathering.We have developed a new tool kit that can address the significance of cation exchange. Our tools are 1) isotope geochemistry, that can trace the rapid chemical reactions, 2) nuclear magnetic resonance that can characterise the mineral surfaces where exchange is occurring and 3) X-ray diffraction that is sensitive to the specific compositions of exchangeable sites in minerals. We have planned a series of experimental studies to quantify the processes in well constrained controlled examples, coupled to a study on the largest rivers in the world (on an archive collection of samples) to determine the global importance of the problem.