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

化学风化是岩石溶解在雨水中的过程，雨水是天然的酸性物质。这是因为大气中的二氧化碳在雨水中溶解形成碳酸，雨水与岩石相互作用使其溶解。溶解的二氧化碳被困在河流和海水中，以重碳酸盐的形式(例如，存在于所有自然水域，如矿泉水中)，在那里稳定存在数千年或数万年，然后以碳酸钙(如石灰石)的矿物形式永久储存，并以石灰石的形式沉积在海洋中。岩石溶解或化学风化是全球碳循环中的一个主要过程，人们认为这种陆地上岩石的化学风化，以及随后以碳酸钙形式埋藏的碳，起到了反馈的作用，控制了地球历史上的碳循环，从而控制了地球历史上的气候。与化学风化有关的碳通量通常是通过河流化学来估计的，假设河流的组成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不只是具有与溶解的岩石相同的化学成分。一系列化学反应称为阳离子交换反应。当带电矿物表面和水之间的化学平衡发展时，它们会迅速发生。粘土是表面带电的最重要的矿物群之一。这些快速反应在土壤和含水层中得到了很好的研究，但从事河流化学工作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，它将显著改变天然水的化学成分和通过化学风化的碳消耗总量。我们已经开发了一种新的工具包，可以解决阳离子交换的重要性。我们的工具有：1)可追踪快速化学反应的同位素地球化学，2)可表征发生交换的矿物表面的核磁共振，以及3)对矿物中可交换位置的特定组成敏感的X射线衍射。我们已经计划了一系列实验研究，以量化受良好约束的受控样本中的过程，并对世界上最大的河流进行研究(关于样本档案收集)，以确定该问题的全球重要性。

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.