Quantifying cation exchange: Re-assessing the weathering signature of continental waters

化学风化作用在地质时间尺度上调节地球的碳循环，从而影响全球气候。在碳酸溶解过程中，硅酸盐矿物中的钙和镁被释放到溶质相。这些溶质钙和镁随后以钙和镁碳酸盐的形式埋藏在海洋盆地中，将碳从大气中转移到碳酸盐岩储集层。这种简单的反应提供了气候反馈，在整个地球历史上保持了地球气候的均衡性和宜居性。因此，要了解地球气候的功能，就必须了解硅酸盐的风化作用，并估算与现代化学风化作用有关的二氧化碳通量。现代的硅酸盐风化通量是根据河流或天然水的化学成分来估计的。天然水含有带正电的元素或阳离子，如钙、镁、钠和钾，几十年来人们一直认为这些元素的相对和绝对浓度取决于被排出的岩石的类型。例如，Ca主要来自石灰岩的风化，而Na和K主要来自长石等硅酸盐矿物的风化。这一区别很重要，因为只有来自硅酸盐风化的钙对二氧化碳的消耗是重要的。因此，硅酸盐风化的钙通量通常以钠为基础，被认为是硅酸盐风化的更可靠的估计。然而，化学风化比简单的矿物溶解更复杂，还会发生一系列其他化学反应，如阳离子交换。这是一个过程，溶液中带正电的阳离子被吸引到粘土上带负电的矿物表面，这一过程被称为缓冲地下水。其中一个关键的化学交换是钙与钠的交换，这意味着钠可能根本无法提供硅酸盐风化的真实估计。最近的同位素数据表明，阳离子交换可能比之前认为的更重要，到目前为止，这一点很难确定。一种方法是使用自然产生的示踪剂或同位素，可以跟踪化学反应。在这项工作中，建议检查元素Li和Mg的自然存在的同位素，以检查阳离子交换在全球预算中的作用。然而，为了能够成功地做到这一点，建议进行一系列的实验工作来研究镁和锂的同位素在阳离子交换过程中的行为。一旦我们了解了示踪剂的工作原理，我们就可以使用它们来重新评估我们对自然水域的理解，并更好地估计与化学风化相关的二氧化碳通量，最终目的是更好地了解地球气候。

Chemical weathering mediates Earth's carbon cycle and hence global climate over geological time-scales. Ca and Mg from silicate minerals are released to the solute phase during dissolution with carbonic acid. This solute Ca and Mg gets subsequently buried as Ca and Mg carbonates in ocean basins transferring carbon from the atmosphere to the carbonate rock reservoir. This simple reaction has provided the climatic feedback that has maintained Earth's climate equable and inhabitable over the entire history of the Earth. To understand how Earth's climate functions, it is therefore vital to understand silicate weathering and to estimate the flux carbon dioxide associated with modern chemical weathering. Modern day silicate weathering fluxes are estimated from the chemistry of rivers or natural waters. Natural waters contain positively charged elements or cations such as Ca, Mg, Na and K, and it has been understood for decades that the relative and absolute concentrations of these elements depend of the type of rocks that are drained. For example, Ca is mainly derived from the weathering of limestones, whereas Na and K are mainly derived from the weathering of silicate minerals such as feldspar. This distinction is important because only the Ca derived from silicate weathering is important for carbon dioxide consumption. Therefore, the Ca flux from silicate weathering is usually estimated based on Na, which has been thought to a more reliable estimate of silicate weathering. However, chemical weathering is more complex than simple mineral dissolution and a series of other chemical reactions also occur such as cation exchange. This is a process whereby the positively charged cations in solution are attracted to negatively charged mineral surfaces on clays, a process known to buffer groundwaters. One of the key chemical exchanges is Ca for Na, meaning that Na may not provide a true estimate of silicate weathering at all. Recent isotopic data suggests that cation exchange might be more significant that previously thought, which until now has been very hard to fingerprint. One method is to use naturally occurring tracers or isotopes, that allow chemical reactions to be tracked. In this work, it is proposed to examine the naturally occurring isotopes of the elements Li and Mg to examine the role of cation exchange in global budgets. However, to be able to do this successfully, a series of experimental work is proposed to examine the behaviour of the isotopes of Mg and Li during cation exchange. Once we understand how our tracers work we can use them to re-evaluate our understanding of natural waters, and better estimate fluxes of carbon dioxide associated with chemical weathering, with the ultimate aim of better understanding Earth's climate.