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

化学风化调节地球的碳循环，从而在地质时间尺度上调节全球气候。硅酸盐矿物中的Ca和Mg在碳酸溶解过程中释放到溶质相中。这种溶质Ca和Mg随后以Ca和Mg碳酸盐的形式被埋藏在海洋盆地中，将碳从大气中转移到碳酸盐岩储层中。这种简单的反应提供了气候反馈，使地球的气候在整个地球历史上保持稳定和适宜居住。为了了解地球的气候功能，了解硅酸盐风化和估计与现代化学风化相关的二氧化碳通量至关重要。现代硅酸盐风化通量是根据河流或自然沃茨的化学性质估算的。天然沃茨含有带正电荷的元素或阳离子，如Ca、Mg、Na和K，几十年来人们已经认识到，这些元素的相对和绝对浓度取决于被排出的岩石类型。例如，Ca主要来自石灰石的风化，而Na和K主要来自硅酸盐矿物如长石的风化。这种区别很重要，因为只有来自硅酸盐风化的Ca对二氧化碳的消耗很重要。因此，硅酸盐风化的Ca通量通常是基于Na来估算的，这被认为是硅酸盐风化的更可靠的估算。然而，化学风化比简单的矿物溶解更为复杂，还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，这是一个已知的缓冲地下水的过程。其中一个关键的化学交换是Ca交换Na，这意味着Na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，阳离子交换可能比以前认为的更重要，直到现在还很难识别。一种方法是使用天然存在的示踪剂或同位素，可以跟踪化学反应。在这项工作中，有人建议检查天然存在的同位素的元素锂和镁研究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，提出了一系列的实验工作，以检查在阳离子交换过程中的Mg和Li的同位素的行为。一旦我们了解了示踪剂的工作原理，我们就可以用它们来重新评估我们对自然沃茨的理解，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的气候。

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.