Fractionation of del30Si and del7Li during supply limited chemical weathering: Towards unified models of stable isotopic responses to weathering

构成固体地球大部分的岩石转化为覆盖地球表面的土壤，这一过程被称为化学风化。这一过程决定了地表环境的许多特征，不仅影响到河流和土壤，还影响到大气和海洋。风化作用在决定全球化学循环的速度方面尤其重要，包括碳的循环，因为固体地球实际上拥有比大气、海洋和生物圈加在一起更多的碳，碳以碳酸盐的形式存在，比如石灰岩。由于化学风化的速度取决于气候，而气候又取决于大气中的碳浓度，因此风化是地球系统如何运行以及它可能如何应对人类干扰的关键一环。在今天的地球上，准确测量风化速率往往是困难的，试图评估这些风化速率随着地质时间的推移发生了怎样的变化就更具挑战性了--这使这成为当今科学家的中心问题。试图了解过去变化的一种方法是查看沉积物中的记录，例如湖泊或海洋中的记录。查看这些记录需要可靠和易于理解的代理，换句话说，提供过去风化过程信息的可测量特征。这不是一门新科学；几十年来，它一直是地质学研究的核心部分，以前工作的主要成果是证明，单凭一个指标很难解释任何一个指标。这一认识刺激了开发多种新的化学风化代理的努力，这些努力得到了技术进步的推动，使风化反应中涉及的关键元素的同位素比率的测量得到了改进。两种特别令人感兴趣的元素是硅(Si)和锂(Li)。这些都是相关的，因为它们在新鲜岩石中的矿物质溶解时释放，但也部分保留在土壤形成过程中产生的粘土中。在这两个过程中，硅和锂的不同同位素优先释放到水中，然后排入溪流和河流。因此，这些河流的同位素组成，以及最终可能形成的沉积物，可能提供有关化学反应和物理去除物质的相对速率的信息(因为这种去除抑制了土壤的形成)。这种平衡被称为“风化强度”。在这项研究的合作伙伴以及国际上其他机构运营的实验室中，经过几年的工作，已经开始定量地了解硅和锂同位素系统的行为，特别是它们如何对自然环境中的风化强度做出反应。这让人对他们的潜力有了一个诱人的一瞥。然而，目前的情况有一个很大的差距，因为当热带环境中的土壤很厚时，它们在非常高的风化强度下的行为尚不清楚。这项研究中提出的研究将试图通过测量喀麦隆和哥斯达黎加热带集水区的溪流的硅和锂同位素组成来填补这一空白。在两个不同的地点工作将使比较对不同类型岩石的影响成为可能。除了测量这些同位素组成外，拟议的工作还将涉及旨在了解产生这些组成的机制的测量和实验室实验，换句话说，是什么化学过程负责，以及这些过程与其他环境中确定的过程有何不同。最后，该项目的结果将被用来建立一个更好的定量模型，该模型可能允许使用硅和锂同位素作为化学风化强度的代用品。

The conversion of the rock that makes up the bulk of the solid Earth into the soil that blankets the planet's surface takes place through a process called chemical weathering. This process is responsible for many characteristics of the surface environment, impacting not only rivers and soils but also the atmosphere and oceans. Weathering is particularly important in setting the pace of global chemical cycles, including the cycle of carbon, because the solid Earth actually holds vastly more carbon, in the form of carbonate like limestone, than the atmosphere, oceans, and biosphere together. Because the pace of chemical weathering depends on climate, which in turn depends on atmospheric carbon concentrations, weathering is a key piece in the how the Earth system operates, and how it may be respond to human disturbance. It is often difficult to accurately measure weathering rates on today's Earth, and it is even more challenging to try to assess how these have changed over geologic time - making this a central question for today's scientists. One way of trying to get at past changes is to look at records in sediments, for example in lakes or the oceans. Looking at these records requires reliable and well-understood proxies, in other words, measurable characteristics that provide information about weathering processes in the past. This is not a new science; it has been a core part of geological research for decades, and the main outcome of previous work has been to demonstrate that it is difficult to interpret any one single proxy on its own. This realisation has stimulated efforts to develop multiple new proxies of chemical weathering, efforts that have been facilitated by technological advancements allowing improved measurement of the isotopic ratios of key elements involved in weathering reactions. Two elements of particular interest are silicon (Si) and lithium (Li). These are relevant because they are released when minerals in fresh rocks dissolve, but also partially retained in clays that are created during soil formation. During both processes, different isotopes of Si and Li are preferentially released into the waters that drain into streams and rivers. As a result, the isotopic composition of these rivers, and of sediments that may eventually form from them, potentially provides information about the relative rates of chemical reaction and physical removal of material (because this removal inhibits soil formation). This balance is referred to as the 'weathering intensity.' Several years of work, in the labs run by the partners in this study as well as other institutions internationally, have helped to begin quantitatively understanding how the Si and Li isotopic systems behave, and particularly how they respond to weathering intensity in natural settings. This provides a tantalizing glimpse of their potential. However there is a major gap in the present picture, because their behaviour at very high weathering intensities, when soils are thick in tropical environments, is not understood. The research that is proposed in this study will try to fill this gap by measuring the Si and Li isotopic composition of streams draining tropical catchments in Cameroon and Costa Rica. Working at two different sites will make it possible to compare effects on different types of rock. In addition to measuring these isotopic compositions, the proposed work will involve measurements and lab experiments designed to understand the mechanism generating these compositions, in other words, what chemical processes are responsible and how these differ from processes identified in other settings. Finally, the results of the project will be used to make a better quantitative model that may allow application of Si and Li isotopes as proxies for chemical weathering intensity.