Searching for direct evidence of the consequences of land plant evolution on silicate weathering and continental sediment retention

泥很重要！泥浆的产生（化学风化）和泥岩的沉积是地球作为一个行星如何运作的基础。化学风化作用通过将固体岩石转化为溶质而将大陆和海洋联系起来。在此过程中，它调节海洋的元素质量收支，并提供维持大陆和海洋生命的营养物质。化学风化作用也可以作为一个行星恒温器，在地质时期调节气候;硅酸盐风化作用为固体地球脱气的大气CO2提供了长期的汇。除了作为地球系统的一个组成部分，泥岩也是一个历史记录。泥岩作为地质信息库的重要性来自于这样一个事实，即它们主要由回收的硅酸盐风化材料形成：化学风化产生的沉积物破坏了暴露的陆地表面。这些材料的特性，特别是粘土矿物，与影响化学风化过程的因素高度协调，如气候，构造和生物进化。因此，泥岩是地球表面最重要和最具进化意义的岩石类型之一，在整个地球历史上，它既充当了档案馆，也充当了调节器。我们的建议取决于我们对地球泥岩记录的新观察：大约4亿年前，河流沉积物中保存的泥岩数量大幅增加。在这一转变之前，冲积层中的泥岩量通常不到沉积岩总量的2%，但在此之后，泥可以占任何给定河流沉积序列的95%。岩石记录性质的这种重大转变在地球历史上只发生过一次，并且发生在新进化的陆地植物开始绿化大陆的同时。我们的提议旨在进一步阐明地球历史上这一关键的进化篇章。在植物出现之前，河流系统中几乎没有保存下来的泥浆，那么泥浆最终是被冲到海洋或湖泊中，还是一开始就产生了更少的泥浆？陆地植物是否有助于促进化学风化和泥浆的产生？还是陆生植物的物理作用，用根茎将泥土束缚在土地上，这才是最重要的？通过结合沉积地质学、岩石学和地球化学技术，追踪3500- 300-Ma前不同环境中保存的泥浆记录，我们将回答上述所有问题，并改变对地球系统演化的认识。我们的地质研究的四个互补链是：1）地质实地考察：我们将访问七个不同的案例地点，选择，因为它们共同涵盖了深，浅海洋，冲积和湖泊沉积环境的沉积物，从1450-360 Ma。我们的实地考察将使我们能够建立关于植物进化前后古代环境中泥分布的概念模型。2)岩石学：通过评估我们现场样品的矿物学，我们将跟踪关键层段泥浆成分的变化。3)地球化学：通过观察我们样本的同位素记录，我们可以解决有关植物进化前后化学风化强度的问题。4)数据库分析：把全世界每一个已知的沉积岩单位的资料结合起来，从已知最古老的沉积物到大陆上植物茂盛时沉积的沉积物，我们将对地球表面的泥岩在其历史的头30亿年中的演变有一个强有力的全球图景。

Mud matters! The production of mud (chemical weathering) and deposition of mudrock is fundamental to how Earth works as a planet. Chemical weathering links the continents and oceans via the transformation of solid rock into solutes. In doing so it regulates the elemental mass budget of the oceans and provides the nutrients that sustain life both on the continents and oceans. Chemical weathering also acts as a planetary thermostat that regulates climate over geological time; with silicate weathering providing the long-term sink for atmospheric CO2 that is degassed by the solid Earth. As well as being an integral part of the functioning Earth system, mudrock also acts as a historical record. The importance of mudrocks as a repository of geological information arises from the fact that they are primarily formed of recycled silicate weathering materials: the sediment that is produced as chemical weathering breaks down exposed land surfaces. The characteristics of these materials, in particular clay minerals, are highly attuned to factors that influenced the chemical weathering process, such as climate, tectonics, and biological evolution. Mudrock is thus one of the most important and evolutionarily revealing rock types at the Earth's surface, and has acted as both as an archive and a regulator throughout the planet's history. Our proposal hinges on our new observation regarding Earth's mudrock record: that there is a massive upsurge in the amount of mudrock preserved in river-deposited sediments at around 400 million years ago. Before this shift, the amount of mudrock in alluvium was typically less than 2% of total sedimentary rock volume, but afterwards mud could account for up to 95% of any given river-deposited succession.This major shift in the nature of the rock record happened only once in Earth history, and occurs at the same time that newly-evolved land plants were beginning their greening of the continents. Our proposal aims to shed further light on this critical evolutionary chapter in Earth history. There was little mud preserved in river systems before plants, so did mud end up washed out into the marine realm, or lakes, or was there just less mud being produced in the first place? Did land plants help to promote chemical weathering and the production of mud? Or was it the physical effect of land plants, trapping and binding mud on land with their roots and stems, which was most important? By combining sedimentary geology, petrography, and geochemistry techniques to track the preserved record of mud through different environments from 3500- to 300-Ma ago, we will answer all the above questions and change the understanding of the evolution of the Earth system. Our four complimentary strands of geological research are: 1) Geological fieldwork: we will visit seven different case localities, chosen because together they encompass the deposits of deep and shallow marine, alluvial and lake sedimentary environments ranging from 1450-360 Ma. Our fieldwork will enable us to build conceptual models about the distribution of mud in ancient environments before and after plant evolution. 2) Petrology: by assessing the mineralogy of samples from our field sites we will track changes to the composition of mud through the key interval. 3) Geochemistry: by looking at the isotopic record of our samples, we can address questions regarding the intensity of chemical weathering before and after plant evolution. 4) Database analysis: by combining information from every single known sedimentary rock unit worldwide, from the oldest known sediment to those deposited when plants were in full flourish on the continents, we will have a strong global picture of the evolution of mudrock on the Earth surface during the first 3 billion years of its history.