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

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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.