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

Lead Research Organisation: University of Cambridge
Department Name: Earth Sciences

Abstract

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.

Planned Impact

The practical, industrial importance of mudrocks (e.g., as sources, reservoirs and seals for conventional and unconventional hydrocarbon resources; natural barriers for geological carbon storage and nuclear waste disposal; influence in groundwater transport pathways; source of metal resources), means that any recognition of a predicatable stratigraphic variation in their abundance will have significant impact outside of academia. Well-established dialogue and discussions of pathways to further engagement and collaboration have already been initiated with contacts in the oil industry, and the results of this project will clearly benefit a broad and receptive industrial audience.

We will expand public interest through outreach. Clearly a topic such as 'mudrock' appears at first glance to have little to commend it to public engagement activities. However, we will spin the innocuousness of such a 'dull' topic to good effect: our outreach will be focussed on explaining how something as inherently 'unexciting' as mudrock actually holds the key to our understanding of ancient climate, the evolution of life, and how mud itself acts as a global thermostat. Additionally, as understanding the evolution of life has enormous public interest, we will emphasise the importance of our research to this topic in all of our outreach strands. The ways in which life has induced knock-on changes to the Earth system have been the subject of popular science and journalistic articles that have covered the team's previous research ventures. The linkages and relevance to high profile topics such as the Anthropocene and planetary geology (see Academic Beneficiaries) will further broaden the public appeal of the research. We will raise awareness in Cambridge through display space in our in-house geological museum (Sedwick Museum of Geology, over 100,000 visitors a year), by framing results of our research in the context of its wider topical implications, with an emphasis on those techniques most readily identifiable with by the interested public (e.g., fieldwork blogs, videos, photographs). We will also promote the results of our work at Cambridge Science Week, where 45,000 visits were made by members of the public including large numbers of school children in 2015. We will actively engage with local museums and organisations in areas specific to our field sites (e.g., the Northwest Highlands Geopark, National Museum of Wales), in order to emphasise the global importance of local geological sites in regions outside of Cambridge.

Publications

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