Lead supervisor: Ed Tipper, Earth Sciences
Co-supervisor: Neil Davies, Earth Sciences; Will McMahon, Earth Sciences
Brief summary:
Clays are a major archive of Earth history. Few studies combine the detailed field understanding with geochemistry to be able to fully exploit such an archive. This project will do exactly that over key areas of geological time.
Importance of the area of research concerned:
As the only tangible repository of ancient silicate weathering, mudrockis a key archive from which the deep time climatic, tectonic, oceanographic, and biological changes at the Earth surface can be reconstructed. As a reservoir, mudrock is a fundamental component of the global chemostat, regulating global element cycles including atmospheric CO2 by mediating cation release from the continents (i.e., clay mineral production through chemical weathering) and deposition. Our existing understanding in all of these fields is predicated upon the notion that the source-to-sink deposition of ancient mudrock is comparable to that of modern muds. However, there are few geochemical studies of well constrained samples from the geological past, where the field setting is understood from a source to sink perspective, and where potential changes in sediment provenance are accounted for. So far this has prevented us from answering first order questions such as did the evolution of land plants fundamentally change the sensitivity of the silicate weathering climate feedback; Earth’s thermostat through geological history.
Project summary :
The fundamental control on the chemistry of any sedimentary rock is the source region that is being eroded. The next major control is mineral sorting induced by physical transport processes. Finally, chemical weathering processes impart a chemical and isotopic signature on a sedimentary rock. Therefore, the reconstruction of any geochemical archive from sedimentary rocks requires a quantification of all these processes. A large archive of mudrocks from well constrained sources from across the world, dating from the latest Precambrian to Permian, provides an ideal opportunity to evaluate the potential changes in chemical composition throughout major evolutionary transitions in continental environments using well established tracers such as major element ratios, radiogenic isotope ratios and stable isotope ratios such as delta7Li.
What will the student do?:
Sample archive and new fieldwork
A large archive of mudrock samples from several global basins from different periods in geological time, with well understood sedimentalogical characteristics is held in Cambridge. There will be opportunities to collect further samples should the student desire.
Geochemical data
The student will measure the strontium and neodymium isotope ratios of subsets of these samples to determine the uniqueness of sediment provenance for each time period. The measurement of major elements, combined with careful use of mixing diagrams, will enable the mineralogical sorting and chemical weathering of samples to be quantified. Lithium isotope ratios will provide a unique characterisation of chemical weathering after provenance and sorting have been properly accounted for by the above techniques. Additionally, there may be opportunities to explore novel isotope ratios such as rubidium and barium isotopes to characterise the history of the clay minerals within the mudrocks.
Modelling and big picture
The data will be interpreted and modelled in the context of global biogeochemical cycles, in particular nutrient cycles and the carbon cycle through time.
References - references should provide further reading about the project:
Spencer, C.J., Davis, N.J., et al (2022), Composition of continental crust altered by the emergence of land plants, Nature Geoscience, vol.15, 735–740
Baronas, J.J., et al, 2020, Integrating suspended sediment flux in large alluvial river channels: Application of a synoptic Rouse‐based model to the Irrawaddy and Salween rivers, Journal of Geophysical Research: Earth Surface, vol. 29, pp. e2020JF005554
Tipper, E.T., et al, 2020, Global silicate weathering flux overestimated because of sediment–water cation exchange, PNAS, vol. 118, pp. e2016430118
Applying
You can find out about applying for this project on the Department of Earth Sciences page.