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C414: The clay mineral history of continental weathering (Lead Supervisor: Oliver Shorttle, Earth Sciences & Institute of Astronomy)

Supervisors: Oliver Shorttle (Earth Sciences & Institute of Astronomy), Neil Davies (Earth Sciences) and Ed Tipper (Earth Sciences)

Importance of the area of research:

Chemical weathering of the continents is a fundamental part of global geochemical cycles, releasing solutes to the biosphere and through its involvement in the carbon cycle forming a core part of Earth's climate thermostat.  However, there is a short circuit in the silicate weathering network: the formation of secondary clays on the continents and in the oceans ‘reverses' the weathering process, sequestering cations into aluminosilicate minerals and releasing CO2 back to the atmosphere.  Depending on where secondary clay formation occurs, cations may in this way be trapped from ever entering the oceans, thus altering ocean chemistry and limiting the supply of bioessential elements to marine life.  Recent research points towards large changes in clay mineral production on Earth's continents during the Palaeozoic, with the pre-vegetated world having substantially lower clay production rates compared with the modern-day.  These observations raise important questions about the efficiency of silicate weathering on the early Earth, and how continental run off influenced ocean chemistry in deep time.  Addressing these questions will be the core of this project.

Project summary:

This project will constrain the weathering processes operating on the early, pre-vegetated, Earth through geochemical and mineralogical study of mudrocks.  Mudrocks form a key archive of the climatic, tectonic and biological history of Earth, yet their detailed character is often overlooked.  The extent to which mudrocks comprise fines produced by mechanical erosion of the continents vs. secondary clay minerals, has major implications for past silicate weathering fluxes.  A complementary isotopic tracer of weathering is the Li isotopic composition of mudrocks, which experiences significant fractionation on clay mineral formation.  This project's focus on an Ediacaran-Cambrian age source-to-sink sedimentary environment will provide an ideal case study to exploit both these mineral and stable isotope proxies and characterise how a key geochemical cycle operated before the rise of plants.

What the student will do:

The project will comprise three main components: 1) Field work will be undertaken in the Series Rouge, a succession of Ediacaran-Cambrian strata in northern France and the Channel Islands, which is well characterised in terms of depositional environment.  The Series Rouge contains sedimentary rocks and basement ranging from weathered protolith, through alluvial, lacustrine, and marine sinks.  Samples collected across these facies will be used to characterise the chemical and mineral evolution of weathering products in different sub-environments of a pre-vegetation regime. 2) Detailed mineral analysis will be performed on the terrestrial and marine mudrock samples.  X-ray diffraction and scanning electron microscopy techniques will be used to identify clay mineralogy. This work is key for constraining the degree and locus of reverse weathering represented by palaeozoic mudrocks.  3) Li isotope analyses will be made on bulk rock and clay separates to constrain the paleo-weathering-intensity experienced by the sediments.  Isotopic fractionation of the sediments from their source rocks will more broadly provide insight into processes driving Li isotopic evolution of seawater.

Please contact the lead supervisor directly for further information relating to what the successful applicant will be expected to do, training to be provided, and any specific educational background requirements.

References:

Dosseto, A., Vigier, N., Joannes-Boyau, R., Moffat, I., Singh, T. & Srivastava, P. 2015. Rapid response of silicate weathering rates to climate change in the Himalaya. Geochemical Perspective Letters, vol. 1, pp.10-19, DOI: 10.7185/geochemlet.1502.

McMahon, W.J., Davies, N.S., Went, D.J., 2017, Negligible microbial matground influence on pre-vegetation river functioning: Evidence from the Ediacaran-Lower Cambrian Series Rouge, France.  Precambrian Research, vol. 292, pp. 13-34.

Tosca, N.J., Johnston, D.T., Mushegian, A., Rothman, D.H., Summons, R.E. & Knoll, A.H. 2010. Clay mineralogy, organic carbon burial, and redox evolution in Proterozoic oceans. Geochimica et Cosmochimica Acta, vol. 74, pp.1579-1592, DOI:10.1016/j.gca.2009.

Follow this link to find out about applying for this project.

Other projects available from the Lead Supervisor can be viewed here.

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