Lead supervisor: Ed Tipper, Earth Sciences
Co-supervisor: Luke Bridgestock, University of St Andrews
Brief summary:
Most estimates of carbon consumption through weathering assume the conservative behaviour of cations in solution. We can now correct for the non-conservative behaviour of cations to make better estimates of the carbon cycle.
Importance of the area of research concerned:
Chemical weathering mediates Earth’s carbon cycle and hence global climate over geological timescales. Ca and Mg from silicate minerals are released to the solute phase during chemical weathering. This solute Ca and Mg subsequently gets buried as Ca and Mg carbonates in ocean basins transferring carbon from the atmosphere to the carbonate rock reservoir. This simple reaction is thought to provide the climatic feedback that has maintained Earth’s climate equable over geological history. Quantitative models of contemporary silicate weathering processes coupled to estimates of modern day carbon fluxes associated with silicate weathering are therefore fundamental to understanding Earth’s carbon cycle, and the feedbacks between the carbon
cycle, climate and chemical weathering. Deciphering exactly how silicate weathering reactions work and how we can trace them is therefore of fundamental importance to understanding climate feedbacks.
Project summary :
Recent work has shown that the cation exchange process may bias estimates of silicate weathering by up to 100% because they assume that cations behave in a conservative way during dissolution and transport. Cation exchange occurs rapidly as a chemical equilibrium develops between charged mineral surfaces and a water. One of the most important mineral groups which have charged surfaces are clays. These rapid reactions are well studied in soils and aquifers, but the scientific community working on river chemistry has largely neglected these reactions. Our work shows that once the cation exchange process is taken into account it changes significantly the chemistry of natural waters and the total amount of carbon consumption through chemical weathering, with major implications for climate feedbacks.
What will the student do?:
This project will determine the impact of cation exchange between clay minerals and river waters in the estuarine environment at the salt-fresh water interface. The main objectives will be achieved by a combination of laboratory experimental studies and work on natural samples. We have an archive of samples from the Mackenzie Delta in the Arctic circle, with potential opportunities for further fieldwork. The student will determine the magnitude of the release of cations from the suspended load exchange pool when rivers enter the ocean and Na replaces Ca, Mg and Sr on exchange sites. We will trace the effects of cation exchange using a suite of very novel isotope ratio tracers such as Ba, Rb and Sr isotope ratios.
References - references should provide further reading about the project:
Tipper E.T., et al. (2020) Global silicate weathering flux overestimated because of sediment–water cation exchangePNAS, https://doi.org/10.1073/pnas.2016430118
Lupker, M et al. 2016. Impact of sediment--seawater cation exchange on Himalayan chemical weathering fluxes, Earth Surface Dynamics, vol. 4, pp 675-684
Cerling, T. E., Pederson, B. L. & Von Damm, K. L. 1989. Sodium-calcium ion exchange in the weathering of shales: Implications for global weathering budgets, vol. 17 pp552-554.
Applying
You can find out about applying for this project on the Department of Earth Sciences page.