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C342: The impact of cation exchange to estimates of chemical weathering fluxes (Lead Supervisor: Ed Tipper, Earth Sciences)

Supervisors: Ed Tipper (Earth Sciences), Sasha Turchyn (Earth Sciences), Mike Bickle (Earth Sciences), and Matt Fantle (Penn State University)

Importance of the area of research:

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.

Project summary:

Estimates of carbon consumption by silicate weathering could be wrong by up to 80% because of cation exchange processes that have not been considered in estimates of carbon consumption by silicate weathering. This is because Ca fluxes from silicate weathering are typically based on Na fluxes, assuming that Na has conservative behavior. It has been suggested that Na and Ca are strongly influenced by cation exchange processes, significantly biasing the quantification of silicate weathering.  This studentship will use stable Ca isotope ratios (both 40Ca/44Ca and 42Ca/44Ca) to quantify the degree of cation exchange in surface waters, and hence re-evaluate surface water geochemistry and silicate weathering carbon budgets.

What the student will do:

The fractionation of Ca isotopes associated with cation exchange will be determined experimentally.  This data will be used to re-evaluate the role of cation exchange on river waters by re-interpreting existing large data-sets of Ca stable isotope data, whilst generating new data sets from appropriate natural systems.  There are three primary research objectives:

1. Quantify cation exchange for Ca to and from different types of clay and determine the attendant Ca isotope fractionation factors,

2. Using a coupled approach between suspended sediments and river waters determine the link between the exchange pool and natural river water compositions,

3. Use existing Ca isotope data from large rivers to re-evaluate the role of ion-exchange reactions in mediating the composition of natural river waters.

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:

Cerling, T. E., et al.,. Sodium-calcium ion exchange in the weathering of shales: Implications for global weathering budgets, vol. 17 pp552-554 (1989)

Tipper, E. T., et al. Positive correlation between Li and Mg isotope ratios in the river waters of the Mackenzie Basin challenges the interpretation of apparent isotopic fractionation during weathering. Earth and Planetary Science Letters,  vol. 333-334, pp.35-40 (2012)

Tipper, E. T., Gaillardet, J., Galy, A., Louvat, P., Bickle, M. J., and Capmas, F., Calcium isotope ratios in the world’s largest rivers: A constraint on the maximum imbalance of oceanic calcium fluxes: Global Biogeochemical Cycles, v. 24, p. 13 (2010)

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