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Cambridge NERC Doctoral Training Partnerships

Graduate Research Opportunities
 

Lead Supervisor: Ed Tipper, Earth Sciences  Co-Lead Supervisor: Oli Shorttle, Earth Sciences

Importance of the area of research concerned: 
Earth is unique in the solar system for having a continental crust. The crust acts as a major reservoir of nutrients, fresh water, and economically essential ores; in forming mountain belts and carving up the oceans. The continents fundamentally alter Earth’s weather and long-term climate; and the continental crust is the oasis, in an where life evolved. Despite the significance of the continents, their creation, evolution, and composition remains controversial. While magmatism has been key in building the crust, its interactions with the hydrosphere and biosphere may also have contributed to its chemical evolution over geological time. Tracing the nature and extent of these interactions remains a major outstanding problem in Earth Sciences. However, new geochemical tools promise a way forward: Novel stable isotope ratios of magnesium and lithium, are fractionated during low temperature reactions, offering the potential to constrain water-rock interaction at a crustal scale. Applying these isotope systems to ancient rocks and modern sediments will allow us to identify how weathering has affected the composition of the continental crust over geological time.
Project summary : 
The crustal composition has likely evolved over time because of weathering. This project will quantify this over Earth history. This has previously been neglected, but could be a major control on crustal composition with implications for the evolution of the surficial environment and for the igneous processes which generate crust. The student will measure stable isotope ratios of Mg and Li on river sediments to characterise average crustal compositions and variability, whilst associated “model” ages will be determined from radiogenic isotope systems such as neodymium. The Li and Mg stable isotope systems are highly-sensitive to water rock interaction, such as weathering, and their fractionations are most prominent at low temperature. Therefore, when combined, these three isotope systems will enable the weathering-driven maturation of the crust to be constrained.
What will the student do?: 
One approach to characterize crustal compositions has been to use suspended sediments from the largest rivers in the world, known to be representative of the continental crust. As sediments undergo successive series of weathering events throughout geological history, their isotope ratios of elements fractionated by weathering become increasingly enriched or depleted. These isotopic fractionations would be expected to correlate both with time and chemical indices of weathering, permitting an assessment of how the continental composition may have been modified by weathering. The project is mostly lab based. The student will be trained in the methods of the stable isotope ratio measurement by state-of-the-art MC-ICP-MS and in the chemical preparation methods that are pre-requisite to this. The student will begin by measuring samples from our extensive suspended sediment collection though there is scope to collect additional samples via fieldwork if required. The student will collect a new elemental and isotopic dataset that will form the basis for the development of a series of numerical models.
References: 
Lee, C. T. A. et al. 2008. Regulating continent growth and composition by chemical weathering Proc. Nat. Acad. of Sciences vol 105 pp. 4981-4986.
Liu, X.M. and Rudnick, R. L. 2011. Constraints on continental crustal mass loss via chemical weathering using lithium and its isotopes Proc. Nat. Acad. of Sciences vol 108 pp. 20873-20880.
Valley, J.W., et al. 2005. 4.4 billion years of crustal maturation: oxygen isotope ratios of magmatic zircon. Contrib. to Min. Pet. vol 150 pp. 561-580.
Applying
You can find out about applying for this project on the Department of Earth Sciences page.