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Cambridge NERC Doctoral Training Partnerships

Graduate Research Opportunities
 

Lead Supervisor: Ed Tipper, Earth Sciences

Co-Supervisor: Oscar BransonEarth Sciences

Brief summary: 
Improving our records of seawater chemistry through Earth history by building a mechanistic understanding of isotope fractionation into carbonate minerals
Importance of the area of research concerned: 
Stable isotopes ratios such as those of Mg, Ca and Sr have emerged as major tracers of bio-physicochemical processes through Earth history. These tracers have been instrumental in revealing changes in the chemistry of seawater, informing us about the inputs and outputs of elements to the oceans through time. One major process that influences global biogeochemical cycles at a planetary scale is carbonate precipitation. Ca, Mg and Sr are all incorporated into carbonates which record the isotope ratios of those elements in seawater, creating an archive of seawater chemistry. However, these isotopes are fractionated during the precipitation of carbonate minerals, and attempts to reconstruct seawater isotope records rely on knowledge of the fractionation factors between seawater and the carbonate mineral. These fractionation factors are poorly understood, mineralogy dependent (e.g. calcite, aragonite), and biogenic and physical processes may cause them to deviate from thermodynamic equilibrium because of kinetic processes. A mechanistic understanding of the fractionation factors is essential to underpin the interpretation of carbonate-derived records of seawater chemistry.
Project summary : 
The purpose of this project is to refine the understanding of thermodynamic equilibrium and kinetic fractionation factors through laboratory experiments and modelling. The novel twist that will be used here to determine the degree of thermodynamic equilibrium is by adding isotopic spikes to the experiments for elements such as Ca, Mg and Sr and exploiting the so-called three isotope method to estimate isotopic fractionation factors at thermodynamic equilibrium. These equilibrium fractionation factors have proved controversial and difficult to determine previously. In addition this project will develop interpretative and mechanistic models of the data, building on the pioneering “transition state theory” models that consider precipitation as a forward and reverse process (the concept of equilibrium).
What will the student do?: 
The student will conduct a series of experiments in the labs at the Dept Earth Science to measure the exchange of ions between carbonate minerals and solutions. These experiments will be conducted at different rates, different environmental conditions and will last for different durations. These conditions will determine the extent of thermodynamic equilibrium precipitation processes versus kinetic processes which control isotopic fractionation factors for isotope ratios such as 26Mg/24Mg, 44Ca/42Ca, and 88Sr/86Sr. The isotopic fractionation factors will be measured using state-of-the-art MC-ICP-MS by measuring the evolution of the precipitation fluids through time, and calculating the difference between the fluid and the solid. The results will inform the development of transition state theory models which capture thermodynamic and kinetic effects on the isotopic fractionation factors of these elements. These models will explore both how fractionation varies with precipitation rate, and the major ion chemistry of the experimental solution. Once established, these models will be applied to re-examine records of seawater chemistry derived from the carbonate record.
References - references should provide further reading about the project: 
DePaolo, D. J. 2011. Surface kinetic model for isotopic and trace element fractionation during precipitation of calcite from aqueous solutions. Geochim. Cosmochim. Act., vol. 75, pp. 1039-1056, doi: 10.1016/j.gca.2010.11.020
Li et al, 2011. Exchange and fractionation of Mg isotopes between epsomite and saturated MgSO4 solution. Geochim. Cosmochim. Act., vol. 75, pp. 1814-1828, doi: 10.1016/j.gca.2011.01.023
Oelkers et al, 2019. The rapid resetting of the Ca isotopic signatures of calcite at ambient temperature during its congruent dissolution, precipitation, and at equilibrium. Chemical Geol., vol 512, pp. 1-10, https://doi.org/10.1016/j.chemgeo.2019.02.035
Applying
You can find out about applying for this project on the Department of Earth Sciences page.
Dr Oscar Branson
Dr Edward Thomas Tipper
Department of Earth Sciences Graduate Administrator