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C319: Biogeochemical heavy metal (Cr, U) cycling in coastal wetlands: Insights from salt marsh sediments (Lead Supervisor: Sasha Turchyn, Earth Sciences)

Supervisors: Sasha Turchyn (Earth Sciences) and Anirban Basu (Royal Holloway, University of London)

Importance of the area of research:

Coastal wetlands are major sinks of heavy metals such as chromium (Cr) or uranium (U). In a changing environment, this sink may become a source and release potentially toxic forms of these metals. Biologically mediated reactions control the speciation of these key metals in the environment and this is often closely linked to solubility of these metals. The exact mechanisms and geochemical factors influencing oxidative Cr release in modern soils and terrestrial environments are not yet well understood. For example, the pathway for Cr oxidation was thought to be a slow process in the presence of oxygen catalysed by MnO2, however, a novel pathway of anoxic Cr oxidation by H2O2 (prevalent in coastal rainwater) has recently been discovered. Similarly, the factors contributing to the immobilization of U in coastal wetlands remain unclear. Therefore, biogeochemical processes in salt mash sediments that control the heavy metal cycling need to be quantified.

Project summary:

This project will address the need to understand and quantify biogeochemical pathways of metal cycling in coastal wetlands. It will use the isotopic fractionation of redox-sensitive (Cr and U) metals to understand biogeochemical activity in the salt marsh sediments. The concentration of the metals, their oxidation state and isotopic signature will be quantified from various pools (e.g., porewater, sediments). The project will determine the dominant pathways of metal cycling and the effects of varying reaction rates, competing reaction mechanisms, variations in fluid and solid chemistry, the effects of transport - all of which control the magnitude of isotopic fractionation and expression of isotopic fractionation.

What the student will do:

The student will obtain sediment from a salt marsh and measure Cr and U concentrations in sediments, porewater, and other operationally defined pools (e.g., adsorbed or complexed with organics) with depth and flow direction. The speciation of the metals will also be determined by spectroscopic methods. For all pools, the isotopic composition of Cr and U will be measured. By integrating the variations in concentrations and isotopic compositions, the student will develop a mechanistic model combining prevalent redox reactions and possible pathways for the observed distribution of metals and their isotopic compositions.

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:

Oze, C., Sleep, N. H., Coleman, R. G., Fendorf, S. Anoxic oxidation of chromium. Geology, 44 (7), 543-546 (2016).

Church, T.M., Sarin, M. M., Fleisher, M. Q., Ferdelman, T.G., Salt marshes: An important coastal sink for dissolved uranium. Geochimical et Cosmochimica Acta, 60 (20), 3879-3887 (1996).

Follow this link to find out about applying for this project

 

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