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E414: Deep time phosphorus cycling: the petrological record (Lead Supervisor: Oliver Shorttle, Earth Sciences & Institute of Astronomy)

Supervisors: Oliver Shorttle (Earth Sciences & Institute of Astronomy), Claire Bucholz (Caltech) and Helen Williams (Earth Sciences)

Importance of the area of research:

Phosphorus is a key bioessential element: by comprising the backbone of DNA, RNA, and in forming ATP, life's core functions of replication and metabolism are dependent on phosphorus.  Life's need for phosphorus places it as the limiting nutrient in many present ecosystems, constraining primary productivity and thus the biosphere's significance in the carbon cycle.  Understanding the geological cycling of phosphorus is therefore fundamental for the emergence of life and the maintenance of planetary habitability.  However, records of phosphorus availability in deep time are notoriously difficult to reconstruct.  Phosphorus has only a single stable isotope, which dramatically limits our ability to constrain the processes that cycle it through surface reservoirs.   This leaves us reliant on bulk sedimentary phosphorus abundances, which may have been affected by diagenesis and alteration at any time since deposition.  The premise of this project is that the hard rock archive of metamorphic rocks and S-type granites may provide a complementary insight into phosphorous availability in the surface environment, one that mitigates some of the problems of the sedimentary archive.

Project summary:

This project will provide new insights into the behaviour of P during metamorphism and partial melting of metasediments.  Crystalline rocks derived from high pressure and temperature transformation of sediments provide a potentially unique window into Earth surface chemistry.  Whilst these geological archives are subject to the same uncertainties as the sedimentary record in terms of early-stage diagenetic alteration of sediment chemistry, the subsequent conversion of the sediment into a high temperature mineral assemblage provides the possibility of ‘locking in' primary geochemical characteristics against late-stage processes. By combining large datasets, undertaking new fieldwork, and performing new geochemical analyses, this project will interrogate the metamorphic, and igneous rock record for evidence of changes in the size of the marine phosphorus reservoir over Earth history.

What the student will do:

This project will comprise three main parts: 1) Developing and analysing a database of P-bearing mineral evolution over Earth history, tracing how changing environmental conditions have affected the mode of P sequestration in the sedimentary and igneous geologic record. 2) A focussed study will be undertaken to characterise P mobility during prograde metamorphism and partial melting of sedimentary rocks.  Fieldwork will be carried out in the central US and Canada to sample sedimentary rocks up grade and through to the point of their melting to identify phosphorus loss and gain to the system during these processes.  Samples will be subsequently analysed at in Cambridge and Caltech to characterise the bulk-rock and accessory phase chemistry.  The resulting chemistry will be combined with forward modelling of these processes to build a model for the interpretation of S-type granite P contents in general. 3) The student will be involved in large analytical campaign to characterise S-type granite chemistry over Earth history, interrogating the record for trends pertaining to evolution of the surface phosphorus reservoir.

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:

Planavsky, N.J., Rouxel, O.J., Bekker, A., Lalonde, S.V., Konhauser, K.L., Reinhard, C.T. & Lyons, T.W. 2010. The evolution of the marine phosphate reservoir. Nature, vol. 467, pp.1088-1090, DOI:10.1038/nature09485.

Ruttenburg, K.C. 2003. The global phosphorus cycle. Treatise on Geochemistry, Eds. Schlesinger, W., Holland, H.D. & Turekian K.K., vol. 8, pp.585-643, Elsevier.

Bea, F., Fershtater, G. & Corretgé, L.G. 1992. The geochemistry of phosphorus in granitic rocks and the effects of aluminium. Lithos, vol. 29, pp.43-56.

Follow this link to find out about applying for this project.

Other projects available from the Lead Supervisor can be viewed here.

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