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Cambridge NERC Doctoral Training Partnerships

Graduate Research Opportunities

Lead Supervisor: Helen Williams, Earth Sciences 

Co-Supervisor: Marie Edmonds, Earth Sciences

Importance of the area of research concerned: 
The Earth's climate is regulated by fluxes of carbon between its surface and interior. Carbon is cycled in the form of sedimentary organic carbon and/or carbonate into the Earth's mantle in subduction zones. For example, carbonates in sediments can be destabilised by fluid-rock reactions taking place during prograde metamorphism, which releases carbonate-bearing fluids to the overlying mantle. These fluids can then contribute to the carbon budgets of arc volcanoes and ultimately, to volcanic gases released into the atmosphere. However, it has also been argued that some of the carbon subducted in sediments is not lost from the slab and is recycled into the mantle, effectively “sequestering” this carbon and removing it from the atmosphere. It has also been shown that the carbon-bearing slab fluids can reprecipitate as graphite at lithological interfaces within the slab, or between the slab and mantle wedge. This process has enormous implications, as this carbon will be recycled into the mantle and not lost to the overriding plate. However it remains to be determined how global this process is, or what its impact on the efficiency on carbon fluxes through the last ~3Ga could be.
Project summary : 
The principal question this project will address is how much carbon is retained in the slab during subduction and buried in the deep mantle versus how much leaves the slab to contribute to the source region of arc lavas and volcanic gases. The project will use novel stable isotope systems in conjunction with fieldwork and petrology in areas such as southern Japan and the Alps to trace carbon-bearing fluids in subduction zones and understand the relationships between subduction zone age, thermal conditions, oxidation state and carbon transfer by slab-derived fluids. Models that link the efficiency of carbon loss from the slab to these parameters will be developed and used to understand how this could have influenced carbon fluxes on Earth through geologic time, and whether these are linked to major changes in surface chemistry such as the rise of atmospheric oxygen ~2.3Ga.
What will the student do?: 
This project presents an exciting opportunity for the student to work on a fundamental question in Earth Sciences: carbon cycling during subduction and the potential impact this has on the chemical evolution of our planet (e.g. Mason et al., 2017), with a particular focus on studying the efficiency of slab decarbonation (e.g. Galvez et al., 2013). The project will use state-of-the-art stable isotope systems as tracers of fluid-rock interaction (e.g. Pons et al., 2016) in conjunction with petrology and fieldwork in areas such as southern Japan and/or the Alps, where they will study and collect samples from carbon bearing metasomatic contacts between subducted ultramafic rocks and carbonate sediments. Depending on the student's interests, this project could expand to consider primitive arc magmas, and the degree to which their volatile contents are controlled by slab fluids, as a potential record of long-term carbon recycling into the deep mantle.
Galvez et al., Graphite formation by carbonate reduction during subduction, 2013. Nature Geoscience, v. 6 pp. 473, DOI: 10.1038/NGEO1827
Mason et al., Remobilization of crustal carbon may dominate volcanic arc emissions, 2017, Science. 2017 357(6348) pp. 290-294. DOI: 10.1126/science.aan5049.
Pons et al., Zinc isotope evidence for sulfate-rich fluid transfer across subduction zones, 2016 Nature Communications. 7:1379, DOI: 10.1038/ncomms13794
You can find out about applying for this project on the Department of Earth Sciences page.