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C321: Calcium isotope exploration of insitu authigenic carbonates (Lead Supervisor: Sasha Turchyn, Earth Sciences)

Supervisor: Sascha Turchyn (Earth Sciences)

Importance of the area of research:

Authigenic carbonate has recently been invoked as a major sink in the global carbon cycle.  Authigenic carbonate forms in marine sediments when anaerobic respiration of organic carbon generates alkalinity; certain processes such as the anaerobic oxidation of methane are particularly important in this regard.  Calcium isotopes may be a novel tracer for authigenic carbonate formation because the lighter calcium isotope (40Ca) is preferentially incorporated into the carbonate mineral, leaving the heavier calcium isotope behind (44Ca).  Understanding the waxing and waning of the authigenic carbonate sink and its importance in the global climate system requires a geochemical fingerprint for authigenic carbonate so that it can be identified in the geological record.

Project summary:

The project will couple measurements of modern marine sediments with simulated microbial processes in flow through reactors in the laboratory to explore the incorporation of trace elements and the calcium isotope fractionation during insitu sedimentary authigenic carbonate precipitation.  Sedimentary grains are often coated with authigenic carbonate and the goal will be to explore the geochemical and specifically calcium isotope signature of this authigenic phase.  The extension of this will be to select and analyse authigenic carbonates from the geological record and to extrapolate the importance of this sink over geological time.

What the student will do:

The student will use existing and ordered marine sediments to develop an extraction method for the authigenic carbonate phase and interrogate its trace element and isotope geochemistry.  This will involve possible sieving and picking of carbonate and other grains and their leaching followed by geochemical analysis. This will be coupled with simulated sediment column work in flow through reactors with different grains but real seawater to see how the authigenic phase can be made as a function of microbial process and what pore water chemistry they preserve.  A final project will involve fieldwork to select and then analyse different authigenic carbonate phases in ancient rocks of varying geological ages.   Depending on the student’s interest there could be a modelling or other numerical component to the project (e.g. extrapolating the authigenic carbonate sink over geological time).

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:

Schrag D.P., Higgins J.A., Macdonald F.A., Johnston D.J. Authigenic Carbonate and the History of the Global Carbon cycle. Science Vol 339 (2013) DOI: 10.1126/science.1229578

Sun X. and Turchyn A.V., Significant contribution of authigenic carbonate to marine carbon burial. Nature Geoscience (2014) DOI: 10.1038/NGEO2070

Fantle M.S., and DePaolo D.J., Ca isotopes in carbonate sediment and pore fluid from ODP Site 807A: The Ca2+(aq)–calcite equilibrium fractionation factor and calcite recrystallization rates in Pleistocene sediments. Geochimica et Cosmochimica Acta Vol 71 (2007) DOI: 10.1016/j.gca.2007.03.006

Follow this link to find out about applying for this project

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