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E316: Calcium isotopes in the oceanic crust, source, sink and significance (Lead Supervisor: Sasha Turchyn, Earth Sciences)

Supervisor: Sasha Turchyn (Earth Sciences)

Importance of the area of research:

It has long been thought that terrestrial silicate weathering is the main process that regulates the surface carbon cycle over million-year timescales but seafloor alteration has also been suggested as an important carbon cycle feedback.  During seafloor alteration cations are released, which increases alkalinity and drives the precipitation of calcium carbonate, removing carbon from Earth’s surface. Understanding the controls on this alteration, how far it persists off axis, and the importance for global chemical cycles, remains challenging.  Recent work in the Department of Earth Sciences has shown that the calcium isotope composition of carbonate veins can be used to asses timing and water-rock interaction in the ocean crust and the calcium isotope composition of altered basalt can be used to model fluid flow through mid ocean ridge systems.  Open questions remain as to how changing the major ion balance of the ocean will impact this water-rock reaction and the sink of carbon in the oceanic crust.

Project summary:

Novel metal isotope systems, such as calcium isotopes, can be used to understand the carbon budget within hydrothermal systems because the sink for carbon through carbonate vein formation is intimately linked to the calcium cycle. This project will expand on the previous work by measuring the calcium and other metal isotopes in a range of carbonate veins and altered oceanic crust with a more comprehensive model for water-rock interaction in the hydrothermal crust to understand the pervasiveness of off-axis chemical reaction and assess its importance in global geochemistry.

What the student will do:

A suite of samples is available in the Department, and more can be available through collaboration and through sampling the ODP repository. The student will select samples that span a range of ages, including ophiolites, and isolate altered basalt and carbonate veins for calcium isotope analysis.  Mixed mineralogy carbonate veins containing various polymorphs will also be examined to explore the influence of recrystallization of aragonite to calcite on the calcium isotope composition.  Mineral separates of the altered basalt will help with ascertaining the range of calcium isotope data for different individual minerals. A comparison from Cretaceous oceanic crust and Cretaceous aged ophiolites will help with our understanding of whether ophiolites are a good analog for the calcium cycle in hydrothermal systems and therefore whether older ophiolites could contribute to our understanding of the long term hydrothermal calcium cycle.  Numerical models for mass balance will be used to explore the sources and sinks within these systems.

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:

Amini M., et al Calcium Isotope Fractionation along Hydrothermal Pathways, Logatchev Field (Mid-Atlantic Ridge 14 45’N). Geochimica et Cosmochimica Acta, http://dx.doi.org/10.1016/j.gca.2008.05.055 (2008)

Turchyn et al., Reconstructing the oxygen isotope composition of late Cambrian and Cretaceous hydrothermal vent fluid. Geochimica et Cosmochimica Acta vol 123, pp 440-458 (2013)

Chen et al., Using Calcium Isotopic Composition of Calcium Carbonate Veins to Assess the Roles of Vein Formation and Seafloor Alteration in Carbon Cycle Regulation. Pending submission (available on request).

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