Leeds Centre for Crystallization

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PhD positions

Controls on the concentration of sulphate in complex natural carbonates

Dr Rob Newton, Prof Liane G. Benning, and Dr Bridget Wade (School of Earth and Environment)

Sulphate is incorporated into the mineral lattice of carbonate minerals in variable amounts during their formation (carbonate associated sulphate, CAS), and its concentration in the lattice is likely to be controlled by some combination of fluid/gas chemistry and temperature thus providing a potential record of important parameters such as ancient seawater chemistry, or pCO2. The geological sulphur cycle exerts a fundamental influence over the redox state of the Earth’s surface and one of the most popular tools of recent years used to understand the ancient sulphur cycle is the measurement of the CAS-sulphur isotope composition of limestones or the carbonate skeletons of individual organisms such as belemnites.

Initial work on the development of micro probe methods to analyse and map sulphate concentrations will be followed by the analysis of a wide range of natural samples to determine the factors that control the concentration of sulphate in carbonates.  This will entail significant method development and synthesis of carbonate minerals to use as standards. The project will utilize the extensive suites of whole rock, foraminifera and belemnite carbonate samples held in the School to establish the concentration and spatial distribution of sulphate in natural carbonate samples.  In addition wet chemical techniques to separate calcite and dolomite components will be used to measure their sulphate isotopic compositions.

Specific objectives would be:

  • The development of wet chemical and microprobe methods to analyse/map sulphate and Mg concentrations in carbonates at a range of scales and in a variety of sample types.  A key step for probe mapping will be the wet chemical synthesis of appropriate standard materials.

  • Employ chemical separation to measure sulphate isotopic composition of dolomite vs calcite.

  • Test the response of biogenic carbonate chemistry to changing seawater sulphate concentration and other factors such as pCO2 and temperature. This will be achieved using a suite of foraminifera and coccolithophore samples from the early Cenozoic to the present day. This time period is thought to bracket a major change in seawater sulphate concentration. Examples of specific avenues of investigation would be records of single foram species from tropical regions and studies of modern coccolithophore samples from a variety of temperature regimes.

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