The Open University: 2011 PhD studentship: Origin and significance of high-Mg calcite for the global carbon budget

1. Project Title
Origin and significance of high-Mg calcite for the global carbon budget

2. List of supervisors and their role in the project
Pallavi Anand (lead supervisor) – chemical analysis techniques and carbonate abundance
Neil Edwards- modelling technique for global calcium carbonate budget calculations
Charles Cockell (University of Edinburgh)- molecular training
Maureen Conte (Bermuda Institute of Ocean Sciences, USA)- Sediment trap samples provider and training in sample collection technique

3. Aspects of the project

  • Field work in Bermuda to learn the sediment trap sampling technique and obtain samples for the project.
  • Training in chemical and molecular analyses techniques.
  • Training in computer modelling of global biogeochemical cycles.
  • Investigate role of high Mg calcite in the global carbon cycle.



4. Project description, objectives and training

Emission of fossil carbon to the atmosphere induces ocean acidification. This poses a threat to marine calcifying organisms that find it harder to grow their calcium carbonate shells in more acidic conditions. Calcium carbonate (CaCO3) exists in three forms, low-Mg calcite, high-Mg calcite and aragonite, but sedimentary deposits are dominated by low-Mg calcite, so this is the only form considered in global carbon cycle models. Of the other two forms, high-Mg calcite was traditionally believed to be produced only in benthic habitats [1]. However, the presence of fine size-fraction (<125µm) high-Mg calcite particles at depths of 500, 1500 and 3200 m in sediment traps moored in the open sea (~100km off Bermuda shelf) poses major questions regarding its origin and significance in the global carbon budget. Bryozoan [2] or bacterial origins [3] have been proposed, and high-Mg calcite has recently been detected in the gut of a certain species of fish [4]. In addition, the production and stability of high-Mg calcite may change in the water column and sediment due to ocean acidification. Understanding its role in the inorganic carbon cycle in the past and future is therefore vital. The main objectives of the project are:

  1. To quantify the nature and origin of high-Mg calcite: firstly by analysis of the detailed chemical composition of sediment-trap samples, core-top samples (from Ocean Drilling Programme sites) and benthic carbonates (calcite particles, fish teeth) using X-ray diffraction and oxygen and carbon isotopic composition. Second, molecular analyses (DNA and lipid) will be applied to sediment-trap and carbonate samples.
  2. To quantify the significance of the high-Mg calcite fraction for the global carbon cycle: a simplified 3D model of the global carbon cycle (GENIE) will be used to test hypotheses concerning the role and significance of high-Mg calcite formation.

The balance of the work may be shifted towards either the analytical geochemistry or modelling components, depending upon the background and expertise of the successful candidate. The successful candidate will join an active research group specialising in geochemistry, microbiology and computer modelling at the Open University where they will receive specialist training in relevant analytical techniques, as well as in oral and written presentation skills. In addition, they will take part in international Ocean Acidification programmes (e.g., European Project on OCean Acidification (EPOCA), Biological Impact of Ocean ACIDification (BIOACID), UK Ocean Acidification Research Programme (UKOARP)).

[1] J. D. Milliman, Marine carbonates. Springer. New York. 375 (1974), [2] V. J. Fabry and W. G. Deuser, Deep Sea Research, 38, 713 (1991), [3] J. B. Thompson et al., Limnol. Oceanogr., 42(I), 133 (1997), [4] R. W. Wilson et al., Science, 323, 359 (2009).

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