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Harold John Bradbury

Harold John Bradbury

Research Associate

Climate Change and Earth-Ocean-Atmosphere Systems

N330 - Downing Site

Office Phone: +44 (0) 1223 333440


The removal of carbon from the surface of the planet is a critical component of the long-term carbon cycle; this removal is through the deposition and subsequent burial of carbon-minerals in the ocean. However, what is deposited at the sediment-water interface is not representative of what is ultimately buried because marine sediments are a dynamic chemical and microbial reactor. The subsurface oxidation of organic carbon into dissolved inorganic carbon occurs mainly due to microbial sulfate reduction in anoxic sediments. This microbial sulfate reduction generates subsurface alkalinity,  and in theory leads to to the formation of authigenic carbonate. Modern studies pinpointing the important microbial processes for authigenic carbonate formation are key for our extrapolation of the importance of authigenic carbonate in the geological record.

Calcium isotopes in carbonate minerals are enriched in the lighter, 40Ca isotope relative to the fluid in which they precipitate.  As such, subsurface authigenic carbonate precipitation should result in an increase in the 44Ca isotope in the remaining sedimentary pore fluid. This is aided by the fact that aqueous calcium isotopes in organic-rich and carbonate-poor sediments remain out of isotopic equilibrium with carbonate minerals likely due to passivation of the mineral surface.  In theory, this means that high-resolution pore water studies should be able to isolate the depths of authigenic carbonate formation and the microbially mediated processes driving this formation. 

Research Interests

Biogeochemistry and Isotope Geochemistry

Key Publications

Bradbury, H.J., Vandeginste, V., John, C.M., 2015. Diagenesis of phosphatic hardgrounds in the Monterey Formation: A perspective from bulk and clumped isotope geochemistry. Geological Society of America Bulletin. September 2015, v. 127no. 9-10, p. 1453-1463

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