Postdoctoral Research Associate
Department of Earth Sciences
University of Cambridge

Title: “Carbonate hardground cements- using geochemical tools to reconstruct Phanerozoic ocean chemistry”

Abstract: Systematic changes in seawater chemistry over the Phanerozoic have been linked to changes in biomineralization and global climate change. These changes have been measured primarily in biogenic carbonate, evaporite minerals, or calcite veins in the oceanic crust. Each has its limitations. Authigenic carbonates, the inorganic precipitation of carbonate within the sediment, represents a new proxy for the reconstruction of seawater Mg/Ca ratios. Forming inorganically, they avoid complications of vital effects and form directly from seawater.

Carbonate hardgrounds represent the ideal medium to investigate this proxy, as its deposition at the sediment-water interface and rapid formation constrain its formational environment. Our results show that the most pristine authigenic carbonate within the carbonate hardgrounds records changes in seawater Mg/Ca throughout the Phanerozic, with low Mg/Ca in the Mesozoic and early Paleozoic, transitional values during the Carboniferous, and higher Mg/Ca in the modern ocean. These results point toward the importance of other variables, such as sulfate, in determining the predominant calcite form through the Phanerozoic.