Evolution of seawater Mg/Ca ratios since the Cretaceous: New constraints from the carbonate 'clumped isotope' thermometer.

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The chemical evolution of the oceans is fundamentally tied to climate change and biological evolution throughout Earth history. Few methods exist for directly reconstructing the past ionic composition of seawater, and thus empirical constraints on seawater magnesium to calcium ratios (Mg/Ca) are very limited. We propose to develop a record of seawater Mg/Ca for the Cretaceous to Pleistocene by combining temperatures reconstructed using the new carbonate 'clumped isotope' thermometer with measurements of benthic foraminiferal Mg/Ca ratios. This record of seawater Mg/Ca will be used to test several hypotheses: 1) We will test whether the Mg/Ca ratio and [Ca2+] of the oceans were much more dynamic than previously thought, as some authors have suggested. We will for the first time be able to precisely determine the timing and magnitude of changes in seawater Mg/Ca, data which are required for both carbon cycle models and Mg/Ca-based paleotemperature reconstructions. 2) We will test whether a major driving force for evolutionary change in biomineralization of marine organisms was secular variation in seawater Mg/Ca. We will compare our record of seawater Mg/Ca with the skeletal mineralogy of dominant reef-builders and sediment producers through time.