Hydrochemical behaviors of Cd in paleoceans and a review of Cd/Ca ratio in foraminifera shells as a paleoeanographyic proxy

As an element sharing linier distribution relation with phosphorous in the ocean, cadmium has great potential to be a proxy for reconstruction of the concentration of labile nutrient during geological history. Further it could be used to trace the water source by calculating the isotope equilibrium caused by the water-atmosphere exchange when connected with the carbon isotope of foraminifera. Also, the concentration of cadmium, as a conservative attribute of water mass, remains considerably constant, thus its variation could record the changes in water mass mixing and the pattern of ocean circulation. The cadmium uptake during the crystallization of foraminifera makes its Cd/Ca ratio a reliable recorder of the concentration variability of cadmium in the sea over the tectonic, orbital and the millennial scales. Because the linear dependence between cadmium and phosphorous is not so perfect and the absorption ratio of cadmium in calcareous shells also changes through different circumstances, and the records has not been adequate for reference, challenge still remains for Cd/Ca as a reliable paleoenvironmental proxy.