Assessment of Cadmium Isotopes as a Paleoclimate Proxy

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It has long been known that the biological activity of the oceans is regulated by the availability of the major nutrient ions phosphate, nitrate, and silicate. More recently, however, it has been recognized that micronutrient trace elements also play an important role in limiting marine productivity. As biological activity draws down CO2 from the atmosphere, micronutrients play a significant role in regulating the Earth's carbon cycle and climate. This study focuses on the micronutrient element cadmium (Cd). The geochemistry of Cd in seawater has attracted significant interest for more than 30 years because its marine distribution mimics the distribution of the macronutrient phosphate. This correlation forms the basis for the application of foraminiferal Cd/Ca ratios as a paleoclimate proxy. The application of this proxy is hindered, however, by our limited understanding of the role and cycling of bioactive Cd in the oceans. A recent pilot investigation of the PI indicates that analyses of Cd stable isotope compositions are able to address such limitations. In particular, the pilot study was the first investigation to identify large Cd isotope variations in seawater, which primarily reflect isotope fractionation from biological uptake of dissolved seawater Cd. The strikingly systematic nature of the fractionations, provide new insights into the marine cycling of Cd and demonstrate that Cd isotopes may be a useful paleoclimate proxy. The present study will build on and verify the results of the pilot investigation. To this end, we will first acquire a significantly larger Cd isotope dataset for seawater. We will then carefully evaluate this new dataset to re-examine the conclusion, that combined analyses of seawater Cd contents and isotope compositions uniquely inform on climate-relevant processes, such as variations in marine productivity. Whilst the interpretation of the new analytical data is expected to be straightforward, we will also address this goal by expanding a currently available global ocean model of Cd cycling to Cd isotopes. This approach will allow us to assess whether the model can reproduce reasonable Cd concentrations and isotope distributions for the oceans, based on known processes and fractionation factors. Any discrepancies between the model results and data will thus help to identify deficiencies in our understanding of the processes that regulate marine Cd contents and isotope compositions. A confirmation of the hypothesis that combined Cd concentration and isotope measurements provide unique constraints on the cycling of Cd and other nutrients in the oceans would be exciting. Such a result provides a basis for the application of Cd isotopes as a paleonutrient proxy in climate research. For example, combined Cd/Ca and Cd isotope data for foraminifera from sediment cores could be used to investigate temporal changes in marine nutrient utilization and the upwelling of nutrient-rich water masses. Such studies are important because they allow an assessment of past changes in the marine carbon cycle and their effect on climate.