Assessment of Cadmium Isotopes as a Paleoclimate Proxy

人们早就知道，海洋的生物活动是由主要营养离子磷酸盐、硝酸盐和硅酸盐的可用性来调节的。然而，最近人们认识到微量营养素微量元素在限制海洋生产力方面也起着重要作用。由于生物活动从大气中吸收二氧化碳，微量营养素在调节地球碳循环和气候方面发挥着重要作用。本研究的重点是微量元素镉（Cd）。30多年来，海水中镉的地球化学研究引起了人们极大的兴趣，因为它的海洋分布模拟了大量营养物质磷酸盐的分布。这种相关性构成了有孔虫Cd/Ca比值作为古气候代用指标的基础。然而，由于我们对生物活性Cd在海洋中的作用和循环的了解有限，这种替代方法的应用受到阻碍。最近对PI进行的一项试点调查表明，对Cd稳定同位素组成的分析能够解决这些限制。特别是，该试点研究首次确定了海水中Cd同位素的大变化，这主要反映了溶解海水Cd的生物吸收的同位素分馏。分馏的惊人系统性质为Cd的海洋循环提供了新的见解，并证明Cd同位素可能是一个有用的古气候指标。本研究将以试点调查的结果为基础并加以核实。为此，我们将首先获得一个更大的海水Cd同位素数据集。然后，我们将仔细评估这个新的数据集，以重新检验结论，即对海水Cd含量和同位素组成的综合分析可以独特地为气候相关过程提供信息，例如海洋生产力的变化。虽然对新的分析数据的解释预计会很简单，但我们也将通过将目前可用的Cd循环全球海洋模型扩展到Cd同位素来实现这一目标。这种方法将使我们能够评估该模型是否能够根据已知的过程和分馏因素再现海洋中合理的Cd浓度和同位素分布。因此，模式结果与数据之间的任何差异将有助于确定我们对调节海洋Cd含量和同位素组成的过程的理解中的不足之处。结合Cd浓度和同位素测量为海洋中Cd和其他营养物质的循环提供了独特的限制，这一假设得到证实将是令人兴奋的。这一结果为将Cd同位素作为古营养元素代用物在气候研究中的应用提供了依据。例如，沉积物岩心中有孔虫的Cd/Ca和Cd同位素组合数据可用于研究海洋养分利用的时间变化和富营养化水体的上涌。这些研究很重要，因为它们可以评估过去海洋碳循环的变化及其对气候的影响。

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.