LIMITATIONS TO THE QUANTITATIVE APPLICATION OF CD AS A PALEOCEANOGRAPHIC TRACER, BASED ON RESULTS OF A MULTI-BOX MODEL (MENU) AND STATISTICAL CONSIDERATIONS

The distribution of cadmium in the modern ocean gives important information about ocean circulation and nutrient distributions. As the Cd/Ca-ratio of foraminiferal shells is proportional to that of seawater, Cd/Ca-records of benthic foraminifera have been used to reconstruct the Glacial oceanic distributions of dissolved Cd and PO4 quantitatively. The results, in turn, have served as boundary conditions for ocean carbon cycle models. This quantitative reconstruction, however, requires that the present oceanic Cd-PO4 relationship and the distribution coefficient converting Cd/Ca-ratios into dissolved Cd and PO4 concentrations, remain constant through geological time. We have constructed a multi-box model (MENU: Metal-nutrients) to test various hypotheses about the biogeochemical mechanisms determining the modern oceanic Cd-PO4 relationship and its behaviour in time. The results indicate that the oceanic Cd-PO4 relationship may change in response to changing oceanic conditions, such as circulation, productivity changes and upwelling intensity, for example during a glacial period. In addition, we discuss the frequent omission in the literature of statistical considerations pertaining to regression analysis, resulting in an unduly optimistic estimate of the distribution coefficient, thereby neglecting uncertainties on the order of 30% or more. In combination with spatial variability of the distribution coefficient, this conceivably poses intrinsic restrictions to its use as a conversion factor. We conclude that, awaiting a significant reduction of the aforementioned uncertainties, inter-pretation of the sedimentary Cd/Ca record should be performed only qualitatively.