The global Cd/phosphate relationship in deep ocean waters and the need for accuracy

Abstract

The relationship between Cd and phosphate is intriguing for unravelling geochemical cycling in the modern ocean. Moreover it is relevant for reconstructing Cd or phosphate distributions in paleoceanography. Whether the global Cd/phosphate data indicate a single linear relationship or whether the data are better described by two different linear relationships, one for the North Atlantic Ocean data and one for the Indo-Pacific Ocean data, is still a matter of debate. Recently, new data have become available for the Southern hemisphere which partly fill a gap that existed between the North Atlantic and Indo-Pacific Ocean data. The model with two different relationships for the North Atlantic and the Indo-Pacific Ocean deviated with statistical significance from a single global linear relationship. Yet, both models are characterized by a large residual variance likely due to apparent differences in accuracy between different laboratories. If these laboratory effects are taken into account in the models, the difference between the relationships of the North Atlantic and the Indo-Pacific Ocean remains statistically significant. However, when only two data sets with the smallest phosphate concentrations (PO4 < 1 μM) are left out the significance disappears. However, it is still risky to be too conclusive on basis of the present data. More accurate Cd- and phosphate-data are needed, especially in the low concentration area. Accuracy become be improved by the availability and use of international reference standards in combination with regular spike-recovery experiments.