Influence of Southern Ocean waters on the cadmium–phosphate properties of the global ocean

Abstract

MEASUREMENTS of the cadmium content of marine CaCO₃ deposits, such as aragonitic corals¹ and foraminiferal shells^2,3, have been used to trace past ocean circulation² and as an indicator of labile nutrient concentrations⁴. In particular, studies of foraminiferal cadmium have provided insight into the oceanic processes controlling atmospheric CO₂ concentration in glacial times^5,6. The use of Cd/Ca ratios in CaCO₃ as a labile nutrient indicator is made possible by a relationship between seawater cadmium and phosphate content that is remarkably uniform throughout the present ocean (Fig.l). In deep waters of the open ocean, [Cd] at a given value of [PO³⁻₄] is consistent within about ±7%; However, there is a unusual kink in the relationship at [PO³⁻₄]≈ 1.3 μmol kg⁻¹ whose cause is not known but which has been suggested² to result from a slightly deeper regeneration cycle for Cd relative to PO³⁻₄. Points to the right of the kink correspond mainly to intermediate and deep waters of the North Pacific, whereas low PO³⁻₄ concentrations to the left of the kink represent mainly Atlantic and upper-ocean waters⁴. Waters of the Southern Ocean have a strong influence on the composition of the global ocean but few Cd–PO³⁻₄ measurements are available for this region. Here we present recent data for Cd and PO³⁻₄ in Southern Ocean waters, which suggest that the kink is created by the input of Cd-depleted Subantarctic waters into the intermediate waters of the global ocean.