Global Contrasts Between Oceanic Cycling of Cadmium and Phosphate

Abstract

AbstractCadmium (Cd) is a trace metal whose distribution in the ocean bears a remarkable resemblance to the nutrient phosphate (PO43−). This resemblance has led to the use of Cd as a proxy for ocean nutrient cycling in paleoceanographic applications, but the processes governing the cycling of Cd in the modern ocean remain unclear. In this study, we use previously published Cd observations and an Artificial Neural Network to produce a dissolved Cd climatology that reproduces the observed subtle deviations between the Cd and distributions. We use the Cd and climatologies, along with an ocean circulation inverse model, to diagnose the biogeochemical sources and sinks of dissolved Cd and . Our calculations reveal that dissolved Cd, like , is removed in the surface ocean and has a source in the subsurface, consistent with the simultaneous incorporation of Cd and into sinking organic particles. However, there are also contrasts between the cycling of dissolved Cd and In particular, the surface export ratio varies 8‐fold across latitudes, reaching highest values in the iron‐limited sub‐Antarctic Southern Ocean. This depletes Cd relative to in the low‐latitude thermocline while adding excess Cd to deep waters by the regeneration of Cd‐enriched particles. Also, Cd tends to regenerate slightly deeper than in the subsurface ocean, and the regeneration ratio reaches a maximum at 700–1,500 m. These contrasts are responsible for a slight concavity in the relationship and should be considered when interpreting paleoceanographic Cd records.