Cadmium-phosphorus cycling at the subtropical convergence south of New Zealand

The water column distributions of cadmium (Cd) and phosphorus (P) in the region of the subtropical convergence (STC) south of New Zealand have been studied. Mixed layer Cd concentrations were extremely low throughout the study area, varying from 0.012 to 0.059 nmol kg−1. In subantarctic waters south of the STC, deep-water Cd concentrations were intermediate between those of the North Atlantic and the North Pacific oceans, consistent with deep-water circulation. Cd varied linearly with P with a slope of 0.35 × 10−3 mol Cd/mol P, but waters in the upper 1000 m were significantly depleted in Cd compared to temperate waters (zero-P intercept at − 0.23 nmol Cd kg−1). The lowest Cd concentrations were observed in subtropical waters north of the STC. Here the Cd-P slope was only 0.08 × 10−3 mol Cd/mol P, indicating that the Cd-P properties in this region are dominated by mixing of low-Cd, high-P subantarctic water across the STC. Freshwater sources of Cd and P were found to be negligible. This work implies that the northern region of the Southern Ocean plays a major role in the global Cd-P relationship. Two mechanisms are suggested: (1) the ventilation of low-Cd-P subantarctic water to intermediate depth through Antarctic Intermediate Water formation, and (2) the remineralisation of low-Cd-P detritus from biota produced in waters formed at the STC.