Continuous transects of cadmium, copper, and aluminium in surface waters of the Atlantic Ocean, 50°N to 50°S: correspondence and contrast with nutrient-like behaviour

Abstract

On three north-south transects across the Atlantic Ocean continuous profiles of Cd, Cu, and Al were measured in surface waters with ultra clean sampling and analytical procedures. Salinity, nutrients, and Al served as tracers to identify the characteristic oceanographic regimes sampled under the different circumstances encountered during three different seasons. The behaviour of the biodepleted element Cd and of the biointermediate element Cu was then investigated in relation to hydrography and nutrient cycling. Trace metals and nutrients are generally uncoupled in surface waters. A Cd/PO 4 relationship only exists at high concentration levels, which were reached in winter under continental influence and in the immediate proximity to intense upwelling. The preferential removal of Cd over phosphate downstream of upwelling regions was confirmed. The reverse situation was observed in the northeastern Atlantic, where Cd depletion occurred after that for phosphate. The difference is tentatively attributed to continental sources of Zn for which Cd substitutes. The continent and the shelves are the major source for Cu. Except in proximity to these sources, the latitudinal Cu distribution is very smooth with open- ocean values around 0.65 to 1 nmol/kg as a result of slow scavenging, probably related to organic complexation. The tropical Atlantic Ocean shows very distinct signatures of dissolved Al and silicate in low-salinity waters. These signatures can be used to trace the origin of freshwater to atmospheric or river/shelf inputs, which have very different AI/Si ratios. The Amazon River was identified as the probable source of a low-salinity zone sampled in October 1990 at 5–9°N, 28°W, with elevated nutrients, particulate organic C, Cu, and Cd values.