Continuous transects of Cd, Cu and Al in surface waters of the Atlantic Ocean (50°N to 50°S): Correspondence and contrast with nutrient-like behaviour



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Continuous transects of Cd, Cu and Al in surface waters of the Atlantic Ocean (50°N to 50°S):
Correspondence and contrast with nutrient-like behaviour.


M. Rutgers v.d. Loeff, E. Helmers, G. Kattner (1997)
Geochim. et Cosmochim. Acta 61(1): 47-61

On three north-south transects across the Atlantic Ocean continuous profiles of cadmium, copper and aluminium were measured in surface waters with ultra clean sampling and analytical procedures. Salinity, nutrients and aluminium 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 cadmium and of the biointermediate element copper was then investigated in relation to hydrography and nutrient cycling.

Trace metals and nutrients are generally uncoupled in surface waters. A Cd/PO4 relationship only exists at high concentration levels, which were reached in winter under continental influence and in the immediate vicinity of intense upwelling. The preferential removal of cadmium over phosphate downstream of upwelling regions was confirmed. In the NE Atlantic, cadmium is supplied by the European continent in excess to phosphate and is depleted later in the season than phosphate. These results imply that cadmium can only be used under very restricted circumstances as a proxy for paleo nutrient levels in surface waters. The continent and the shelves are the major source for copper. Except in the vicinity of these sources, the latitudinal copper 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. Under the influence of the Inter Tropical Convergence Zone (ITCZ) the tropical Atlantic shows very distinct signatures of dissolved aluminium and silicate. As the sources of freshwater have very different Al/Si ratios, these signatures can be used to trace the origin of the freshwater to atmospheric or river/shelf inputs. The Amazon was identified as the probable source of a low-salinity zone sampled in October at 5°-9°N, 28°W, with elevated nutrients, particulate organic carbon, copper and cadmium values.




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