Different behaviour of platinum in the Indian and Pacific Oceans

Abstract

THERE is little known about the chemical behaviour of the platinum-group metals in the ocean. Here we have used cathodic stripping voltammetry1 to determine the concentration of dissolved platinum in the water column of the Indian Ocean. The range of Pt values obtained (0.2–1.6 pM) is similar to that found previously in the Pacific Ocean2–4. But unlike the earlier Pacific Ocean investigations which depicted platinum as having nutrient-like behaviour, our data show that in the Indian Ocean the element is scavenged down the water column and so is depleted with depth. This is consistent with the predominant oxidation state of platinum in oxygenated sea water being Pt(IV), which is also predicted from thermodynamic considerations and the association of platinum with manganese nodules in sediments2. The data show that platinum co-varies in the water column with dissolved manganese, the concentration of Pt being lower than Mn by a factor of about 600. But the residence time of Pt in the oceans is longer than that of Mn, as revealed by a Pt/Mn ratio in the Indian Ocean that is 300 times higher than in crustal rock.