A preliminary study of the distributions of Cd in the South China Sea

The water column distributions of dissolved cadmium and supporting parameters were determined during a cruise in the South China Sea in October, 2002. The investigation was focused on waters of the shelf and the central basin which have depths of <200 m and >2000 m, respectively. In the surface waters, concentrations of Cd were 0.07-0.95 and 0.05-0.15 nM for the shelf and central basin, respectively. The spatial distributions in the surface waters show that concentrations of Cd were relatively high in the Pearl River Estuary. In contrast, Cd was totally depleted in surface waters of the deep water basin. This is attributed to the riverine inputs from the East Asian continent in the shelf regions. The influence of riverine input is reduced by dilution process with waters containing low concentrations of Cd waters. In the central basin, vertical profiles of Cd show a nutrient-like distribution which is depleted in the surface, increases with depth to about 1000 m, and that is finally maintained within a narrow concentration range of 0.94-0.99 nM in the deep water. Here, the Cd/PO4 ratio was 0.369±0.011 nM Cd/μM PO4, indicating that the biogeochemical processes of Cd and PO4 are similar to that in marginal seas of the western Pacific Ocean. However, the concentration gradients in surface waters show a southward increase. The results indicate that the relative enrichment of nutrients and the nutricline uplifted sharply in surface waters of the southern SCS.