The isotopic composition of Cadmium in the water column of the South China Sea

We determined the Cd isotopic composition of seawater and sinking particles collected in the deep basin of the northern South China Sea (SCS) to investigate the controlling mechanisms on the Cd isotopic composition in the water column. The isotopic composition in the water column decreased with depth, with epsilon Cd-114/110 values (epsilon Cd-114/110 = [(Cd-114/Cd-110)(sample)/(Cd-114/Cd-110)(JMC Cd Munster) - 1] x 10(4)) ranging from +8.7 to +9.9 in the top 80 m, from +4.6 to +5.5 between 100 and 150 m, decreasing from +5.5 to +3.6 at depths from 150 to 1000 m, and remaining at +3.4 +/- 0.5 from 1000 to 3500 m. The isotopic composition and concentrations of Cd observed in the deep waters of the SCS are similar to the values that were previously reported in the North Pacific Ocean. In the thermocline, the variations in the Cd isotopic composition and concentrations were consistent with the relative volumetric percentages of the subsurface water, the intermediate water, and the deep water in the water column, indicating that water mixing is the dominant process determining the isotopic composition in the thermocline. Comparable to the isotopic composition value in the seawater of the mixed layer, the epsilon Cd-114/110 in the sinking particles collected at 30 m was +9.3 +/- 0.9. Because our previous studies demonstrated that the particulate Cd was predominantly biogenic organic matter, the comparable isotopic composition between the surface seawater and the sinking particles indicates that net biological isotopic fractionation on Cd in the surface water was insignificant. The result indicates that phytoplankton do not necessarily take up relatively light Cd in the oceanic surface waters. It is necessary to directly and systematically investigate how marine phytoplankton fractionate Cd isotopes. (C) 2012 Elsevier Ltd. All rights reserved.