Processes controlling Cadmium isotopic fractionation in the surface water of the northern South China Sea

We have determined Cd isotopic composition among various Cd pools, including aerosols, dissolved Cd, phytoplankton, zooplankton, and sinking particles, to investigate the major biogeochemical processes controlling Cd isotopic fractionation in the euphotic zone of the northern South China Sea. We found that the composition in aerosols, dissolved Cd, and sinking particles were all comparable and relatively heavy in the surface water, supporting that aerosol deposition is the major source of both dissolved Cd and sinking particles in the surface water. The isotopic composition in phytoplankton was distinctly lighter than dissolved Cd, validating that phytoplankton prefer to take up relatively light Cd isotopes. In addition, there was no significant isotopic difference between phytoplankton and zooplankton, suggesting that Cd isotopic fractionation effect through trophic transfer may be relatively small. The elevated isotopic composition in sinking particles with depths suggest that the remineralization of organic matter preferentially decompose relatively light Cd. The isotopic composition of sinking particles was also much heavier than plankton but comparable with dissolved Cd in the mixed layer. We propose that the fractionation effects through phytoplankton uptake and remineralization take place at similar magnitudes and result in insignificant net biological fractionation effects for Cd in the mixed layer.