Cd isotopic composition in the suspended and sinking particles of the surface water of the South China Sea: The effects of biotic activities

- We determine Cd isotopic composition in plankton and sinking particles.
- Phytoplankton assimilate relatively light Cd isotopes in seawater.
- Zooplankton possess similar Cd isotopic composition as phytoplankton.
- Heterotrophic processes decompose relatively light Cd in sinking particles.
- Cd isotopic composition in sinking particles increase with depth.

We determined the Cd isotopic composition of size-fractionated phytoplankton, zooplankton and sinking particles collected in the top 160 m of a deep water time series station in the South China Sea (SCS) to investigate the fractionation effects of major biotic activities. The isotopic composition (εCd114/110=[(Cd114/Cd110)sample/(Cd114/Cd110)NIST3108−1]×104) ranged from −2.5 to +7.0, from −9.3 to +6.7, from −1.2 to +4.5, and from −0.2 to +3.2ε in the plankton of size fractions 10-60, 60-150, >150, and >100 μm, respectively. The composition in the two smallest fractions, dominated by phytoplankton, was lower than the ambient seawater value (+8 to +9ε), indicating that the phytoplankton preferentially take up relatively light Cd isotopes. The insignificant difference between the composition of phytoplankton and zooplankton suggests that the fractionation effect through zooplankton grazing may be relatively small. The composition in the sinking particles at 30 m, ranging from +8.1 to +12.8ε, was significantly heavier than the composition in the suspended particles and the composition increased with depth, ranging from +11.5 to +20.8ε at 100 m, and from +16.0 to +18.0ε at 160 m. The increasing isotopic value with depth indicates that the processes of microbial degradation and/or zooplankton repackaging preferentially decomposed relatively light Cd in the sinking particles. The comparable isotopic composition between sinking particles and dissolved Cd indicates that the fractionation effects of major biotic activities, including phytoplankton uptake, microbial degradation and zooplankton repackaging, take place at similar magnitudes and result in insignificant net biological fractionation effects of Cd in the mixed layer.