Comment on “The isotopic composition of cadmium in the water column of the South China Sea”

Yang et al. (2012) found that marine biogenic particles and corresponding surface seawater from the South China Sea were characterized by essentially identical Cd isotope compositions, and inferred that biological Cd isotope fractionation is insignificant at this location. Based on results obtained for a box model that represents Cd cycling in the surface layer, and using published data to constrain the Cd source and sink fluxes, we show that this conclusion is likely to be incorrect. The modeling results indicate that the heavy Cd isotope signature of ε114/110Cd ≈ +9 observed by Yang et al. (2012) for the surface South China Sea is most likely a consequence of substantial isotope fractionation (of about 4–6 ε) during uptake of dissolved seawater Cd by phytoplankton.