Reply to the "Comment by Murphy et al. (2014) on 'the isotopic composition of Cadmium in the water column of the South China Sea'"

Abstract Our previous study has shown that the Cd isotopic composition in the surface water and the sinking particles in the South China Sea (SCS) are identical at e 114/110 Cd ∼ +9. Therefore, net biological isotopic fractionation of Cd in the surface water was insignificant (Yang et al., 2012). Using a box-model calculation, Murphy et al. (2014) suggested that the particles should have relatively light eCd, +3 to +5, if the aerosol inputs were characterized with eCd from −4 to +4. Consequently, the observed +9 eCd is not representative to the overall SCS phytoplankton exports, and the heavy eCd observed in the surface water of the SCS must be the results of preferential uptake of light Cd by phytoplankton. Here we argue that their assumption about the aerosol eCd is most likely incorrect as in the case of SCS. In addition, it is questionable for the heavy eCd in the surface water reflecting solely by the preferential uptake of phytoplankton. Besides phytoplankton uptake, eCd in the surface water may also be influenced by microbial degradation and zooplankton grazing. Consequently, an integrated study focusing on the fractionation effects of the processes is necessary to fully understand the major controlling mechanisms on Cd isotopic fractionations in the oceans.