Cadmium isotopic fractionation in the South China Sea: the roles of biotic and abiotic processes
Date Issued
2014
Date
2014
Author(s)
Yang, Shun-Chung
Abstract
In this study, we determined the Cd isotopic composition in seawater, suspended plankton, and sinking particles of the South China Sea (SCS) to better understand the relative contributions of biotic and abiotic processes that control Cd cycling in the ocean. The isotopic composition in the water column decreased with depth, with ε114/110CdNIST values ranging from +8 to +9 in the top 80 m, +4 to +5 between 100 and 150 m, decreasing from +5 to +3 at depths from 150 to 1000 m, and remaining at around +3 from 1000 to 3500 m. The isotopic composition in seawater below 150 m varies conservatively, indicating that the Cd cycling is mainly controlled by advection and mixing of water masses. Comparable to the isotopic composition value in surface seawater, the ε114/110CdNIST in the sinking particles collected at 30 m ranged from +8 to +9 suggesting that the net biological isotopic fractionation in the surface water is insignificant. In the SCS, aerosol depositions are known to be the dominant source of the Cd in the surface water. Relatively heavy Cd isotopic composition can be found in the seawater and particles, which suggests that the Cd originating from aerosols is composed of similarly heavy composition. This finding indicates that the heavy Cd isotopic composition in oceanic surface water cannot necessarily be attributed to biological fractionation. The isotopic composition of Cd in suspended and sinking particles further sheds light on how the major biotic activities, including phytoplankton uptake, microbial degradation, and zooplankton grazing, control the net Cd isotopic fractionation in the surface water. The ε114/110CdNIST in the suspended plankton of the euphotic zone ranged from -9 to +7, which is significantly lighter than the ambient seawater. The isotopic difference between plankton and seawater indicates that plankton preferentially assimilate light Cd isotopes from seawater. Additionally, the composition of the plankton was also lighter than the composition of the sinking particles at 30 m. Because microbial degradation and zooplankton grazing are the major processes controlling the transformation from suspended to sinking particles, the isotopic difference between suspended and sinking particles indicates that the processes disseminate relatively light Cd through the transformation. The ε114/110CdNIST in the sinking particles increased with depth, ranging from +8 to +13 at 30 m, from +12 to +21 at 100 m, and from +16 to +18 at 160 m. The increasing isotopic composition with depth indicates that microbial degradation and zooplankton repackaging also disseminate relatively light Cd from biogenic particles during particle sinking. Consequently, the Cd isotopic fractionations via phytoplankton uptake, microbial degradation, and zooplankton repackaging take place at similar magnitudes. The combined effects of these processes play critical roles in regulating the isotopic composition of dissolved and particulate Cd in the surface water. It is traditionally believed that the dissolved Cd in deep waters predominantly originates from the biogenic sinking particles generated in surface waters. However, our study has found that the isotopic composition of Cd in the sinking particles in the SCS surface waters is ≥ +8 ε, which is not comparable to the composition in global deep waters, which is ~ +3 ε. The isotopic difference indicates that the sinking particles exported from surface waters may not serve as the direct source of Cd in deep waters. Future studies should focus on the Cd isotopic composition in suspended and sinking particles at various water depths to determine the source of Cd in deep waters and decipher the interactions of Cd cycling between surface and deep waters.
Subjects
cadmium
trace metal cycling
stable isotope fractionation
biogeochemistry
South China Sea
Type
thesis
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