An Investigation of Rare Earth Element Patterns and an Application of Using Zn and Cd Isotope Ratios in Oysters to Identify Contamination Sources in an Estuary in Southern China

Description

Environmental monitoring and investigation of metal biogeochemical cycling has been
carried out in the Pearl River Estuary (PRE), an important and complex system in Southern
China. In this study, rare earth element (REE) patterns as well as isotope ratios (i.e., Zn and Cd)
were evaluated as tools to identify contamination sources in environmental compartments (i.e.,
water and suspended particles (SP)) as well as in oysters collected from estuarine sites. Results
show elevated concentrations (also called anomalies) of Pr, Nd, Dy and Ho, relative to other
REE elements, in water samples, potentially from REE recycling and other industrialized
activities in this area. Unlike water samples, no REE anomalies were found in SP or oysters,
suggesting that the dominate REE uptake pathway in oysters is from particles. Secondly, site to
site variations in Zn isotope ratios were found in water and SP, showing the complexity of the
source inputs in this area. Also, in estuarine locations, larger spatially differences in Zn isotope
ratios were found in water collected in wet season than those in dry season, which may due to
mixing of different source inputs under the water circulations in different seasons.
A series of laboratory experiments were conducted during which changes in Zn isotope
ratios were measured during uptake under varying salinity and Zn concentrations and during
depuration. Neither in vivo Zn transportation among the various tissues within the oysters nor
water exposure conditions (i.e., different salinities or Zn concentrations) caused Zn isotopic
fractionation in the oysters.
Cd and Zn isotope ratios were also determined in oysters obtained from the PRE. Large variations in Cd and Zn isotope ratios suggest that oysters were receiving contaminants from different input sources within the PRE. A consistent difference (approximately 0.67‰) was observed for Zn isotope ratios in oysters collected from the east side of the PRE compared to those from sampling locations on the western side of the PRE, suggesting different Zn sources in these two areas. Ultimately, by combining biogeochemistry with physiology, this study represents a first attempt to assess pollution status, monitor contaminants using oysters and model/identify contamination sources using both REEs and metal isotope ratios.
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