Anomalous mercury isotopic compositions of fish and human hair in the Bolivian Amazon

Abstract : We report mercury (Hg) mass-dependent isotope fractionation (MDF) and mass-independent isotope fractionation (MIF) in hair samples of the Bolivian Esse Ejjas native people and in several tropical fish species that constitute their daily diet MDF with delta Hg-202 ranging from -0.40 to -0.92 parts per thousand for fish and +1.04 to +1.42 parts per thousand for hair was observed. Hair samples of native people with a fish-dominated diet are enriched by +2.0 0.2 parts per thousand in 6212 Hg relative to the fish consumed. Both odd Hg isotopes, Hg-199 and Hg-201, display MIF in fish (from -0.14 to +0.38 parts per thousand for Delta Hg-201 and from -0.09 to +0.55 parts per thousand for Delta Hg-199) and in hair (from +0.12 to +0.66 parts per thousand for Delta Hg-201 and from +0.14 to +0.81 parts per thousand for Delta Hg-199). No significant difference in MIF anomalies is observed between Hg in fish and in human hair, suggesting that the anomalies act as conservative source tracers between upper trophic levels of the tropical food chain, Fish Hg MIF anomalies are 10-fold lower than those published for fish species from midlatitude lakes. Grouping all Amazonian fish species per location shows that Delta Hg-199: Delta Hg-201 regression slopes for the clear water Itenez River basin (0.95 +/- 0.08) are significantly lower than those for the white water Beni River basin (1.28 +/- 0.12). Assuming that the observed MIF originates from aquatic photoreactions, we calculated limited photodemethylation of monomethylmercury (MMHg) in the Beni River floodplains and insignificant photodemethylation in the Itenez River floodplains. This is possibly related to lower residence times of MMHg in the Itenez compared to the Beni River floodplains. Finally, a significantly negative Delta Hg-201 of -0.14 parts per thousand in Beni River fish suggests that the inorganic Hg precursor to the MMHg that bioaccumulates up the food chain defines an ecosystem specific non-zero Delta Hg-201 baseline. Calculation of photodemethylation intensities from Hg or MMHg MIF, therefore, requires a baseline correction.
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L. Laffont, J. E. Sonke, Laurence Maurice, H. Hintelmann, Marc Pouilly, et al.. Anomalous mercury isotopic compositions of fish and human hair in the Bolivian Amazon. Environmental Science and Technology, American Chemical Society, 2009, 43 (23), pp.8985-8990. ⟨10.1021/es9019518⟩. ⟨ird-00452959⟩

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