Zinc isotopes as tracers of anthropogenic sources and biogeochemical processes in contaminated mangroves

Abstract

Recent work has shown that variations in zinc (Zn) isotope ratios enable us to identify contamination sources in the terrestrial environment and uptake processes in higher plants. Here in this study, we demonstrate that this also holds true for mangrove forests, which play an important role in the biogeochemical cycling of metals in tropical coastal ecosystems and that are seriously threatened by anthropogenic pollution. To this end, we determined zinc concentration and isotope composition (expressed using the δ66Zn notation relative to the JMC 3-0749-L standard) in sediments and tree leaves collected from a mangrove close to Rio de Janeiro in Brazil. The δ66ZnJMC values of sediments vary between +0.36 and + 0.84‰ and fall on a mixing line between detrital terrestrial sources (characterized with δ66ZnJMC = +0.28 ± 0.12‰, 2σ) and metallurgical ore sources (δ66ZnJMC = +0.86‰ ±0.15‰, 2σ). Leaves of Laguncularia racemose, in contrast, showed δ66ZnJMC values ranging between +0.08 and + 0.23‰, suggesting that processes including uptake, translocation and bioavailability in the rhizosphere control the isotope composition of zinc in the mangrove plant.