Historical variations in the isotopic composition of atmospheric zinc deposition from a zinc smelter

In order to test the usefulness of stable zinc isotopes as an atmospheric source tracer, we analyzed the zinc isotopic composition of two sediment cores, taken at 1 km distance of the former zinc smelter in Lommel, Belgium. The peat bog lake sediments accumulate mainly atmospheric particles, have high organic matter contents (12–60 wt.%), are anoxic and highly contaminated with heavy metals (up to 4.7 wt.% Zn, and 1.1 wt.% Pb) with a sulfide mineralogical control on mobility. Down core variations in δ66Zn (relative to the JMC 3-0749L standard) were small, ranging from + 0.07‰ to + 0.39‰, but are nevertheless eight times the external reproducibility of 0.04‰. Good agreement was found between the two cores, and despite 30% Zn mobilization to deeper layers, no evidence of associated diagenetic Zn isotope fractionation was found. Sediments deposited in the early 20th century have δ66Zn of + 0.30‰ ± 0.05‰ (2SD, n = 5) and a shift takes place between 1945 and 1950 to δ66Zn values of + 0.14‰ ± 0.09‰ (2SD, n = 7) in the 2nd half of the century. In order to understand this shift in δ66Zn we analyzed 32 ore-grade sphalerite (ZnS) samples from African, Australian and European origin. Together with 29 published δ66Zn values for ore-grade ZnS, we find remarkably homogeneous isotopic compositions, which when averaged for mining location gives δ66Zn of + 0.16‰ ± 0.20‰ (2SD, n = 10 mines, n = 61 analyses). Early 20th century Zn deposition with δ66Zn of +0.30‰ is significantly different (p < 0.001) from average ZnS δ66Zn of +0.16‰. We suggest that this reflects the presence of an atmospheric smelting residue (slag) component, enriched in the heavy Zn isotopes due to Rayleigh type fractionation during Zn refining.