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 delta(66) Zn (relative to the JMC 3-0749L standard) were small, ranging from +0.07 parts per thousand to + 0.39 parts per thousand, but are nevertheless eight times the external reproducibility of 0.04 parts per thousand. 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 delta(66) Zn of +0.30 parts per thousand +/- 0.05 parts per thousand (2SD, n=5) and a shift takes place between 1945 and 1950 to 666 Zn values of +0.14 parts per thousand +/- 0.09 parts per thousand (2SD, n=7) in the 2nd half of the century. In order to understand this shift in delta Zn-66 we analyzed 32 ore-grade sphalerite (ZnS) samples from African, Australian and European origin. Together with 29 published delta Zn-66 values for ore-grade ZnS, we find remarkably homogeneous isotopic compositions, which when averaged for mining location gives delta Zn-66 of +0.16 parts per thousand +/- 0.20%. (2SD, n=10 mines, n = 61 analyses). Early 20th century Zn deposition with delta Zn-66 of +0.30 parts per thousand is significantly different (p < 0.001) from average ZnS delta Zn-66 of +0. 16 parts per thousand. 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. (c) 2008 Elsevier B.V. All rights reserved.