Puzzling Zn isotopes in spruce tree-ring series

Zinc (Zn) isotopes have shown promise for investigating biogeochemical processes and tracing sources of Zn for environmental studies. This study, the first in its kind, investigates Zn isotopic ratios in tree rings and soils of two sites in the Northern Athabasca Oil Sands Region (NAOSR) of Alberta, Canada. To this end, we have developed an appropriate protocol to analyze tree-ring δ66Zn values by pooling year-equivalent tree rings of four individual trees for both sites. The results of combined tree-ring Zn concentrations show minimal variation in heartwood, with a statistically significant decrease after 1979 and 1986 for site 1 and site 2, respectively. For site 1, tree-ring δ66ZnJMC Lyon ratios vary between 0.83 ± 0.08 to 0.54 ± 0.07‰ with a statistically significant decrease from 1877 to 2008. For site 2, δ66ZnJMC Lyon ratios range from 0.78 ± 0.02 to 0.59 ± 0.07‰ with the lowest values obtained for the outermost ring closest to the bark. However, this site does not display statistically significant long-term trend. In comparison to the tree rings, adsorbed Zn within soil organic horizons is relatively enriched in heavy isotopes (δ66ZnJMC Lyon of 1.00 ± 0.20 to 1.12 ± 0.10 and 0.81 ± 0.10 to 0.86 ± 0.06‰ for sites 1 and 2, respectively). Tree-ring and soil organic horizons δ66Zn values are also substantially heavier than the value reported for adsorbed Zn in NAOSR tailing sands (δ66ZnJMC Lyon = 0.35 ± 0.06‰). The heavy signature in the organic horizons may be a product of Zn biogeochemical cycling through soil processes and uptake by trees, although Zn atmospheric deposition from the NAOSR cannot be discounted. On the other hand, tree physiological processes, particularly radial translocation, could have potentially influenced the studied tree-ring Zn concentration and isotopic characteristics. However, this mechanism has not received substantial research. The δ66Zn analyses in different wood components are required before its influence on δ66Zn values can be properly assessed and tree-ring δ66Zn can be used as environmental indicators.