Zinc isotopes in sediment core from urban and reference lakes across the United States

Zinc (Zn) contamination from energy production, tire wear, galvanized steel, and other anthropogenic sources is on the rise globally. Lake sediment cores can provide a historical record of this contamination, but we are often unable to identify unique sources of Zn in sediment core based on concentration data alone. We are evaluating the application of stable Zn isotopes for resolving historical Zn inputs in sediment cores collected from urban and reference lakes across the US. The δ⁶⁶Zn (⁶⁶Zn/⁶⁴Zn of the sample relative to the Johnson Matthey Zn standard batch 3-0749-L) from three lakes investigated to date (two urban and one reference lake) ranged from 0.00 to 0.30 per mil (± 0.06 2σ). For the two urban lakes, Zn concentrations ranged from 70 to 600 mg/Kg and increased over time (ages were estimated from cesium-137 dating). Zinc concentrations in the reference lake ranged from 60 to 160 mg/Kg and generally decreased over time from the late 1960s to top of core (2008). The trends in δ⁶⁶Zn in all the lakes generally correlated (or inversely correlated) with changes in Zn concentrations and even more so with Zn concentrations normalized to aluminum concentrations. This indicates that Zn isotopes are likely sensitive to changes in metal sources. Additionally, organic carbon increased in both urban lake cores from depth toward the sediment-water interface, while organic carbon decreased from depth toward the sediment-water interface in the reference lake. The changes in organic carbon concentrations were abrupt and appeared to correspond to abrupt changes in δ⁶⁶Zn for two of the lakes, indicating possible diagenetic effects. Despite this potential complication, our initial results are encouraging in that Zn isotopes appear to track metal sources and, therefore, have the potential to improve our understanding of the historical sources of Zn contamination in sediment core.