Zinc Isotope Study of Baseline Occurrence Patterns and Potential Contamination of Sediments from the Great Lakes Basin

The Laurentian Great Lakes are a globally unique freshwater resource, playing a pivotal role in public water supply and agriculture, transportation, hydroelectric power, and ecosystem functioning. However, the Great Lakes also have a long history of metal contamination, especially in so-called Areas-of-Concern near urban areas and industrial sites. One major anthropogenic contaminant in the Great Lakes is Zn, derived from point-sources such as metal mining, smelting, and chemical industries, as well as diffuse sources such as fertilizer application or urban runoff, in addition to natural inputs from atmospheric deposition and natural weathering. Disentangling the importance of these geogenic versus anthropogenic sources and processes is critical to improving our understanding of the cycling and environmental fate of Zn in the Great Lakes region.

We examined spatiotemporal variations in Zn concentrations and isotopic compositions, as well as bulk physicochemical and mineralogical properties, of a total of 72 surface sediments and sediment core samples across nearshore-to-offshore gradients and depositional environments with distinct limnological conditions spanning >50 years.

Our results reveal spatial variations in both Zn concentrations (18 to 580 mg/kg) and isotopic compositions across oligotrophic (Lake Huron) to mesotrophic (Lake Erie) environments. Interestingly, the intra-lake heterogeneity is comparable to or higher than the variability observed at the inter-lake (basin-scale) level, with no upstream-to-downstream accumulation being evident. The isotopic signatures of surface sediment, measured as δ⁶⁶Zn and ranging from -0.09‰ to +0.41‰, suggest a predominantly geogenic source for Zn. Furthermore, an examination of temporal trends in sediment cores from Lake Huron and Lake Erie indicates consistent metal concentrations and Zn isotopic signatures, implying minimal biogeochemical fractionation within the lakes themselves. Across all sediments and both lakes, metal concentrations and isotopes are positively but not very strongly (R²<0.49 overall) correlated to TOC, TN, and TP, but not to chlorophyll a.

In summary, our findings indicate that sedimentation plays a crucial role as a repository for metals in the Great Lakes, exerting significant influence on the distribution patterns of metals throughout the basin. Furthermore, the higher levels of metals at locations with historical contamination remain spatially constrained and do not seem to disrupt the interconnected system of the Great Lakes.