Thermokarst Disturbance Drives Concentration and Composition of Metals and Polycyclic Aromatic Compounds in Lakes of the Western Canadian Arctic

Abstract

When assessing the environmental impact of petroleum hydrocarbon exploitation, it can be challenging to differentiate anthropogenic from natural hydrocarbon sources. For example, areas underlain by permafrost may be affected by erosion of hydrocarbon-rich deposits from thermokarst activity, complicating environmental assessments of human impacts from petroleum extraction. Here we examined polycyclic aromatic compounds (PACs) and metals in sediment cores from lakes affected to varying degrees by thawing permafrost. We used a paired-lake design on lakes with pronounced shoreline retrogressive thaw slumps and compared them to nearby lakes in undisturbed (no retrogressive thaw slump) systems in the Mackenzie Delta uplands (Northwest Territories, Canada). Total organic carbon (TOC)-normalized concentrations of parent and alkylated PACs were higher in surface sediments of slump-affected lakes. Slump-affected lakes were also enriched in metals related to local shale-based, Quaternary deposits (e.g., Ca, Sr, and Mn) when compared to reference lakes where surficial materials were not exposed by thermokarst activity. Diagnostic ratios of specific PACs suggested that slump-affected lakes also had a greater influence from petroleum-based compounds, likely sourced from the local geology. Higher PAC concentrations and petrogenic composition were best explained as a combination of low TOC availability and increased inputs of previously bound hydrocarbons from the catchment due to permafrost erosion. Plain Language Summary We examined how permafrost thaw, in the form of large disturbances called slumps (a type of "landslide"), impacted lakes in the western Canadian Arctic, near the Mackenzie Delta. We used lake sediment records of organic molecules called polycyclic aromatic compounds, some of which are known to have negative environmental impacts, and metals to determine if thaw slumping introduced these materials into lakes. We show that lakes with thaw slumps have higher concentrations of these materials. The specific metals found in lakes with thaw slumps are associated with the type of geology in the area. The PACs elevated in lakes with thaw slumps are also those of geological origin. This suggests that thaw slumps may be an important source of potential contaminants in areas of permafrost thaw.