Abstract:
Use of dated lake sediment cores to quantify metals deposition over time
Human activities have increased the release of metals into the atmosphere for millennia, with most marked increases occurring after the industrial revolution and attributable mainly to metal extraction, fossil fuel combustion, and waste incineration. Mercury (Hg) is one of the metal pollutants of greatest concern globally as it can be transported over large distances and after deposition to landscapes and waterbodies, it can be methylated to the more toxic form, methylmercury, which can be bioaccumulated and biomagnified through food webs. In the absence of long-term measurements, environmental proxies are the only methods to determine the magnitude of change from environmental disturbances. Dated lake sediment cores are one such proxy and are reliable for reconstructing changes in atmospheric metals deposition over time. As part of the Canadian federal government’s Climate Change and Air Pollutants Program (CCAP), we have collected ~100 dated lake sediment cores from across Canada, including nearby point sources and in remote areas, to examine long-term records of contaminant deposition. To generate quantitative depositional metals fluxes, we “calibrated” and corrected sediment derived values using a variety of methods including: 1. Comparison of sediment derived metals fluxes to those from the limited Canadian precipitation-monitoring network (CAPMoN) and, for mercury, to fluxes from Canada’s primary atmospheric model (GEM-MACH-Hg) 2. Development of correction factors using lithogenic elements or catchment:lake area ratios to account for catchment contributions, allowing reconstruction of purely atmospheric metals deposition over time. Using these methods, we show that dated lake sediment cores faithfully record metals deposition over time and can be used to quantifiably assess the effectiveness of regional and global emission reductions. For mercury, we also applied various statistical analyses to quantify recent (1990−2018) spatial and temporal trends in anthropogenic atmospheric Hg deposition. Temporal trend analysis shows significant synchronous decreasing trends in post-1990 anthropogenic Hg fluxes in western Canada in contrast to increasing trends in the east, with spatial patterns largely driven by longitude and proximity to known point source(s). Various other applications of dated lake sediment cores will be discussed, including analyses of a toxic high tech sector element, Tellurium, and analyses of multiple lake sediment cores from a single region to “scale up” measurements and estimate metals fluxes to the landscape over hundred year time spans.