Long-term records of atmospheric deposition of mercury in peat cores from Arctic, and comparison with bogs

Because of the strong tendency of mercury (Hg) to bioaccumulate in the food chain, one of the greatest challenges faced by environmental mercury research in the Arctic is to quantify the relative contribution of anthropogenic sources of mercury to the contamination of this environment, as anthropogenically elevated mercury deposition over the past 150 years in Arctic ecosystems is potentially a serious environmental problem. To determine the magnitude of this concern, it is necessary to quantify the natural “background” of atmospheric mercury deposition and its variation over a millennial-scale period of time. A second problem is to understand the role which is played by the unique climate of the Arctic on the deposition of atmospheric mercury.<br />Total mercury concentrations were determined in peat cores from the High Arctic of Canada, the Faroe Islands and southern Ontario. The cores were dated using 210Pb and 14C. The mercury concentrations were used to calculate rates of atmospheric mercury accumulation in the peat in order to quantify rates of atmospheric deposition of mercury in the Arctic. In addition, rates of atmospheric mercury accumulation in the Arctic were compared with peat cores from the temperate zone of North America (southern Ontario).<br /><br />Mercury concentration measurements and age dating of two peat hummocks from Bathurst Island, Nunavut indicate rather constant natural “background” mercury flux of ca. 1 µg m-2 per year from 5900 to 800 calibrated year BP. The values are well within the range of the Hg fluxes reported from other Arctic locations (Greenland and the Faroe Islands) but also by peat cores from southern Canada (Ontario) which provide a record of atmospheric Hg accumulation extending back to eight thousand years. Thus, pre-anthropogenic Hg fluxes in the Arctic were not significantly different from atmospheric Hg fluxes in the temperate zone. In pre-industrial times, therefore, the High Arctic was not more important as a sink for global atmospheric mercury than the temperate zone. Therefore, other processes have to be invoked as chief mechanism for transferring atmospheric Hg to the Arctic environment, possibly made more efficient in recent years by environmental changes, resulting in the mercury contamination of the Arctic food chain.<br /><br />The beginning of mercury contamination from anthropogenic sources dates from AD 1475 at the Luther Bog in southern Ontario, corresponding to biomass burning for agricultural activities by Native North Americans. During the late 17th and 18th centuries, deposition of anthropogenic mercury was at least equal to that of mercury from natural sources. Anthropogenic inputs of mercury to the bogs have dominated continuously since the beginning of the 19th century.<br />The records from southern Ontario and the Faroe Islands show similar chronologies to one another, with mercury accumulation rates peaking in the 1950`s with 54 to 141 and 34 µg m-2 per year respectively. At both locations, in these (modern) samples, the Hg concentration profiles resemble those of lead (Pb), an element which is known to be immobile in peat bogs. The correlation between these two metals suggests that the predominant anthropogenic source of Hg (and Pb) was coal burning. While Hg accumulation rates have gone into strong decline since the late 1950`s, Hg deposition rates today still exceed the average natural background values by 7 to 13 times in southern Ontario. The increase in atmospheric mercury rates since the pre-industrial times in all these records is larger than the predicted by current global mercury cycling models.