Predominant anthropogenic sources and rates of atmospheric mercury accumualtion in southern Ontario recorded by peat cores from three bogs:comparison with natural 'background' values (past 8000 years)

摘要

Peat cores form three bogs in southern Ontario provide a complete,quantitative record of net rates of atmospheric Hg accumulation since pre-industrial times.For comparison with modern values,a peat core extending back 8000 years was sued to quantify the natural variations in Hg fluxes for this region,and their dependence on cliamtic change and land use history.The net mercury accumulation rates were separated into "natural"a nd "excess" components by comparing the Hg/Br ratios of modern samples with the long-term,pre-anthropogenic average Hg/Br.The average background mercury accumulation rate during the pre-anthropogenic period (from 5700 years BC to 1470 AD) was 1.4+-1.0 mug m~(-2) per year (n=197).The beginning of Hg contamination from anthropogenic sources dates from AD 1475 at the Luther Bog,corresponding to biomass burning for agricultural activities by Nativer North Americans.During the late 17th and 18th centuries,deposition of anthropogenic Hg was at least equal to that of Hg from natural sources.Anthropogenic inputs of Hg to the bogs have domianted continuously since the beginning of the 19th century.The maximum Hg accumualtion rates decrease in the order Sifton Bog,in the City of London,Ontario (141 mug Hg m~(-2) per year),Luther Bog in an agricultural region (89 mug Hg m~(-2) per year),and Spruce Bog which is in a comparatively remote,forested region (54 mug Hg m~(-2) per year).Accurate are dating of recent peat samples using the bomb pulse curve of ~(14)C shows that the maximum rate of atmospheric Hg accumulation occurred during AD 1956 and 1959 at all sites.In these (modern) samples,the Hg concentration profiles resemble those of Pb,and element which is kneon to be immobile in peat bogs.The correlation between these two metals,together with sulfur,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.