Anthropogenic contributions to atmospheric Hg, Pb and As accumulation recorded by peat cores from southern Greenland and Denmark dated using the 14C 'bomb pulse curve'

摘要

Mercury concentrations are clearly elevated in the surface and sub-surface layers of peat cores collected from a minerotrophic ("groundwater-fed") fen in southern Greenland (GL) and an ombrotrophic ("rainwater-fed") bog in Denmark (DK). Using C-14 to precisely date samples since ca. AD 1950 using the "atmospheric bomb pulse," the chronology of Hg accumulation in GL is remarkably similar to the bog in DK where Hg was supplied only by atmospheric deposition: this suggests not only that Hg has been supplied to the surface layers of the minerotrophic core (GL) primarily by atmospheric inputs, but also that the peat cores have preserved a consistent record of the changing rates of atmospheric Hg accumulation. The lowest Hg fluxes in the GL core (0.3 to 0.5 mug/m(2)/yr) were found in peats dating from AD 550 to AD 975, compared to the maximum of 164 mug/m(2)/yr in AD 1953. Atmospheric Hg accumulation rates have since declined, with the value for 1995 (14 mug/m(2)/yr) comparable to the value for 1995 obtained by published studies of atmospheric transport modelling (12 mug/m(2)/yr). The greatest rates of atmospheric Hg accumulation in the DK core are also found in the sample dating from AD 1953 and are comparable in magnitude (184 mug/m(2)/yr) to the GL core; again, the fluxes have since gone into strong decline. The accumulation rates recorded by the peat core for AD 1994 (14 mug/m(2)/yr) are also comparable to the value for 1995 obtained by atmospheric transport modelling (18 mug/m(2)/yr). Comparing the Pb/Ti and As/Ti ratios of the DK samples with the corresponding crustal ratios (or "natural background values" for preanthropogenic peat) shows that the samples dating from 1953 also contain the maximum concentration of "excess" Pb and As. The synchroneity of the enrichments of all three elements (Hg, Pb, and As) suggests a common source, with coal-burning the most likely candidate. Independent support for this interpretation was obtained from the Pb isotope data (Pb-206/Pb-207 = 1.1481 +/- 0.0002 in the leached fraction and 1. 1505 0.0002 in the residual fraction) which is too radiogenic to be explained in terms of gasoline lead alone, but compares well with values for U.K. coals. In contrast, the lowest values for Pb-206/Pb-207 in the DK profile (1.1370 +/- 0.0003 in the leached fraction and 1.1408 +/- 0.0003 in the residual fraction) are found in the sample dating from AD 1979: this shows that the maximum contribution of leaded gasoline occurred approximately 25 yr after the zenith in total anthropogenic Pb deposition. Copyright (C) 2003 Elsevier Ltd. [References: 87]