Modeling the past atmospheric deposition of mercury using natural archives

Historical records of mercury (Hg) accumulation in lake sediments and peat bogs are often used to estimate human impacts on the biogeochemical cycling of mercury. On the basis of studies of lake sediments, modern atmospheric mercury deposition rates are estimated to have increased by a factor of 3-5 compared to background values: i.e., from about 3-3.5 μg Hg m^-2 yr ^-1 to 10-20 μg Hg m^-2 yr^-1. However, recent studies of the historical mercury record in peat bogs suggest significantly higher increases (9-400 fold, median 40x), i.e., from about 0.6-1.7 μg Hg m^-2 yr^-1 to 8-184 μg Hg m^-2 yr^-1. We compared published data of background and modern mercury accumulation rates derived from globally distributed lake sediments and peat bogs and discuss reasons for the differences observed in absolute values and in the relative increase in the industrial age. Direct measurements of modern wet mercury deposition rates in remote areas are presently about 1-4 μg m^-2 yr^-1, but were possibly as high as 20 μg Hg m^-2 yr ^-1 during the 1980s. These values are closer to the estimates of past deposition determined from lake sediments, which suggests that modern mercury accumulation rates derived from peat bogs tend to overestimate deposition. We suggest that smearing of ²¹⁰Pb in the uppermost peat sections contributes to an underestimation of peat ages, which is the most important reason for the overestimation of mercury accumulation rates in many bogs. The lower background mercury accumulation rates in peat as compared to lake sediments we believe is the result of nonquantitative retention and loss of mercury during peat diagenesis. As many processes controlling time-resolved mercury accumulation in mires are still poorly understood, lake sediments appear to be the more reliable archive for estimating historical mercury accumulation rates.