Modeling the Past Atmospheric Deposition of Mercury Using Natural Archives

Abstract

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 microg Hg m(-2) yr(-1) to 10-20 microg 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 microg Hg m(-2) yr(-1) to 8-184 microg 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 microg m(-2) yr(-1), but were possibly as high as 20 microg 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.

Over three millennia of mercury pollution in the Peruvian Andes

“…3). This level of relative Hg enrichment is in agreement with the vast majority of sediment- core studies from remote lakes, which collectively suggest an average increase in global Hg deposition rates of 3-5x background values (10). In contrast, Hg flux ratios at both LY1 and LY2 in 1975 AD were 105 times background.…”

“…A growing number of cores from remote lakes suggest an approximately 3-fold increase in Hg deposition over the last Ϸ100-150 years (10,24). These records are predominantly from the northern hemisphere, and do not reveal any significant preindustrial Hg enrichment.…”

Polar firn air reveals large-scale impact of anthropogenic mercury emissions during the 1970s

“…From May 25 to May 31, 2006, we investigated GEM in the firn air at a remote location 10 km away from Summit Station, central Greenland (72.6°N, 38.5°W, 3200 m elevation). We sampled firn air using established methods (18,19,56) from one borehole at depths of 15,25,30,40,50,58,63,66,70,72,74,76,78, and 79.5 m. A 4-m long bladder was lowered into the borehole after drilling to the sampling depth and pressurized with air from the bottom of the hole, effectively sealing the hole. Two Dekabon (polyethylene/aluminum composite) lines were used to pump firn air from a space left immediately below the bladder.…”

“…The first direct measurements of total gaseous mercury (TGM, including GEM and divalent gaseous mercury species) were reported in the late 1970s during ship cruises across the Atlantic Ocean (7), but real-time atmospheric continuous monitoring at various European and Arctic locations was only initiated in the early 1990s (8). Lake sediment (9, 10), peat bog (11,12), and ice and snow (13,14) records have been powerful tools to reconstruct past evolution of atmospheric deposition of Hg 2ϩ species at specific locations, although their interpretation is still debated (15,16). These archives, however, do not inform directly about GEM, the major atmospheric Hg species.…”

“…In the Arctic, the record of atmospheric deposition of Hg 2ϩ species in lake sediments exhibits a similar global trend as atmospheric GEM concentrations inferred from Greenland firn: lake sediments show an increase in mercury deposition which parallels increasing industrialization (see ref. 15 for a critical review). However, deposition patterns differ among Arctic lakes, particularly within the last few decades where either increases or decreases in Hg accumulation rates have been reported in Alaska (52) as well as in Greenland (11).…”