Hg Accumulation Rates Research Articles

Use of Br and Se in Peat To Reconstruct the Natural and Anthropogenic Fluxes of Atmospheric Hg: A 10000-Year Record from Caribou Bog, Maine

Using Br and Se as reference elements, the natural and anthropogenic fluxes of atmospheric Hg were reconstructed for the past 10,000 years using peat cores from Caribou Bog, ME. In the ombrotrophic peat layers, the average background Hg accumulation rate (AR) was 1.7 +/- 1.3 microg m(-2) year(-1) which is comparable with the natural rate of atmospheric Hg accumulation reported in other retrospective studies. The average Hg AR determined using all peat samples dating from preindustrial times, including minerotrophic peat, was slightly greater (3.1 +/- 2.3 microg m(-2) year(-1)) which may reflect differences in canopy interception due to the changes in plant communities, aquatic inputs, or possibly climatic factors. The maximum Hg AR (32 microg m(-2) year(-1)) occurred ca. 1961 A.D. In samples predating the settlement by Europeans, there is a linear correlation between the AR of Hg and those of Br and Se; this relationship allows both Br and Se to be used to calculate the natural AR of Hg (Hgnat). The difference between Hg AR and Hg(nat) is the Hg AR in excess of background (Hg(ex)). Because Hg(ex) was positive only after ca. 1840 A.D., it is assumed to represent the anthropogenic Hg component. By the late 19th century, Hg(ex) deposition was equal to the natural flux. At the peak in Hg deposition in 1961 A.D., Hgex made up >90% of total atmospheric Hg deposition. The AR in the uppermost peat decreased to 25% of peak values by 2000 A.D.

Accumulation rates of the heavy metals lead, mercury and cadmium in ombrotrophic peatlands in the west of Ireland

The vertical distributions of three heavy metals: Hg, Pb and Cd were determined in 3 cores sampled from two ombrotrophic bogs in the west of Ireland, one at Knockroe Co. Mayo, and the second at Letterfrack National Park, Co. Galway. Core chronologies were established using 210Pb dating techniques and were checked with fallout radionuclides from weapons testing. Variations were found in metal concentrations and cumulative inventories of each of the metals within each site and between the two sites. Maximum accumulation rates of the anthropogenically derived elements Hg, Pb, and Cd, were found in peat sediments dated between 1950 and 1970s at both sites. Pb and Hg accumulation rates are slightly lower than those found in similar studies from remote sites in Europe. Hg accumulation rates are fairly similar to those found in peatlands in America. Unlike the Pb and Hg concentration profiles, the Cd concentration profiles at the Letterfrack site were dominated by a surface enrichment, thought to be due to biological cycling of Cd in the peat. However Cd accumulation rates calculated at the Knockroe site are lower than those observed in Eastern Europe. Local meteorological conditions at the sites chosen for this study may account for the lower concentrations profiles observed when compared with some of the European studies. The similarity between the timing of the increase in metal accumulation rates in peat bogs in Northern America and this study could indicate that long range transportation of trace metals from Northern America may be occurring. Lead accumulations in the surface peat sediments (1993–1996) were between 1.5–3.0 mg m −2 yr −1 and 4–5 mg m −2 yr −1 at Knockroe and Letterfrack, respectively. Mercury accumulation rates for the same period at Knockroe were found to be between 6–11 μg m −2 yr −1, and between 19–24 μg m −2 yr −1 at Letterfrack. A greater variation in surface Cd accumulation rates was observed at both sites, with surface layer accumulation rates varying from 25 to 50 μg m −2 yr −1 at Knockroe and between 166 and 405 μg m −2 yr −1 at Letterfrack. Higher metal concentrations were found at the Letterfrack site, which are most likely due to local sources and the history of the site.

Changes in Concentrations of Major Elements and Trace Metals in Northeastern U.S.-Canadian Sub-Alpine Forest Floors

The forest floor at high elevation spruce-fir sites from southern Vermont, U.S. to the Gaspe Peninsula, Quebec, Canada was sampled and analyzed in 1979 and re-sampled and analyzed in 1996 to study temporal changes in the impacts of atmospheric pollutants. We determined organic matter mass, pH, and concentrations of Al, Ca, Fe, K, Mg, Na, Cd, Cu, Hg, Pb, and Zn for the litter (L = fresh litter plus Oi horizon) and fermentation plus humic horizons (F+H) (= Oe plus Oa horizons) of the forest floor. There were no trends for Al or Fe concentrations in the 1979 or 1996 L along the transect. Several sites had significantly lower Al and Fe values in 1996 than in 1979, likely indicating less mineral soil in the 1996 samples. The 1996 concentrations of Ca in L increased along the transect from 0.22% dry weight (dw) in Vermont to 0.60% dw in Quebec. Concentrations of Mg in L were relatively constant along the transect. Neither Ca nor Mg changed at sites from 1979 to 1996, indicating unchanged base status. Concentrations of Cd did not vary spatially along the transect but decreased at all sites from 1979 to 1996. Cu and Zn did not vary spatially or with time. In 1979, the concentration of Hg in L ranged between 150 and 300 μg kg−1 dw, with no spatial gradient. By 1996, Hg concentrations were 25 to 50% lower in L, with decreases generally proportional to the concentration in 1979. The concentration of Pb in 1979 L decreased significantly from 200 mg kg−1 dw in southern Vermont to 60 mg kg−1 dw in Quebec. By 1996, the Pb concentration in L ranged between 32 and 66 mg kg−1 dw with no spatial trend along the transect. Decreases in Pb concentrations at sites were proportional to the absolute value in 1979. The concentrations of Cd, Hg, and Pb have declined in litter from 1979 to 1996, indicating a decline in atmospheric deposition. Higher Hg and Pb accumulation rates to the southwest are suggested for the past as indicated by (F+H) concentrations and inventories of Hg and Pb. The decline of Pb in L is consistent with the decreased use of leaded gasoline starting in the 1970s; the declines in Cd and Hg probably reflect lower emissions over the same period. Declining concentrations of Cd, Hg, and Pb in L parallel those documented in recent lake and peat sediments in the northeastern United States.

Deconstruction of Historic Mercury Accumulation in Lake Sediments, Northeastern United States

Total atmospheric contribution of mercury (Hg(T)) to lake sediment was estimated using 210Pb-dated sediment cores. Algorithms based on estimates of lake and watershed processes were applied to more accurately assess anthropogenic contributions of Hg to the environment and Hg(T). Factors addressed include: lake-specific background accumulation rates of Hg (Hg(B)), variability of sediment accumulation rates that caused variation in Hg accumulation during the last 100-150 years (Hgv), and variable flux of anthropogenic Hg from the atmosphere (Hg(A)). These fluxes were normalized for sediment focusing using a regional, unsupported 210Pb correction factor to yield Hg(A,F). Time series maps of Hg(A,F) allow for comparison across time and space, and are provided for 1900, 1950, 1975 and 1990 across eastern New York and New England, USA. Deconstruction algorithms reduce inter-/intra-lake variability in Hg accumulation rates and improve temporal coherence. Hg(A,F) started to increase near the end of the 19th century to a maximum between 1970 and 1990, depending on the lake. Maximum Hg(T) across the region ranges from 27.1 to 175 microg/m2 year. Maximum Hg(A,F) ranged from 10.4 to 66.3 microg/m2 year. The timing of Hg(A,F) declines in response to decreased atmospheric deposition may be controlled by in-lake and in-watershed storage and transport of Hg-bearing sediment.

Accumulation rates and predominant atmospheric sources of natural and anthropogenic Hg and Pb on the Faroe Islands

A monolith representing 5420 14C yr of peat accumulation was collected from a blanket bog at Myrarnar, Faroe Islands. The maximum Hg concentration (498 ng/g at a depth of 4.5 cm) coincides with the maximum concentration of anthropogenic Pb (111 μg/g). Age dating of recent peat accumulation using 210Pb (CRS model) shows that the maxima in Hg and Pb concentrations occur at AD 1954 ± 2. These results, combined with the isotopic composition of Pb in that sample ( 206Pb/ 207Pb = 1.1720 ± 0.0017), suggest that coal burning was the dominant source of both elements. From the onset of peat accumulation (ca. 4286 BC) until AD 1385, the ratios Hg/Br and Hg/Se were constant (2.2 ± 0.5 × 10 -4 and 8.5 ± 1.8 × 10 -3, respectively). Since then, Hg/Br and Hg/Se values have increased, also reaching their maxima in AD 1954. The age date of the maximum concentrations of anthropogenic Hg and Pb in the Faroe Islands is consistent with a previous study of peat cores from Greenland and Denmark (dated using the atmospheric bomb pulse curve of 14C), which showed maximum concentrations in AD 1953. The average rate of atmospheric Hg accumulation from 1520 BC to AD 1385 was 1.27 ± 0.38 μg/m 2/yr. The Br and Se concentrations and the background Hg/Br and Hg/Se ratios were used to calculate the average rate of natural Hg accumulation for the same period, 1.32 ± 0.36 μg/m 2/yr and 1.34 ± 0.29 μg/m 2/yr, respectively. These fluxes are similar to the preanthropogenic rates obtained using peat cores from Switzerland, southern Greenland, southern Ontario, Canada, and the northeastern United States. Episodic volcanic emissions and the continual supply of marine aerosols to the Faroe Islands, therefore, have not contributed significantly to the Hg inventory or the Hg accumulation rates, relative to these other areas. The maximum rate of Hg accumulation was 34 μg/m 2/yr. The greatest fluxes of anthropogenic Hg accumulation calculated using Br and Se, respectively, were 26 and 31 μg/m 2/yr. The rate of atmospheric Hg accumulation in 1998 (16 μg/m 2/yr) is comparable to the values recently obtained by atmospheric transport modeling for Denmark, the Faroe Islands, and Greenland.

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”

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 14C 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 μg/m 2/yr) were found in peats dating from AD 550 to AD 975, compared to the maximum of 164 μg/m 2/yr in AD 1953. Atmospheric Hg accumulation rates have since declined, with the value for 1995 (14 μg/m 2/yr) comparable to the value for 1995 obtained by published studies of atmospheric transport modelling (12 μg/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 μg/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 μg/m 2/yr) are also comparable to the value for 1995 obtained by atmospheric transport modelling (18 μg/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 ( 206Pb/ 207Pb = 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 206Pb/ 207Pb 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.

A 6,000-years record of atmospheric mercury accumulation in the high Arctic from peat deposits on Bathurst Island, Nunavut, Canada

There is a growing interest in the atmospheric transport, deposition, and accumulation of anthropogenic Hg in the Arctic. To quantify the impact of industrial Hg emissions, the natural rate of atmospheric Hg accumulation must be known. Mercury concentration measurements and age dating of peat from the Canadian Arctic show that natural background Hg flux rather constant (ca. 1 microgram per sq. m per yr.) throughout the past 6000 years. Mercury concentrations in surface peat layers are much higher, but chronology of these changes cannot be interpreted until more age dates are available. The elevated Hg concentrations in surface layers, however, are out of proportion with Br and Se, suggesting that there has been a significant human impact. Peat cores from southern Canada provide a record of atmospheric Hg accumulation extending back nine thousand years, with similar background fluxes. Thus, pre-anthropagenic Hg fluxes in the High Arctic were not significantly different from atmospheric Hg fluxes in the temperate Zone.

Mercury Concentrations in the Food Web of Lake Malawi, East Africa

Little is known about the concentrations of mercury (Hg) and factors affecting this contaminant in tropical freshwater biota. Lake Malawi, an East African Rift Valley lake, is internationally renowned for having the highest diversity of fish species in the world, and the lake supports subsistence fisheries for the three riparian countries. In this study the concentrations of Hg in fish and invertebrates collected from the lake in 1996 and 1997 were examined, and tissue stable carbon (δ 13C) and nitrogen (δ 15N) isotopes were used to distinguish and contrast those consumers relying upon carbon fixed by either benthic or pelagic primary producers, and their trophic positioning. Concentrations of Hg were generally low (2 to 200 ng/g wet weight) in fish from Lake Malawi. In addition, pelagic fish contained significantly higher concentrations of Hg than the benthic species. As in temperate systems, log-transformed Hg concentrations were significantly predicted by δ 15N and the highest concentrations of Hg were found in the largest fish within each species. The rate of Hg accumulation was not significantly different between the pelagic and benthic fishes but pelagic species had significantly higher Hg concentrations at a common weight and trophic position. The slope of the Hg-δ 15N relationship in Lake Malawi was comparable to what has been found in temperate and arctic lakes, suggesting that Hg accumulation in freshwater food webs is independent of climatic factors and species composition.

Mercury contamination chronologies from Connecticut wetlands and Long Island Sound sediments

Sediment cores were used to investigate the mercury deposition histories of Connecticut and Long Island Sound. Most cores show background (pre-1800s) concentrations (50–100 ppb Hg) below 30–50 cm depth, strong enrichments up to 500 ppb Hg in the core tops with lower Hg concentrations in the surface sediments (200–300 ppb Hg). A sediment core from the Housatonic River has peak levels of 1,500 ppb Hg, indicating the presence of a Hg point source in this watershed. The Hg records were translated into Hg contamination chronologies through 210Pb dating. The onset of Hg contamination occurred in ~1840–1850 in eastern Connecticut, whereas in the Housatonic River the onset is dated at around 1820. The mercury accumulation profiles show periods of peak contamination at around 1900 and at 1950–1970. Peak Hg* (Hg*= Hg measured minus Hg background) accumulation rates in the salt marshes vary, dependent on the sediment character, between 8 and 44 ng Hg/cm2 per year, whereas modern Hg* accumulation rates range from 4–17 ng Hg/cm2 per year; time-averaged Hg* accumulation rates are 15 ng Hg/cm2 per year. These Hg* accumulation rates in sediments are higher than the observed Hg atmospheric deposition rates (about 1–2 ng Hg/cm2 per year), indicating that contaminant Hg from the watershed is focused into the coastal zone. The Long Island Sound cores show similar Hg profiles as the marsh cores, but time-averaged Hg* accumulation rates are higher than in the marshes (26 ng Hg/cm2 a year) because of the different sediment characteristics. In-situ atmospheric deposition of Hg in the marshes and in Long Island Sound is only a minor component of the total Hg budget. The 1900 peak of Hg contamination is most likely related to climatic factors (the wet period of the early 1900s) and the 1950–1970 peak was caused by strong anthropogenic Hg emissions at that time. Spatial trends in total Hg burdens in cores are largely related to sedimentary parameters (amount of clay) except for the high inventories of the Housatonic River, which are related to Hg releases from hat-making in the town of Danbury. Much of the contaminated sediment transport in the Housatonic River Basin occurs during floods, creating distinct layers of Hg-contaminated sediment in western Long Island Sound. The drop of about 40% in Hg accumulation rates between the 1960s and 1990s seems largely the result of reduced Hg emissions and to a much lesser extent of climatic factors.

Effect of peat decomposition and mass loss on historic mercury records in peat bogs from patagonia.

Ombrotrophic peat bogs have been widely used to evaluate long-term records of atmospheric mercury (Hg) deposition. One of the major aims of these investigations is the estimation of the increase in atmospheric Hg fluxes during the industrial age compared to preindustrial fluxes. Comparability of Hg accumulation rates calculated from density, peat accumulation rates, and Hg concentrations requires linearity between these parameters. Peat formation is a dynamic process accompanied by intense mass loss and alteration of the organic material. Our investigations on three peat cores from the Magellanic Moorlands, Chile, indicate that Hg concentrations in peat strongly depend on peat humification. Moreover, differences in mass accumulation rates during peat evolution are not compensated by linear changes in density, peat accumulation, or Hg concentrations. We suggest that Hg accumulation rates be normalized to carbon accumulation rates to achieve comparability of Hg accumulation rates derived from differently altered peat sections. Normalization to the carbon accumulation rates reduces Hg accumulation rates in less degraded peat sections in the upper peat layers by factors of more than 2. Our results suggest that the increase in Hg deposition rates during modern times derived from ombrotrophic peat bogs are potentially overestimated if Hg accumulation rates are not corrected for mass accumulation rates.

Elevated mercury accumulation in a peat bog of the Magellanic Moorlands, Chile (53°S) – an anthropogenic signal from the Southern Hemisphere

Increasing mercury deposition rates in the Northern Hemisphere recorded in natural archives such as peat bogs or lake sediments have been documented in numerous studies. However, data on atmospheric Hg deposition in the Southern Hemisphere dating back to pre-industrial times are rare. Here, we provide a continuous record of atmospheric Hg deposition in the Southern Hemisphere recorded by an ombrotrophic peat bog of the Magellanic Moorlands, Chile (53°S), extending back 3000 yr. Pre-industrial mercury accumulation rates range between 2.5 and 3.9 μg/m 2/yr. In the past 100 yr, Hg accumulation rates increased 18-fold from about 3 μg/m 2/yr to a maximum of 62.5 μg/m 2/yr. If Hg accumulation rates were normalized to peat accumulation rates, maximum rates were 7.9 μg/m 2/yr, which is only 2.5 times the pre-industrial rates. Thus, Hg accumulation rates normalized to peat accumulation rates are more comparable to the three-fold net increase in atmospheric Hg concentrations estimated for the same period. We suggest that the increase in Hg accumulation rates in the Magellanic Moorlands within the past 100 yr is at least partly attributed to global dispersion of Hg derived from anthropogenic sources in the Northern Hemisphere. The finding that no increase of atmospheric deposition of Pb could be observed in the bog indicates the extraordinary long-range transport and ubiquitous dispersion of anthropogenic derived gaseous Hg compared to other metals.

Accumulation rates and mass calculations of Zn and Hg in recent sediments, Göta älv estuary, Sweden

Sediment masses and heavy-metal accumulation rates for the recent deposits of the Gota alv estuary were calculated using a GIS. Total masses of 4,000 t Zn and >20 t Hg were recorded, of which 367 t Zn and >0.72 t Hg occur in the inner estuary, the Goteborg harbour, and 3,657 t Zn and 19 t Hg are in the outer estuary, suggesting a high trapping efficiency of the estuary. The accumulation rates are 5 g m–2 year–1 Zn and 0.01 g m–2 year–1 Hg in the harbour, and 1.5 g m–2/year–1 Zn and 0.007 g m–2 year–1 Hg in the outer estuary. The Zn and Hg accumulation rates are highest close to the outlet of a sewage treatment plant, and decrease by a factor of 5 for Zn, and a factor of 2 for Hg in the outer estuary. The accumulation rates in the outer estuary decrease after 1980 A.D.

Total gaseous mercury concentration and flux over a coastal saltmarsh vegetation in Connecticut, USA

The objective of this study is to characterize the ambient total gaseous mercury (TGM) concentration (C) at a coastal salt marsh and the TGM flux (F) between the marsh and the atmosphere using data collected on a near-continuous basis from 5 Jun to 8 July, 1997 and from 8 March to 23 July, 1998. In general, C was lower in the afternoon than at night, which is believed to be a result of diurnal variation in the mixing efficiency of the atmospheric boundary layer. The lack of a significant upward trend from March to July 1998 implies that TGM was removed from the boundary layer at a rate equal or greater than the surface emission. Three episodes of low C (0–1ngm−3) occurred in July 1998, each lasting about 3h. The TGM flux, F, was determined with the micrometeorological aerodynamic method. An episodic event of large positive F (emission) occurred in early spring of 1998 and appears to have been triggered by ice melting. It is proposed that three competing mechanisms – Hg vaporization, oxygen transport via roots, and diffusion into plants via stomata – may have played a role in the Hg exchange over other periods, resulting in bi-directional exchange of Hg with the atmosphere. Extrapolation of the 1998 data to the whole year suggests a weak sink removing 4±7μg Hgm−2yr−1 from the atmosphere, a rate similar in magnitude to wet or particulate deposition to the site but much smaller than the rate of Hg accumulation in the sediment.

History of Mercury Loading in the Upper Mississippi River Reconstructed from the Sediments of Lake Pepin

An array of sediment cores was analyzed to determine historical trends in mercury (Hg) accumulation in Lake Pepin, a natural lake on the Upper Mississippi River. Whole-basin Hg accumulation rates increased from 3 kg/yr before European settlement (ca. 1830) to a maximum of 357 kg/yr in the 1960s. The recent Hg accumulation rate (110 kg/yr, 1990−1996) is experimentally indistinguishable from measured Hg loadings in the river entering the lake, indicating that accumulation rates in Lake Pepin correspond quantitatively to river loadings. The modern accumulation rate represents a decline of almost 70% from the peak value, reflecting large decreases in Hg inputs to the Mississippi River from industrial and municipal point sources in the metropolitan Minneapolis/St.Paul area upstream. A total of 18.1 t of Hg has been deposited in Lake Pepin since 1800; half of that (9.0 t) was deposited between 1940 and 1970, when regional growth accelerated rapidly but pollution control mechanisms were inadequate. Point sources accounted for approximately 60% of the Hg accumulating in Lake Pepin in the 1960s, but these inputs have been virtually eliminated since then.

Uncertainty in Paleoecological Studies of Mercury in Sediment Cores

Increased recognition of the ecological damage of mercury (Hg) has focused attention on quantifying spatial and temporal patterns of Hg deposition. Studies are commonly based on core chronologies and use a combination of techniques to measure parameters such as bulk density, percent solids, Hg concentration, and radionuclide activity. Little attention is generally devoted to the propagated error associated with these measurements. We identified the impact of sources of uncertainty on stratigraphic Hg determinations for Florida Everglades and Lake Erie cores. Large errors may be introduced by converting wet sample Hg content to dry-weight concentrations. Drying of sediments at 55 °C caused Hg losses of 18%. Samples, air-dried at room temperature, retained considerable moisture and required corrections for remaining water content. Frozen sediments did not lose Hg during a 72-day storage. Random error in radionuclide analysis of cores resulted in dating uncertainty of ±1.2 yr in 10 yr old deposits. This error increased to ±20 yr in 100 yr old sediments. Propagation of small errors in each step of the analysis (while adhering to strict QA/QC criteria) produced compounded uncertainties of ±11 and ±29% in Hg concentrations under different analytical rigor, and errors of up to ±73% in Hg accumulation rates in older sediments. Enrichment factors, comparing uncertain recent and historic Hg accumulation rates, differed by as much as ±48%. Uncertainty in paleoecological studies of mercury needs to be documented in order to correctly evaluate trends and remediation efforts.

Historical atmospheric mercury emissions and depositions in North America compared to mercury accumulations in sedimentary records

Gold and silver production in North America (included United States, Canada and Mexico) released a large amount of mercury to the atmosphere until well into this century when mercury (Hg) amalgamation was replaced by cyanide concentration. Since then, emissions from industries have been the dominant anthropogenic sources of atmospheric Hg in North America as a whole. Past Hg emissions from gold and silver extractions in North America during the 1800s do not show a clear evidence of atmospheric deposition occurred at the coring sites considered in this study. Estimated atmospheric emissions of Hg in North America peaked in 1879 (at about 1708 t yr −1) and 1920 (at about 940 t yr −1), primarily due to Hg emissions from gold and silver mining. After the Great Economic Depression (1929) Hg emissions peaked again in the 1947 (274 t yr −1), in 1970 (325 t yr −1) and in 1989 (330 t yr −1) as result of increased Hg emissions from industrial sources, though improvements in the emissions control technology in United States and Canada have been substantial. Estimates of total atmospheric deposition fluxes of Hg to water and terrestrial receptors were in the range of 14.3–19.8 μg m −2 yr −1 in North America as a whole, and averaged 135 μg m −2 yr −1 (global background + local emissions) in the Great Lakes. These values were in good agreement with recent estimates reported in literature. The comparison of atmospheric Hg deposition fluxes with Hg accumulation rates in sediment cores suggests that atmospheric deposition was the major source of Hg entering the lakes system at coring sites, however, important contributions to Lake Ontario sediment cores sites from 1940 to 1970 were likely originated from local point sources (i.e. direct discharges).

Mercury accumulation in Devils Lake, North Dakota — Effects of environmental variation in closed-basin lakes on mercury chrologies

Sediment cores were collected from lakes in the Devils Lake Basin in North Dakota to determine if mercury (Hg) accumulation chronologies from sediment-core data are good indicators of variations in Hg accumulation rates in saline lakes. Sediment cores from Creel Bay and Main Bay, Devils Lake were selected for detailed analysis and interpretation. The maximum Hg concentration in the Creel Bay core was 0.15 micrograms per gram at 8 to 9 centimeters. The maximum Hg concentration in the Main Bay core was 0.07 micrograms per gram at 5 to 7 centimeters. The general decreases in Hg concentrations with depth are attributed to historic variations in atmospheric Hg deposition rate. Hg stratigraphies combined with210 Pb and 137Cs dating analyses yield Hg chronologies that indicate a general increase in Hg accumulation rates in Devils Lake since the middle of the 19th century.

Mercury accumulation trends in Florida Everglades and Savannas Marsh flooded soils

Global and regional increases in atmospheric mercury (Hg) concentrations have previously been identified as the cause of increased mercury accumulation rates in north temperate lakes in Sweden, Wisconsin, and Minnesota. Atmospheric deposition can often account for elevated Hg concentrations in fish from these systems. Mercury levels in sportfish collected from some areas of the Florida Everglades and Savannas Marsh exceed limits that are acceptable for human consumption. Forty five soil cores and soil grab samples were retrieved from the Everglades and Savannas Marsh wetlands. Eighteen sediment cores were dated radiochemically with 210Pb and 137Cs using γ-ray spectroscopy to determine modem and historic mercury accumulation rates for these subtropical wetland systems. Recent (“post-1985”) Hg accumulation rates averaged 53 μg m−2 y−1 (23 to 141, n=18) corresponding to an average rate increase of 4.9 times (1.6 to 19.1) over those observed around the turn of the century. This accumulation seems to result more from either global or regional atmospheric deposition rather than from lateral transport via overlying surface water. The trends for mercury accumulation match those reported for lakes in Sweden and the northern United States, even though these systems are distinctly different in their climate, vegetational composition, and location. We provide the first data on accumulation of mercury in subtropical wetland systems, and demonstrate the feasibility of radiochemical dating of wetland sediment.

Influence of Dietary Levels of Protein and Sulfur Amino Acids on the Subacute Toxicity of Methylmercury in Mice.

The influence of diet on the subacute toxicity of methylmercury (MeHg) was investigated using C57BL/6N male mice, each fed a 24.8% protein diet (normal protein diet, NPD), a 7.5% protein diet (low protein diet, LPD) or LPD supplemented by methionine and cystine to NPD level (amino acid supplemented diet, ASD). When MeHg was consecutively administered to each group of mice (20 μmol/kg per day), median survival times were 11, 18 and 30 d in ASD-, LPD- and NPD-fed mice, respectively. Toxic symptoms including ataxia and loss of body weight were in that order. Hg accumulation rate in brain was faster in LPD-fed mice than in NPD-fed mice, and was further enhanced in ASD-fed mice. The fact that Hg in plasma low molecular weight (LMW) fraction was in the order of ASD->LPD->NPD-fed mice, would be one reason for the difference in the brain Hg accumulation. Hg levels in liver, blood and plasma showed a monotonous increase, and their terminal Hg levels were similar in the three groups. Renal Hg levels increased rapidly and almost never changed from the 7th d in the three groups. Urinary excretion of Hg in LPD-fed mice was markedly lower than in NPD-fed mice throughout the experimental period ; that in ASD-fed mice, which was the highest at least for the first few days, began to decline earliest. No difference was observed in fecal Hg levels during the early part of the experiment. Our results suggest that diet is an important factor determining the susceptibility to subacute MeHg toxicity, and that accelerated Hg accumulation rate in the brain caused by diet would result in higher susceptibility to subacute MeHg toxicity.

Mercury content in relation to sex, size, age and growth in two scorpionfish (Helicolenus dactylopterus and Pontinus kuhlii) from Azorean waters

Levels of total Hg were determined in the muscle tissue of Helicolenus dactylopterus (mean=0.29 ug.g−1 ±0.025 S.E.; range=0.041.10 ug.g−1) and Pontinus kuhlii (mean=0.16 ug.g−1 ±0.092 S.E.; range=0.05-0.50 ug.g−1) caught in the Azores between August 1989 and May 1990. Mercury concentrations were related to sex, length, weight, age, growth and condition of both fish species. There were highly significant positive correlations between Hg levels and these variables. In P. kuhlii a significant sex-related difference in Hg levels was found; the rate of Hg accumulation is significantly faster in females than in males and mean Hg levels were higher in females. Relationships describing Hg vs size and age dependence were determined for both species and sexes and patterns and rates of Hg accumulation were discussed in relation to a number of biological and ecological factors with influence on it. A comparison of Hg content of Azorean and Mediterranean populations of H. dactylopterus was conducted and its suitability as an indicator in pollutant monitoring of benthic compartment of marine ecosystems is discussed.