1g: Mercury through time – Reconstructing Hg-deposition using natural archives

Environmental regulations on mercury (Hg) emissions and associated ecosystem restoration are closely linked to what Hg levels we consider natural. It is widely accepted that atmospheric Hg deposition has increased by a factor 3 ± 1 since pre-industrial times. However, no long-term historical records of actual atmospheric gaseous elemental Hg (GEM) concentrations exist. In this study we report Hg stable isotope signatures in two Pyrenean peat records (southwestern Europe) that are used as tracers of Hg deposition pathway (Δ200Hg, wet vs dry Hg deposition) and atmospheric Hg sources and cycling (δ202Hg, Δ199Hg). Three cores from the Pinet peat bog (880m asl) were used to reconstruct Hg deposition for the past 10,000 years. A comparison with Estibere (2100m asl) peat record reveals both similarities and differences in terms of Hg deposition. By anchoring peat-derived GEM dry deposition to modern atmospheric GEM levels we are able to reconstruct the first millennial-scale atmospheric GEM concentration record, which is in good agreement with post-1970 monitoring data and a reconstruction based on polar firn air from 1940 to 2005. Historical variations in peat Hg stable isotope signatures reveal different periods of anthropogenic influence over the past millenniums. We suggest that deforestation and associated biomass burning Hg emissions caused a first shift in δ202Hg during the medieval and Renaissance periods. The maximum GEM concentration found during the 20th Century and exceeds by a factor of 15 the natural Holocene background. Our findings suggest therefore that Human impacts on the global mercury cycle are subtler and substantially larger than currently thought.

Atmospherically mobile Hg(0) can be converted, post-deposition, to toxic methylmercury in terrestrial and aquatic environments posing significant risks to human and ecosystem health in regions far from emission sources. Accurate projections of future Hg exposure and impacts will partly depend on our ability to understand changes in the atmospheric Hg pool, but long-term knowledge of atmospheric Hg variability is limited to ~2 decades of instrumental monitoring. Natural Hg archives such as ice cores, tree-rings and lake sediments have the potential to fill this knowledge gap. Tree-rings are ideal since they are annually resolved, span multiple centuries, and cover large areas of Earth’s surface. Few studies concerning a small number of species have shown the potential of tree-rings to record local Hg emissions, but additional proof-of-concept is needed. We present an annually resolved tree-ring Hg record from a stand of white spruce ~100 m from the Bear Creek Gold Room site, Klondike Goldfields, where the Au-Hg amalgam method was used to recover fine gold from placer ore. Bear Creek operations began in 1905, but were greatly expanded in 1930 when most Klondike recovery operations were consolidated to the Bear Creek site. Results from 15 trees reveal a common pattern in tree-ring Hg over the last ~150 years (mean r = 0.60, p<0.01), extending into the pre-mining era. The lowest Hg concentrations occur prior to the Klondike Gold Rush (1896-1899). Initial Bear Creek operations mark a rise in tree-ring Hg concentrations, but this signal is small compared to a major step to peak values in the 1930s coinciding with consolidated operations. Tree-ring Hg declines rapidly after the cessation of Bear Creek operations in ca. 1966. This result suggests the principle Hg intake pathway is likely air-leaf assimilation rather than root uptake from contaminated soil, as there is no delayed decline in tree-ring Hg after the cessation of Bear Creek operations, despite a legacy of elevated soil Hg concentrations to this day. Finally, we observe that post-1966 Hg concentrations are slightly higher than pre-industrial values, likely reflecting higher global atmospheric Hg(0) concentrations. This is the first study of white spruce tree-ring Hg, and only the second to measure this variable at annual resolution. These results demonstrate that this species is a reliable recorder of local emissions, and offers great promise for longer-term reconstructions of atmospheric Hg at potentially broader spatial scales.

Large-scale, semi-continuous volcanism in India produced the Deccan Traps and is coincident with the Chicxulub impactor and extensive end-Cretaceous extinction, including the demise of dinosaurs. Linking the timing of volcanism and impact events with local records of climate change and extinction has proven difficult with existing methods. We present total mercury concentrations (THg) derived from the biogenic carbonate of marine mollusks at intervals spanning before, during, and after the period of active Deccan volcanism. THg values were determined by cold vapor atomic fluorescence spectroscopy after digesting sample carbonate material in 4N HNO3 at 70°C. The same shells were measured for their clumped isotopic composition (Ca13C18O16O2 abundance), an isotopic quantity directly related to the water temperature where the bivalves lived. We document a pattern of increased temperatures co-occurring with THg anomalies of 10 to 43 ng/g relative to a background of 0.5 to 4 ng/g in shells from numerous localities proximal to and distal from the proposed volcanic source. The beginning of the warm interval (~100 to 250 ka prior to the end Cretaceous boundary) coincides with the onset of Deccan Traps volcanism. At one locality, with the highest temporal sample resolution (Seymour Island, Antarctica), we find evidence for multiple THg anomalies (~10 to 17 ng/g), potentially corresponding to separate phases of Deccan Traps activity. Taken together, measuring paleotemperature and THg in the same material allows for direct comparison between indicators of volcanism and records of climate change. These data strongly suggest volcanic forcing caused end-Cretaceous climate change, and shed new light on concurrent extinction patterns. The combined THg and paleotemperature records also provide new insight into long-standing questions regarding the role of volcanism as opposed to bolide impact on observed extinction patterns at the end of the Cretaceous. This method could similarly be applied to other past volcanic events to further constrain links between climate change and volcanism through Earth history.

Concentrations of atmospheric mercury (Hg) in the past are not known. As part of a larger project, our team is investigating whether tree rings are reliable archives of changing atmospheric Hg concentrations over time on local, regional, and/or global scales. In general it is thought that trees take up gaseous elemental mercury (GEM) through their foliage (via the stomatal openings and through the cuticle). Gaseous oxidized mercury (GOM) is not thought to be taken up by the foliage. Additionally, the effect of different atmospheric GOM composition ontree ring Hg is uncertain. We tested the hypothesis that Hg concentrations of tree growth rings and needles of coniferous tree species would be influenced by air chemistry, Hg concentration, and field conditions at different locations.

To do this, 5-6 year old Austrian pines (Pinus nigra) purchased from a farm in rural Oregon, USA, were moved in 25 gallon pots in their original soil to 3 locations: an urbanized site in Reno, Nevada (approximately 1370 m), experiencing proximate effects of petroleum-based combustion along a busy interstate; a mountaintop site at Peavine Peak (approximately 2518 m), 12 km west of Reno, influenced by the free troposphere; and an oceanside site at the UCSC Arboretum, Santa Cruz California (approximately 140 m), interacting directly with maritime fog thought to carry relatively high concentrations of methylmercury. Each location has been identified as having different compounds of GOM (HgCl2, HgBr2, Hg-nitrogen and sulfur compounds, Hg-organic compounds). Needle data collected from the first year (n=5 trees from each site) showed significant increases at all sites relative to those from trees as originally received. Highest concentrations were measured at the coastal location (p<0.05). We also tested for whorl age and effect of tree orientation at the mountaintop and urban location on Hg concentration in foliage. After a year there was no influence of whorl age or orientation on needle concentration. The wood of the trees showed an increase in Hg in the most recent year of growth at 2 locations (Peavine Peak and Santa Cruz) and no increase at the urban Reno location. Tree orientation does not appear to be a factor in the radial distribution of Hg within the annual growth rings for any location.

High mountain intercontinental glaciers are very suitable natural archives to reconstruct current and historical Hg deposition reflecting changes of atmospheric Hg concentrations both on the global and regional scale. Belukha glacier in the Siberian Altai provided a 300 years high-resolution record of Hg concentrations allowing to consider not only the long-term trend (based on an average of 5-10 year period values), but also the dynamics of current changes (1-2 years), as well as to fix the short-term events, both natural and anthropogenic. Hg concentrations in ice core range from < 0,04 to 3,87 and from 0,07 to 8,89 ng/L for Hg reactive and Hg total, respectively, and reflect seasonal, intra- and inter-annual variations and long-term trend of concentrations in the period 1700-2001. Large variability of Hg concentrations is observed partly due high resolution and seasonality. Short-term Hg changes in ice-core mostly related to dust or volcanic input, while long-term changes of Hg levels in the Altai are mainly determined by global Hg sources. Although the Hg concentration in the ice core from 1740 to 1850 apparently increased also due to the contribution of the regional component, i.e. mining and metallurgical industry of Rudny Altai, as evidenced by correlation between Hg concentrations and volumes of ore smelting and silver production. Since 1850 regional contribution decreased and the global one increased, and after 1880 mercury concentrations in ice core reflected in a greater extent the global atmospheric mercury changes. The most significant increasing of Hg concentrations observed from 1940 with maximum in 1970th and decline during the 1980s that is in agreement with general trends of other environmental archives. Concentration levels are in the same order of magnitude as values from firn and ice cores of the Tibetan Plateau, Canada, Canadian Arctic, and Greenland. High Hg concentrations at the end of the 20th century founded in Belukha ice core can be explained by rising emissions from coal combustion in Asia. Hg concentrations in ice core change synchronously with modelled global atmospheric mercury emissions over the past 150 years. Average THg fluxes in Belukha ice core changed from 0,30 (preindustrial) to 0,84 (industrial) μg/m2y, hence atmospheric Hg deposition in industrial time has been enriched by a factor of 2.8.

Heavy metals previously accumulated in soils and biomass can be remobilized by wildfires and deposited in aquatic environments by atmospheric transport or subsequent catchment erosion. Because wildfire frequency and intensity are predicted to increase in northwestern Canada in response to climate change, understanding the impact of wildfires on freshwater ecosystems is important to better plan for and manage the potential consequences of climate change. Increasing concentrations of mercury have recently been reported in fishes in the Northwest Territories (Canada) and this rise could be attributed to increased atmospheric mercury deposition from long-range sources, changes in bioavailability of mercury from climate warming or remobilisation of previously deposited mercury. We explore the hypothesis that forest fires are a significant source of mercury to Northwest Territory lakes using a paleoenvironmental approach. We reconstructed mercury deposition history and forest fire history in 3 dated lake sediments cores and 2 dated peatland cores from the Great Slave Lake region in the southern Northwest Territories. Radioisotope dating methods (210Pb and 14C) were used in combination to build an age-depth model for each core. Wildfire history was inferred through macroscopic charcoal analysis and trace mercury concentrations in the sediments and peat were measured in laboratory. Time series analysis will be used to determine the potential contribution of wildfire to mercury deposition. Whereas ombrotrophic peatlands solely record the influence of atmospheric depositions, lake sediments also record the effect of increased catchment erosion that usually follows a wildfire. Therefore, these complementary records of deposition (lake and ombrotrophic peatland) will be used in a single analysis to distinguish the relative influence of different mercury transportation pathways to aquatic ecosystems through time.

Contemporary and historical information obtained from examination of natural archives has been critically important to the assessment and understanding of the biogeochemical cycling of Hg and influence of human activities. Few archival studies of Hg, however, include other geochemical indices that can aid and constrain the interpretation. Here we present a high-resolution chronology of Hg accumulation between 1727 and 1996 as determined in a well-studied sediment core from the Pettaquamscutt River Estuary in Rhode Island, northeast U.S. The Hg accumulation in this scrupulously dated, varved repository (1-3 y resolution) provides a quantitative means for assessing the magnitude, timing, and relative source strengths of human-related Hg emissions and deposition over three centuries. The significance of these findings is examined relative to (1) other temporal reconstructions of Hg deposition in urban, regional, and remote settings, and (2) historical depositional patterns of pyrogenic polycyclic aromatic hydrocarbons (PAHs), lead (Pb) and its isotopes (206Pb/207Pb) that have been determined in the same core.Broadly, Hg deposition in Pettaquamscutt sediment parallels the temporal patterns of PAHs, which track growth in industrialization and decentralized regional coal use between 1850 and 1950, as well as increased use of petroleum after 1950. There is little suggestion of a North American Hg signal predicted to be associated with extensive silver and gold mining in the western U.S. between 1850 and 1900 suggesting such releases have been slow to participate in global cycling. A broad maximum in sedimentary Hg accumulation is evident during the 1926-1990 time period. The largest flux ratios for Hg, which occurred during 1949-1976, are consistent with the predicted influence of additional sources and commercial products. In contrast, the temporal patterns for the sedimentary accumulation of Hg reported for remote lakes show a gradually increasing trend from about 1850 to modern times. The question of whether Hg accumulating in the estuary is derived primarily from either local or more regional sources is explored given the importance of atmospheric deposition in the environmental cycling of PAHs, Hg, and Pb.

High-resolution records of anthropogenic mercury (Hg) deposition were constructed from dated sediment cores from nine lakes located 5-75 km from the Flin Flon, Manitoba Cu-Zn smelter (formerly one of North America’s largest atmospheric Hg point sources) and five lakes in Experimental Lakes Area (ELA), Ontario; a region remote from Hg point sources. For the Flin Flon lake records we have taken a novel approach integrating the relation between Hg inventories (minus that contributed from long-range transport [i.e. that observed at the ELA] and the residual normalized for prevailing wind direction) and distance from the smelter to scale up the measurements to get a landscape scale picture of Hg deposition over time. To our knowledge this is the first time such a quantitative spatial-temporal reconstruction of contaminant deposition and inventory with respect to a point source has been performed. The sediment records of anthropogenic Hg flux and inventory were remarkably consistent among the ELA lakes and are also in good quantitative agreement with monitoring data (post-1992) supporting the veracity of our record. The anthropogenic Hg flux to the ELA was 0.6 µg m-2 year-1 in the late 1800s increasing to 2.0 circa 1900 rising steadily till late-1930s thereafter rising sharply and peaking in the early-1950s at 6.1, followed by a slight decline in the 1960s and 70s (5.3 µg m-2 year-1). A trend similar to long-term Hg emissions estimates for the Great Lakes region over the last century. That is until sediment recorded anthropogenic Hg deposition began rising post-1990, reaching a maximum of 6.5 µg m-2 year-1 in the most recent time interval (2005-2010), suggesting that increasing Hg emission from very distant sources is offsetting reductions of Hg emissions in the Great Lakes region and North America. In contrast to the ELA, the anthropogenic Hg deposition and inventories of the Flin Flon region lakes varied by 2 orders of magnitude. The relation between Hg inventories and distance from the smelter was used to estimate the total smelter sourced Hg fallout within a 50-km radius (64 tonnes) and in five-year time-steps for the same. The propensity for Hg to undergo long-range, even global transport explains why Hg deposition within 50 km was ~11% of estimated releases. That is until smelter releases were reduced >10-fold (post-2000), after which observed deposition equaled or exceeded smelter releases, suggesting landscape re-emission /remobilization of legacy Hg is a major ongoing regional source of Hg.