Article

Comparison of co-located ice-core and tree-ring mercury records indicates potential radial translocation of mercury in whitebark pine

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Abstract

Tree-ring records are a potential archive for reconstructing long-term historical trends in atmospheric mercury (Hg) concentrations. Although Hg preserved in tree rings has been shown to be derived largely from the atmosphere, quantitative relationships linking atmospheric concentrations to those in tree rings are limited. In addition, few tree-ring-based Hg records have been evaluated against co-located proxies of atmospheric Hg deposition or direct atmospheric measurements. Here we develop long-term Hg records extending from 1800 to 2018 CE using cores collected from two stands of whitebark pine located near the Upper Fremont Glacier in the Wind River Range, Wyoming, where a long-term record of atmospheric Hg deposition previously was developed from an ice core. The tree ring record showed that Hg concentrations increased beginning in 1800 CE to a broad peak centered at ~1960 CE, before decreasing to present, generally paralleling the ice-core record of Hg deposition. The exact timing and magnitude of the Hg increases in the trees, however, is offset earlier relative to the ice-core record. These discrepancies potentially arise from biotic processes that impact Hg uptake and preservation in whitebark pine, and results from an advection-diffusion model indicate that the temporal differences are consistent with radial movement of Hg within the trees. The forms of atmospheric Hg and seasonality may also impact the Hg record preserved by each archive, but are less likely to affect long-term trends. Further work is needed to assess radial Hg translocation in more controlled studies with larger sample sizes.

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... Understanding the response between Hg induced risk in the environment and long-term anthropogenic Hg emissions is the prerequisite for evaluating the impact of Minamata Convention. Monitoring of atmospheric Hg 0 concentration began only 20-30 years ago, and the absence of long-term measurements has limited assessment on atmospheric Hg pollution (Chellman et al., 2020;Clackett et al., 2018). The ability to infer the level of historical pollution using a readily available proxy offers opportunities to better understand the long-term and large-scale pollution of a global pollutant such as Hg. ...
... Notwithstanding the advantages and demonstrated applications to reconstruct the centennial atmospheric Hg changes at local or regional scale (Chellman et al., 2020;Clackett et al., 2018Clackett et al., , 2020Ghotra et al., 2020;Kang et al., 2018;Navratil et al., 2018;Peckham et al., 2019;Scanlon et al., 2020;Schneider et al., 2019), the Hg dendrochemistry is still an emerging field of research (Arnold et al., 2018;Chellman et al., 2020;Clackett et al., 2018;Peckham et al., 2019). Previous studies had mixed assessment on the suitability of using tree rings as archives for atmospheric Hg pollution trends. ...
... Notwithstanding the advantages and demonstrated applications to reconstruct the centennial atmospheric Hg changes at local or regional scale (Chellman et al., 2020;Clackett et al., 2018Clackett et al., , 2020Ghotra et al., 2020;Kang et al., 2018;Navratil et al., 2018;Peckham et al., 2019;Scanlon et al., 2020;Schneider et al., 2019), the Hg dendrochemistry is still an emerging field of research (Arnold et al., 2018;Chellman et al., 2020;Clackett et al., 2018;Peckham et al., 2019). Previous studies had mixed assessment on the suitability of using tree rings as archives for atmospheric Hg pollution trends. ...
Article
The accuracy of mercury (Hg) dendrochemistry has been questioned because significant knowledge gaps exist in understanding the Hg translocation and mobility in tree-ring. In this study, we evaluated Hg concentrations and isotopic profiles in the tree-ring at a Hg artisanal mining site and a control site with the documented local Hg production inventory. Results show that the Hg concentration accumulated in tree-ring fails to reconstruct the temporal trend of Hg production due to confounded tree physiological and environmental factors, specifically, the radial translocation and tree age effects occurring during the fast-growing period. The temporal profiles of δ²⁰²Hg exhibit pronounced tree-specific variabilities due to the complexity of Hg isotopic mass dependent fractionation during atmospheric Hg uptake and translocation in vegetation. The Hg odd-MIF (mass independent fractionation) profiles in tree-ring can reconstruct a decadal-scale temporal trend of the atmospheric Hg⁰ pollution level, and also be used as a tracer to distinguish the emission source shifts of atmospheric Hg⁰. However, the radial translocation would result in uncertainties at the higher resolution because of the mixing of odd-MIF signatures with active rings. Caution should be taken and additional supporting evidence collected from independent methods should be used for verifying the tree-ring records.
... Cutter and Guyette (1993) suggested that radial xylem-to-xylem translocation of elements could possibly occur throughout the sapwood, including within intermediate wood leading to the transition zone at the heartwood-sapwood boundary where moisture content in conifer sapwood tends to decrease prior to the accumulation of extractives during heartwood formation (Hillis, 1987;Taylor et al., 2002) depending on the rate of change in xylem features (e.g., pit closure and encrustation) that influence conductance and permeability (Cutter and Guyette, 1993;Hillis, 1987;Taylor et al., 2002). In fact, it was recently suggested that radial translocation of Hg may occur in whitebark pine (Chellman et al., 2020). Keeping in mind practical considerations, as well as physiological characteristics important to the distribution of elements among tree rings, Cutter and Guyette (1993) recommended several tree species likely to be most useful for dendrochemical studies, with preference given to long-lived species with wide geospatial distribution that are also characterized by a thin sapwood zone and low-moisture, non-permeable heartwood. ...
... On an individual-tree basis, this suggests that the much greater and age-dependent number of pine sapwood tree-rings promotes radial translocation that results in variably shifting Hg peaks backward in time through the tree-ring record. Similarly, Chellman et al. (2020) recently observed a 20-year backward shift in the Hg peak in the tree-ring record of whitebark pine and also suggested that it was the result of radial translocation. More broadly, this suggests that sapwood-heartwood distribution may be an important driver of the high intra-site variability in individual treering records commonly observed in tree-ring Hg studies. ...
... Based on our previous discussion, we suggest that the backward shift in peak Hg concentrations in pine by approximately 60 years and the overall decoupling of the pine tree-ring Hg record from the peat Hg accumulation record has resulted from translocation of Hg through the active xylem of the pine sapwood and is influenced by the great width of the sapwood region in addition to the timing of occurrence of the sapwood-heartwood boundary. Similarly, Chellman et al. (2020) observed that the Hg peak in the tree-ring Hg record of whitebark pine occurred 20 years earlier with respect to the Hg accumulation peak in an ice core at Fremont Glacier in the Rocky Mountains (USA), and used an advection-diffusion model to show that the distribution of Hg within tree rings of whitebark pine was plausibly influenced by translocation. The larch tree-ring Hg record in our study, on the other hand, is more consistent with peat Hg accumulation trends and appears to provide a more reliable record of historical atmospheric Hg levels. ...
Article
Tree ring records are increasingly being used as a geochemical archive of past atmospheric mercury (Hg) pollution. However, it is not clear whether all tree species can be used reliably for this purpose. We compared tree-ring Hg records of two coniferous species-widely used Scots pine (Pinus sylvestris) and less frequently used European larch (Larix decidua) at 6 study sites across the Czech Republic. Site-specific mean Hg concentrations in tree-ring segments of larch ranged from 2.1 to 5.2 µg kg-1 , whereas pine had higher mean Hg concentrations (3.6-8.3 µg kg-1). Temporal records of Hg concentrations in tree rings of larch and pine differed significantly. Comparisons with previously documented peat Hg records showed that larch tree-ring Hg records more closely agreed with peat archive records. For pines, which had a large, tree-age dependent number of sapwood rings (62 ± 17, 1SD), we found a strong relationship between the year of peak Hg and the number of sapwood tree rings (p = 0.012, r 2 = 0.35), as well as between peak Hg year and the sapwood-heartwood boundary year (p < 0.001 , r 2 = 0.65), rather than with temporal changes in atmospheric Hg levels. The much greater number of pine sapwood tree-rings appears to promote radial Hg translocation, resulting in the shift of Hg peaks backward in time through the tree-ring record. In contrast, Larch consistently had a low number of sapwood tree rings (19 ± 6, 1SD), and more closely agreed with peat Hg records. This study suggests that European larch, a tree species characterized by a relatively low and consistent number of sapwood tree rings, records changes in atmospheric Hg concentrations more reliably than does Scots pine, a species with a relatively high and variable number of sapwood tree rings.
... This confirms the importance of phloem Hg translocation from foliage. Studies of Hg distribution in tree-ring have reported the radial Hg translocation from the sapwood to the heartwood (Arnold et al., 2018;Chellman et al., 2020;Novakova et al., 2021;Schneider et al., 2019;Wang et al., 2021b). The radial translocation in woody biomass depends on the tree species and tree-age related sapwood rings. ...
... The radial translocation in woody biomass depends on the tree species and tree-age related sapwood rings. The tree species with relatively low and consistent number of sapwood tree rings typically exhibit a weaker radial translocation (Chellman et al., 2020;Novakova et al., 2021;Wang et al., 2021b). ...
Article
Forest ecosystem accounts for 31% of global land areas and plays a key role in the global biogeochemical cycling of mercury (Hg). In this critical review, datasets of Hg flux measurements and Hg isotopic compositions in the environmental compartments of forests in the last three decades are synthesized to examine the budgets of Hg mass balance and storages. The primary goal of this synthesis is to provide insight into the source, transportation, translocation and fate of legacy Hg in forests. Existing data indicate that forests represent the largest atmospheric Hg sink in the terrestrial ecosystem, with atmospheric total Hg deposition of 2200–3400 Mg yr⁻¹ (i.e., relative to 40–65% atmospheric Hg pool size) and 500–1100 Gg of Hg stored in surface soils and vegetation. The climate and land cover changes, deforestation and wildfire re-volatilize several hundred tons of Hg into the atmosphere, thus increasing the ecological risk to the regional and global environments. Vegetative uptake of Hg⁰ vapor from air predominantly controls Hg accumulation and isotopic fractionation in the atmosphere and in global forests. With the ongoing Hg emission reduction from anthropogenic sources required by the Minamata Convention, an integrated assessment on the changing biogeochemical processes and isotopic fractionation in response to human and natural perturbations of emissions, climate, and land use is needed.
... As the best alternative to these, annual plants that are not evergreen solved this problem. Their leaves can easily give data on heavy metal pollution during vegetation season (Bonanno & Pavone, 2015;Chellman et al., 2020;Nakazato et al., 2016). Previous research stated that the healthiest information is obtained using biomonitors by using the tissues of species such as pine, spruce, and fir, whose needles have remained on the tree for many years (Alahabadi et al., 2017;Isinkaralar, 2021;Savas et al., 2021). ...
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Ambient air pollution in industrial areas is one of the significant problems increasing high emissions since the 1990s. Atmospheric trace metal deposition in the industrial estate has occurred, and some have shown toxic effects on humans and non-human biota. Tree bark and annual rings have been used as convenient biomonitors due to the highly absorbing capacity of several metals for passive samplers in recent decades. It gives information about the speciation of some pollutants and their transition between organs in the trees. Systematic differences in air pollution degrees can be recorded spatial or temporal by analyzing the annual rings of the trees. In this study, Cupressus arizonica, Platanus orientalis, and Robinia pseudoacacia have been selected as landscape trees to identify Al, Cr, and Mn pollution during the past 30 years in the urban environments of the İzmit, Türkiye. The widespread type of landscape species was compared to perform the deposition degree of several sites. Metal concentrations have been detected in the barks and wood due to the industrialization of the territory year-to-year. The study’s framework indicated that the applicability of Robinia pseudoacacia L. is the better choice for assessing the degree of atmospheric metal pollution, both historical and near-past, on local scales.
... Other coniferous species such as Larix decidua, Picea glauca and Pinus ponderosa have been shown by many studies to be useful for understanding trends in Hg tree ring records (Clackett et al., 2021;Eccles et al., 2020;Ghotra et al., 2020;Gustin et al., 2022;Navrátil et al., 2018;Nováková et al., 2021;Peckham et al., 2019a;Wright et al., 2014). Overall, the advantages of tree rings as archives include (i) their wide geographical availability, (ii) the ability to track the most abundant species of Hg in the atmosphere (GEM), and (iii) reliable and low-cost dating methods (Chellman et al., 2020;Cooke et al., 2020). ...
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The Chemical Factory in Marktredwitz (CFM) is known as the oldest chemical factory in Germany (1778–1985), and from the beginning of the 20th century focused primarily on the production of mercury (Hg) compounds. Due to extensive pollution, together with employee health issues, the CFM was shut in 1985 by a government order and remediation works proceeded from 1986-1993. In this study, tree ring archives of European Larch (Larix decidua Mill.) were used to reconstruct changes of air Hg levels near the CFM. Mercury concentrations in larch boles decreased from 80.6 μg kg−1 at a distance of 0.34 km–3.4 μg kg−1 at a distance of 16 km. The temporal trend of atmospheric Hg emissions from the CFM reconstructed from the tree ring archives showed two main peaks. The first was in the 1920s, with a maximum tree ring Hg concentration 249.1 ± 43.9 μg kg−1 coinciding with when the factory had a worldwide monopoly on the production of Hg-based seed dressing fungicide. The second peak in the 1970s, with a maximum tree ring Hg concentration of 116.4 ± 6.3 μg kg−1, was associated with a peak in the general usage and production of Hg chemicals and goods. We used the tree ring record to reconstruct past atmospheric Hg levels using a simple model of Hg distribution between the larch tree rings and atmosphere. The precision of the tree ring model was checked against the results of air Hg measurements during the CFM remediation 30 years ago. According to the tree ring archives, the highest air Hg concentrations in the 1920s in Marktredwitz were over 70 ng m−3. Current air Hg levels of 1.18 ng m−3, assessed in the city of Marktredwitz, indicate the lowest air Hg in the past 150 years, underscoring the effective remediation of the CFM premises 30 years ago.
... One important concern for using tree rings as archives is the potential for radial translocation. For example, through comparison with an ice core archive, Chellman et al. (2020) demonstrated that radial translocation occurred in whitebark pine in the Wind River Range, Wyoming, USA. Navokova et al. (2021) compared tree ring concentrations for European larch and Scots pine with peat bog Hg concentrations and found that radial translocation impacted Scots pine records, resulting in the shift of Hg peaks backward in time. ...
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Full-text available
Because there are limited locations where atmospheric mercury (Hg) measurements are made, a method is needed for characterizing air concentrations locally, regionally and globally. Research has shown growth rings of some tree species may be used as archives that record concentrations of gaseous elemental Hg. However, there are still questions regarding the most appropriate sampling method(s), which tree species to use, and the physiological factors that influence atmospheric Hg uptake and translocation to the rings. To address these questions, tree cores were collected from different tree species at two locations on the eastern side of the Sierra Nevada, California, USA. An assessment was made as to whether there was correlation of ring concentrations between co-located deciduous and coniferous trees, as well as between concentrations measured on the east versus west sides of individual trees, and whether tree ring Hg concentrations correlated with local climate, tree ring width, and total tree ring mass. Broadleaved and coniferous tree ring Hg concentrations were not correlated. Based on results obtained in this and other studies, Pinus species of relatively the same age are recommended for tree ring monitoring in the Western United States; however, the potential for radial translocation still needs to be assessed. Data collected from the two sites focused on were aligned with the overall regional trends in Hg concentrations established from previous studies. Climatic factors, including dew point, temperature, and relative humidity, were negatively correlated with ring Hg concentrations. Tree tissues are active samplers of atmospheric Hg, and are affected by climate and local growing conditions.
... The time offset of 4 to 5 years implies the possibility of high 450 mobility of Hg in pine tree rings.Cutter and Guyette (1993) suggested that radial 451 xylem-to-xylem translocation of elements could occur throughout the sapwood due to 452 active transport between the tissues. Hg records in white-bark pine trees also revealed 453 the potential for radial translocation(Chellman et al., 2020).Nováková et al. (2020) 454 found that the year of peak Hg was determined more strongly by the year when the 455 sapwood-heartwood boundary formed rather than the atmospheric Hg level. This ...
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Permafrost is a potential mercury (Hg) pool released by thawing, which can raise the risk of Hg pollution under global warming. Tree rings are useful archives of environment-specific Hg exposure over long periods. We determined Hg concentrations in tree rings of two dominant tree species (Larix gmelinii and Pinus sylvestris) at permafrost sites in northeastern China. The biweighted mean Hg concentrations ranged from 0.36 ng/g to 3.96 ng/g from 1840 to 2014. The tree-ring width had no significant influence on the Hg concentration. Larch Hg increased slightly before 1970s and peaked in 1990s. However, the pine Hg concentration increased continuously until the 1930s, decreased rapidly until 1970, then rose to a peak in the late 1980s. The change of Hg concentrations in larch and pine revealed a time offset of 4 to 5 years, which implied possibly high mobility of Hg in pine tree rings. Higher Hg concentrations from 1930 to 1950 and subsequent decreases in isolated permafrost forests revealed the local geographical Hg cycling history. Lower Hg concentrations and faster increases in larch suggest the role of additional winter Hg loading for the evergreen pine and species-specific differences in root absorption in response to melting permafrost. Our results highlight possible geographical impacts on tree-ring Hg records, improve understanding of Hg cycles in permafrost forest, and suggest a need to sample additional species in a range of permafrost environments.
... In summary, a number of southern African tree species contain datable, annual tree rings and may reach multi-century age (Gebrekirstos et al., 2014;Woodborne et al., 2015). Although reported issues around radial translocation (Hagemeyer, 2000;Chellman et al., 2020) may make interpretation of highresolution records complex, studies elsewhere on mercury (Schneider et al., 2020), and 129 I and 14 C from nuclear weapons tests (Turney et al., 2018;Zhao et al., 2019) do suggest potential for Anthropocene records. ...
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The process for the formal ratification of the proposed Anthropocene Epoch involves the identification of a globally isochronous stratigraphic signal to mark its starting point. The search for a Global Boundary Stratotype Section and Point (GSSP), a unique reference sequence that would be used to fix the start of the epoch, is in progress but none of the candidate sections are located in Africa. We assessed the currently available stratigraphic evidence for the possible markers of the Anthropocene in southern Africa and found that, although most markers have been identified in the region, the robustly dated, high resolution records required for the GSSP are very sparse. We then assessed the extent and stratigraphic resolution of a range of potential natural archives and conclude that a small number of permanent lakes, as well as marine sediments, corals and peats from selected locations in southern Africa could provide the temporal resolution required. With sufficient chronological control and multi-proxy analyses, one of these archives could provide a useful auxiliary stratotype thereby helping to confirm the global reach, and extending the utility, of the selected Anthropocene GSSP.
... The jump was observed precisely before the last eight tree rings. Stable distributions over time were also observed for some heavy metals (Hg, Cd, As, Cu, Fe, Ni, Zn) in a previous study (Chellman et al., 2020). We observed that Zn, Cr, Cd and Ni varied significantly between individuals. ...
Article
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... Zhang (2019) suggested that the accumulation of Pb in the crust is extremely high, which is evidence that Pb concentrations are absorbed from the atmosphere rather than from the soil. Similarly, it is stated that the Hg in tree ARs is mostly of atmospheric origin (Chellman et al. 2020). The fact that HMC such as Pb, Cd, Cr, and Ni were found at a much higher level in barks than in wood in different studies supports this thesis (Akarsu 2019;Cesur 2019;Sevik et al. 2020). ...
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Mercury (Hg) is an extremely toxic pollutant, and its biogeochemical cycle has been perturbed by anthropogenic emissions during recent centuries. In the atmosphere, gaseous elemental mercury (GEM; Hg degrees ) is the predominant form of mercury (up to 95%). Here we report the evolution of atmospheric levels of GEM in mid- to high-northern latitudes inferred from the interstitial air of firn (perennial snowpack) at Summit, Greenland. GEM concentrations increased rapidly after World War II from approximately 1.5 ng m(-3) reaching a maximum of approximately 3 ng m(-3) around 1970 and decreased until stabilizing at approximately 1.7 ng m(-3) around 1995. This reconstruction reproduces real-time measurements available from the Arctic since 1995 and exhibits the same general trend observed in Europe since 1990. Anthropogenic emissions caused a two-fold rise in boreal atmospheric GEM concentrations before the 1970s, which likely contributed to higher deposition of mercury in both industrialized and remotes areas. Once deposited, this toxin becomes available for methylation and, subsequently, the contamination of ecosystems. Implementation of air pollution regulations, however, enabled a large-scale decline in atmospheric mercury levels during the 1980s. The results shown here suggest that potential increases in emissions in the coming decades could have a similar large-scale impact on atmospheric Hg levels.
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This study investigated the methodology and utility of dendrochemistry for assessment of spatial and temporal concentrations of gaseous elemental mercury. Tree cores from Pinus species in California and Nevada, USA, were collected from previously sampled areas to test the stability of tree ring concentrations over time. Cores were collected from 2 new locations to assess spatial variability among trees within and between stands located at two elevations in the same watershed. Results indicated using 2-to-3 cores from ~10 or more trees provided the best framework for understanding tree ring concentrations within a population of trees located in uncontaminated areas. At the least 2 sides of a tree should be cored to account for radially asymmetric variation associated with growing conditions or injury. Agreement of concentrations and trends measured in trees cored with previous research indicated tree rings are suitable proxies for historical air mercury concentrations, and that mercury concentrations have increased since the Industrial Revolution. Data collected demonstrate that tree rings record regional gradients in GEM concentrations. In addition, temporal consistency may vary within a geographic location due to differing biotic and abiotic factors influencing ring growth since trees are active samplers of atmospheric Hg.
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We propose tree rings of European Larch (Larix decidua) as a widely available and reliable geochemical archive of local and regional changes in atmospheric mercury (Hg). Mean Hg concentrations in larch tree rings from 8 background sites across the Czech Republic ranged from 2.2 to 4.8 µg kg-1; the maximum concentrations occurred in the period 1951-1970. At 3 sites impacted by Hg emission sources -- gold amalgamation processing, caustic soda production and lead (Pb) ore smelting -- mean larch tree ring Hg concentrations were significantly elevated relative to background sites. Changes in larch tree ring Hg concentrations were temporally coherent with known activities at the sites that would alter Hg emissions; the nearly simultaneous response in tree rings indicated little or no translocation of Hg within the larch bole. Based on the present-day atmospheric Hg concentration of 1.63 ng m-3 at the intensively monitored Czech Global Mercury Observation System site, and the most recent mean tree ring Hg concentration of 2.8 µg kg-1 in co-located larch trees, we developed a simple distribution model of Hg between the atmosphere and larch tree rings. We applied the model using observed changes of Hg in larch tree rings from the countrywide background sites to reconstruct past atmospheric Hg concentrations in central Europe. Modelled Hg concentrations were in agreement with annual means from the European Monitoring and Evaluation Programme observatories.
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Tree-rings are a promising high-resolution archive for gaseous atmospheric mercury (comprised primarily of Hg⁰) reconstruction, but the influence of cambial age (ring number from pith) and tree-specific differences are uncertainties with potential implications for interpreting tree-ring Hg signals. We address these uncertainties and reconstruct the last 400 years of Hg⁰ change using a tree-ring Hg dataset from 20 white spruce (Picea glauca) trees from a pristine site in central Yukon. Cambial age has no significant influence on tree-ring Hg concentration, but tree-specific differences in mean concentration are prevalent and must be normalized to a common mean to accurately constrain long-term trends in the mean tree-ring Hg record. Our record shows stable, low Hg⁰ concentrations prior to ~1750 CE, a persistent rise from ~1750-1950 (increasing more rapidly post-1850), a pause from ~1951-1975, and then a resumed increase to record-high levels at present. This general pattern is reflected in other proxy-based Hg reconstructions worldwide. This study provides a novel long-term Hg⁰ reconstruction in the Western subarctic from one of the most widely distributed boreal tree species in North America and, therefore this proxy may also hold potential for investigating broader spatial patterns in Hg⁰ cycling across the subarctic and northern boreal forest.
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Natural levels of heavy metals (HM) have increased during the industrial era to the point of posing a serious threat to the environment. The use of tree species to record contamination is a well-known practice. The objective of the study was to compare HM levels under different pollution conditions: a) soil pollution due to mining waste; b) atmospheric pollution due to coal-fired power plant emissions. We report significant HM enrichment in Pinus halepensis tissues. Near a burning power plant, Pb content in a tree wood was 2.5-fold higher that in natural areas (no pollution; NP). In mining areas, Cd content was 25-fold higher than NP. The hypothesis that HM contents in tree rings should register pollution is debatable. HM uptake by pines from soil, detoxification mechanisms and resuspended local soil dust is involved in HM contents in wood and bark.
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To determine whether trees are reliable biomonitors of air mercury (Hg) pollution concentrations were measured in bark, foliage, and tree rings. Data were developed using 4-year old Pinus and Populus trees grown from common genetic stock in Oregon and subsequently transferred to four air treatments differing in gaseous oxidized mercury (GOM) chemistry and total gaseous Hg (TGM) concentrations. Soil of a subset of trees was spiked with HgBr2 in solution to test for root uptake. Results indicate no significant effect of the soil spike or GOM compounds on tree tissue Hg concentrations. TGM treatment had a significant effect on Pinus and Populus foliage, and Pinus year 5 growth ring concentrations. Populus foliar Hg concentrations were highest in the exposure where 24 h TGM concentrations were highest, indicating the importance of the nonstomatal pathway for uptake. Pinus tree ring concentrations were correlated to day time TGM concentrations suggesting Hg accumulation into tree rings is by way of the stomata and subsequent translocation by way of phloem. Populus leaves and Pinus rings can be used as biomonitors for TGM concentrations over space. However, the use of trees as temporal proxies requires further investigation due to radial translocation observed in active sapwood tree rings.
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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 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). 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. Reconstructed GEM levels from 1970-2010 agree with monitoring data, and maximum 20th Century GEM levels of 3.9±0.5 ng m⁻³ were 15±4 times the natural Holocene background of 0.27±0.11 ng m⁻³. We suggest that a -0.7‰ shift in δ²⁰²Hg during the medieval and Renaissance periods is caused by deforestation and associated biomass burning Hg emissions. Our findings suggest therefore that human impacts on the global mercury cycle are subtler and substantially larger than currently thought.
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The Upper Fremont Glacier (UFG), Wyoming, is one of the few continental glaciers in the contiguous United States known to preserve environmental and climate records spanning recent centuries. A pair of ice cores taken from UFG have been studied extensively to document changes in climate and industrial pollution (most notably, mid-19th century increases in mercury pollution). Fundamental to these studies is the chronology used to map ice-core depth to age. Here, we present a revised chronology for the UFG ice cores based on new measurements and using a novel dating approach of synchronizing continuous water isotope measurements to a nearby tree-ring chronology. While consistent with the few unambiguous age controls underpinning the previous UFG chronologies, the new interpretation suggests a very different time scale for the UFG cores with changes of up to 80 years. Mercury increases previously associated with the mid-19th century Gold Rush now coincide with early-20th century industrial emissions, aligning the UFG record with other North American mercury records from ice and lake sediment cores. Additionally, new UFG records of industrial pollutants parallel changes documented in ice cores from southern Greenland, further validating the new UFG chronologies while documenting the extent of late 19th and early 20th century pollution in remote North America.
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The ability to accurately measure interannual variability of the stable isotope of nitrogen, ¹⁵N, in tree rings could greatly improve understanding of marine-derived nitrogen delivery to terrestrial ecosystems, soil nitrogen dynamics, and the influences of climate on nitrogen cycles. It could also provide a tool for reconstructing past salmon abundances. However, the mobility of nitrogen throughout the entire tree including between rings has confounded attempts to make full use of ¹⁵N in dendrochemistry. We report that despite solvent-based extraction pretreatment to remove nonstructural, mobile nitrogen from increment core samples, an elevated ¹⁵N signal is detected in rings formed up to about ten years prior to application of a ¹⁵N-enriched tracer in four western redcedar trees. We conclude that the interpretation of the heavy nitrogen signal in tree rings via this pretreatment extraction method is confounded. These findings corroborate work already done in the field.
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We assessed > 100 years of mercury (Hg) pollution recorded in the tree rings of Scots Pine near a Czech chlor-alkali plant operating since 1941. Hg concentrations in tree rings increased with the launching of plant operations and decreased when Hg emissions decreased in 1975 due to an upgrade in production technology. Similar to traditional bioindicators of pollution such as pine needles, bark and forest floor humus, Hg concentrations in Scots Pine boles decreased with distance from the plant. Mean Hg in pine bole in the 1940s ranged from 32.5 μg/kg Hg at a distance of 0.5 km from the plant to 5.4 μg/kg at a distance of > 4.7 km, where tree ring Hg was the same as at a reference site, and other bioindicators also suggest that the effect of the plant was no longer discernible. Tree ring Hg concentrations decreased by 8–29 μg/kg since the 1940s at all study sites including the reference site. The lack of exact correspondence between changes at the plant and tree ring Hg indicated some smearing of the signal due to lateral translocation of Hg from sapwood to heartwood. Bole Hg concentrations reflected local and regional atmospheric Hg concentrations, and not Hg wet deposition.
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For the Western North America Mercury Synthesis, we compiled mercury records from 165 dated sediment cores from 138 natural lakes across western North America. Lake sediments are accepted as faithful recorders of historical mercury accumulation rates, and regional and sub-regional temporal and spatial trends were analyzed with descriptive and inferential statistics. Mercury accumulation rates in sediments have increased, on average, four times (4×) from 1850 to 2000 and continue to increase by approximately 0.2μg/m(2) per year. Lakes with the greatest increases were influenced by the Flin Flon smelter, followed by lakes directly affected by mining and wastewater discharges. Of lakes not directly affected by point sources, there is a clear separation in mercury accumulation rates between lakes with no/little watershed development and lakes with extensive watershed development for agricultural and/or residential purposes. Lakes in the latter group exhibited a sharp increase in mercury accumulation rates with human settlement, stabilizing after 1950 at five times (5×) 1850 rates. Mercury accumulation rates in lakes with no/little watershed development were controlled primarily by relative watershed size prior to 1850, and since have exhibited modest increases (in absolute terms and compared to that described above) associated with (regional and global) industrialization. A sub-regional analysis highlighted that in the ecoregion Northwestern Forest Mountains, <1% of mercury deposited to watersheds is delivered to lakes. Research is warranted to understand whether mountainous watersheds act as permanent sinks for mercury or if export of "legacy" mercury (deposited in years past) will delay recovery when/if emissions reductions are achieved.
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A peat core from a bog in northwest Spain provides a record of the net accumulation of atmospheric mercury since 4000 radiocarbon years before the present. It was found that cold climates promoted an enhanced accumulation and the preservation of mercury with low thermal stability, and warm climates were characterized by a lower accumulation and the predominance of mercury with moderate to high thermal stability. This record can be separated into natural and anthropogenic components. The substantial anthropogenic mercury component began ∼2500 radiocarbon years before the present, which is near the time of the onset of mercury mining in Spain. Anthropogenic mercury has dominated the deposition record since the Islamic period (8th to 11th centuries A.D.). The results shown here have implications for the global mercury cycle and also imply that the thermal lability of the accumulated mercury can be used not only to quantify the effects of human activity, but also as a new tool for quantitative paleotemperature reconstruction.
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Past emissions of the toxic metal mercury (Hg) persist in the global environment, yet these emissions remain poorly constrained by existing data. Ice cores are high-resolution archives of atmospheric deposition that may provide crucial insight into past atmospheric Hg levels during recent and historical time. Here we present a record of total Hg (HgT) in an ice core from the pristine summit plateau (5340 m asl) of Mount Logan, Yukon, Canada, representing atmospheric deposition from AD 1410 to 1998. The Colonial Period (~1603-1850) and North American "Gold Rush" (1850-1900) represent minor fractions (8% and 14%, respectively) of total anthropogenic Hg deposition in the record, with the majority (78%) occurring during the 20th Century. A period of maximum HgT fluxes from 1940 to 1975 coincides with estimates of enhanced anthropogenic Hg emissions from commercial sources, as well as with industrial emissions of other toxic metals. Rapid declines in HgT fluxes following peaks during the "Gold Rush" and the mid-20th Century indicate that atmospheric Hg deposition responds quickly to reductions in emissions. Increasing HgT fluxes from 1993 until the youngest samples in 1998 may reflect the resurgence of Hg emissions from unregulated coal burning and small-scale gold mining.
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Centuries of anthropogenic releases have resulted in a global legacy of mercury (Hg) contamination. Here we use a global model to quantify the impact of uncertainty in Hg atmospheric emissions and cycling on anthropogenic enrichment and discuss implications for future Hg levels. The plausibility of sensitivity simulations is evaluated against multiple independent lines of observation, including natural archives and direct measurements of present-day environmental Hg concentrations. It has been previously reported that pre-industrial enrichment recorded in sediment and peat disagree by more than a factor of 10. We find this difference is largely erroneous and caused by comparing peat and sediment against different reference time periods. After correcting this inconsistency, median enrichment in Hg accumulation since pre-industrial (1760 to 1880) is a factor of 4.3 for peat and 3.0 for sediment. Pre-industrial accumulation in peat and sediment is a factor of ~5 greater than the pre-colonial era (3000 BC to 1550 AD). Model scenarios that omit atmospheric emissions of Hg from early mining are inconsistent with observational constraints on the present-day atmospheric, oceanic, and soil Hg reservoirs, as well as the magnitude of enrichment in archives. Future reductions in anthropogenic emissions will initiate a decline in atmospheric concentrations within one year, but stabilization of subsurface and deep ocean Hg levels requires aggressive controls. These findings are robust to the ranges of uncertainty in past emissions and Hg cycling.
Article
Tree rings are a useful long term monitor of climate change indicators and certain atmospheric constituents. Since we have no long-term historical record of air mercury (Hg) concentrations, and previous research demonstrates atmospheric uptake of Hg into vegetative biomass, we hypothesized that tree rings would be a useful archive for monitoring changes in local, regional, and global air Hg concentrations. We developed and tested a method for measurement of Hg in tree rings, and then investigated our hypothesis using species within the genus Pinus. Mercury concentrations declined along a coast-to-interior (California-to-Nevada, USA) gradient with highest mean concentrations (5.7 ng/g) at the coast, and lowest (
Article
In the past, ice-core records from mid-latitude glaciers in alpine areas of the continental United States were considered to be poor candidates for paleoclimate records because of the influence of meltwater on isotopic stratigraphy. To evaluate the existence of reliable paleoclimatic records, a 160-m ice core, containing about 250 yr of record was obtained from Upper Fremont Glacier, at an altitude of 4000 m in the Wind River Range of south-central North America. The δ18O (SMOW) profile from the core shows a -0.95‰ shift to lighter values in the interval from 101.8 to 150 m below the surface, corresponding to the latter part of the Little Ice Age (LIA). Numerous high-amplitude oscillations in the section of the core from 101.8 to 150 m cannot be explained by site-specific lateral variability and probably reflect increased seasonality or better preservation of annual signals as a result of prolonged cooler temperatures that existed in this alpine setting. An abrupt decrease in these large amplitude oscillations at the 101.8-m depth suggests a sudden termination of this period of lower temperatures which generally coincides with the termination of the LIA. Three common features in the δ18O profiles between Upper Fremont Glacier and the better dated Quelccaya Ice Cap cores indicate a global paleoclimate linkage, further supporting the first documented occurrence of the LIA in an ice-core record from a temperate glacier in south-central North America.
Article
Human activities over the last several centuries have transferred vast quantities of mercury (Hg) from deep geologic stores to actively-cycling earth-surface reservoirs, increasing atmospheric Hg deposition worldwide. Understanding the magnitude and fate of these releases is critical to predicting how rates of atmospheric Hg deposition will respond to future emission reductions. The most recently-compiled global inventories of integrated (all-time) anthropogenic Hg releases are dominated by atmospheric emissions from preindustrial gold/silver mining in the Americas. However, the geophysical evidence for such large early emissions is equivocal, because most lake-sediment records - reliable repositories of past Hg deposition - cover only the industrial period (1850-present). Here we evaluate historical changes in atmospheric Hg deposition over the last millennium from a suite of lake-sediment cores collected from remote regions of the globe. Along with recent measurements of Hg in the deep ocean, these archives indicate that atmospheric Hg emissions from early mining were modest as compared to more recent industrial-era emissions. Although large quantities of Hg were used to extract New World gold and silver beginning in the 16th century, a reevaluation of historical metallurgical methods indicates that most of the Hg employed was not volatilized, but rather was immobilized in mining waste.
Article
For many types of forest studies, it is essential to identify the exact years of formation of annual rings in increment cores taken from living trees. To accomplish this, dendrochronologists employ cross dating, which involves both ring counting and ring-width pattern matching, to ensure against counting error, or errors, caused by missing or false rings. To date, published accounts of the cross-dating process generally describe a graphical method for achieving cross dating, known as skeleton plotting. However, when working with cores from living trees, skeleton plotting is seldom necessary. Such cores can commonly be cross-dated more quickly and easily by listing the narrow rings that are present in each core in a laboratory notebook and then comparing core notes for shared narrow rings. The latter approach permits faster recognition of ring-width patterns because calendar-year, rather than relative-year, dates are assigned to rings in cores. It also allows cross-dating records to be stored in a more concise manner.
Article
Rubidium solution was injected in the sapwood of a Japanese cedar cultivar in the growing period, and its radial movement in stem was traced to investigate the accumulation of alkali metals in the heartwood. Sapwood-injected Rb was detected in outer heartwood at 10 days after the treatment, and continued increasing at 20 days after. Radial movement of Rb toward heartwood was considered to occur soon after the treatment, and to decline at a certain point of the time after Rb injection ceased. However, Rb continued moving in heartwood probably by diffusion even after the cease of Rb injection. In a series of injection experiment, radial movement of injected Rb is not corresponding to the seasonality of both cambial activity and cytological changes of ray parenchyma accompanied with heartwood formation. From the results on Rb’s behavior, we conclude that accumulation of K and other alkali metals in heartwood of Japanese cedar has two steps, active transport from sapwood to outer heartwood via ray, and diffusion in heartwood, and that these processes proceed independently from both cambial activity and cytological changes of ray parenchyma.
Article
Two activable tracers, Rb and Eu, were injected into the sapwood of Japanese cedars (Cryptomeria japonica D. Don) to investigate the radial movement of minerals in their stems in the resting period. Eight trees of four cultivars, two of which genetically form wet heartwood, were treated near the end of the growing period. At 40 days after the treatment, Rb was detected in the outer heartwood, whereas Eu was not. Radial movement of Rb was more rapid in trees with wet heartwood than in those with normal heartwood. At 204 days after the treatment, more Rb was detected in the heartwood than was found on the first sampling, whereas no Eu was detected in the heartwood. The difference in radial movement between Rb and Eu was considered mainly to be the result of selective transport of beneficial minerals by Japanese cedar. The difference in the rate of radial movement of Rb between wet and normal heartwood became more conspicuous at 204 days after treatment. We concluded that the movement of Rb from the sapwood to the outer heartwood was by active transport through the rays, whereas that in the heartwood was by diffusion due to the gradient of Rb concentration. Key wordsRb–Eu–Neutron activation analysis–Radial transport–Heartwood formation
Article
To determine if a correlation exists between mercury concentrations in lichens and wood from tree cores, tree core and lichen samples were taken from sites under mercury advisories and analyzed for mercury using a Bacharach 50B Mercury Analyzer System employing the Manual Cold Vapor Atomic Absorption EPA method 7471A. Wood from tree cores was dated using tree ring data. Wood samples from the cores were analyzed for various depths in the tree core (periods of time) to determine if variations occur throughout the cores. Preliminary data indicate that a strong correlation exists between mercury concentrations in tree core and lichen samples. Samples were analyzed for the 6-month period of summer 2003–winter 2004. A correlation coefficient of 0.882 was found between the tree core and lichen data sets. The derived correlations were used to estimate concentrations of mercury in tree rings for sites in lichen samples which were previously analyzed. These predicted values compared favorably to recently determined concentrations of Hg in tree cores. The strong correlation between lichen and tree core Hg concentrations suggests similar uptake mechanism for the two types of biota.
Article
Observed wet deposition fluxes of mercury in the United States show a maximum in the Southeast, and a consistent seasonal variation (maximum in summer, minimum in winter) that increases in amplitude from north to south. We simulate these patterns successfully with a global 3-D chemical transport model (GEOS-Chem) including our best estimates of sources and processes. We attribute the high wet deposition over the Southeast in summer to scavenging of upper-altitude Hg(II) by deep convection. Seasonal variation at higher latitudes is attributed to a combination of enhanced summertime oxidation of Hg(0) and inefficient scavenging of Hg(II) by snow. Scavenging of Hg(II) from above the boundary layer contributes over half of wet deposition to the US in the model. Even within the boundary layer, we find that most of Hg(II) originates from the global mercury pool. Wet deposition in the model accounts for only 30% of total mercury deposition in the US, the remainder being from dry deposition, including 42% from Hg(0) uptake. North American anthropogenic emissions contribute 20% of total mercury deposition in the US (up to 50% in the industrial Midwest and Northeast).
Article
Subglacial conditions of large polar ice sheets remain poorly understood, despite recent advances in satellite observation. Major uncertainties related to basal conditions, such as the temperature field, are due to an insufficient knowledge of geothermal heat flow. Here, a hybrid method is presented that combines numerical modeling of the ice sheet thermodynamics with a priori information using a simple assimilation technique. Additional data are essentially vertical temperature profiles measured in the ice sheet at selected spots, as well as the distribution of subglacial lakes. In this way, geothermal heat-flow datasets are improved to yield calculated temperatures in accord with observations in areas where information is available. Results of the sensitivity experiments show that 55% of the grounded part of the Antarctic ice sheet is at pressure melting point. Calculated basal melt rates are approximately 65 Gt year− 1, which is 3% of the total surface accumulation. Although these sensitivity experiments exhibit small variations in basal melt rates, the impact on the ice age of basal layers is quite important. In areas that are characterized by relatively low melt rates, this may lead to standard deviations up to 50% (in a model run over a period of 106 years). Subglacial water flow is concentrated underneath the large outlet glaciers of Antarctica. The larger subglacial lakes are perched at the head of these subglacial water systems. An exception, however, is the lake system of the Recovery Ice Stream Catchment, that receives a substantial amount of subglacial melt water from the upstream catchment.
Article
One of the most critical measurements needed to understand the biogeochemical cycle of mercury, and to verify atmospheric models, is the rate of mercury wet-deposition. The Mercury Deposition Network (MDN) operates sites across North America to monitor total mercury in wet-deposition. MDN's primary goal is to provide both spatial and temporal continental-scale observations of mercury wet-deposition fluxes to support researchers, modelers, policy-makers and the public interest. MDN represents the only continental-scale mercury deposition database with a >10-year record of continuous values. This study provides analysis and interpretation of MDN observations at 10 years (1996–2005) with an emphasis on investigating whether rigorous, statistically-significant temporal trends and spatial patterns were present and where they occurred. Wet deposition of mercury ranges from more than 25 μg m−2 yr in south Florida to less than 3 μg m−2 yr in northern California. Volume-weighted total mercury concentrations are statistically different between defined regions overall (Southeast ≈ Midwest > Ohio River > Northeast), with the highest in Florida, Minnesota, and several Southwest locations (10–16 ng L−1). Total mercury wet-deposition is significantly different between defined regions (Southeast > Ohio River > Midwest > Northeast). Mercury deposition is strongly seasonal in eastern North America. The average mercury concentration is about two times higher in summer than in winter, and the average deposition is approximately more than three times greater in summer than in winter. Forty-eight sites with validated datasets of five years or more were tested for trends using the non-parametric seasonal Kendall trend test. Significant decreasing mercury wet-deposition concentration trends were found at about half of the sites, particularly across Pennsylvania and extending up through the Northeast.
Article
The distribution of total mercury (THg) within common deciduous trees and the applicability of tree cores as biomonitors of historical environmental THg trends were assessed for both contaminated and reference sites around Kingston, Ontario. Samples were collected from Acer spp., Quercus spp. Populus spp. and Salix spp. Bark and wood THg concentrations were found to be highly correlated whereas soil and wood THg concentrations were not. There were no temporal relationships for THg in dated tree rings corresponding with any other known environmental Hg trends. The shoreline species, Populus and Salix spp., had the greatest bark and wood Hg concentrations reaching 18 ng/g, significantly higher than for inland trees Quercus and Acer spp. with maximum values of 7 and 1.2 ng/g for bark and wood respectively. While tree cores cannot be reliably used as temporal THg biomonitors, there is promise for tree species such as Populus spp and Salix spp as spatial indicators of local long-term Hg contamination.
Article
Determining atmospheric deposition rates of mercury and other contaminants using lake sediment cores requires a quantitative understanding of sediment focusing. Here we present a novel approach that solves mass-balance equations fortwo cores algebraicallyto estimate contaminant contributions to sediment from direct atmospheric fallout and from watershed and in-lake focusing. The model is applied to excess 210Pb and Hg in coresfrom Hobbs Lake, a high-altitude lake in Wyoming. Model results for excess 210Pb are consistent with estimates of fallout and focusing factors computed using excess 210Pb burdens in lake cores and soil cores from the watershed and model results for Hg fallout are consistent with fallout estimated using the soil-core-based 210Pb focusing factors. The lake cores indicate small increases in mercury deposition beginning in the late 1800s and large increases after 1940, with the maximum at the tops of the cores of 16-20 microg/m2 x year. These results suggest that global Hg emissions and possibly regional emissions in the western United States are affecting the north-central Rocky Mountains. Hg fallout estimates are generally consistent with fallout reported from an ice core from the nearby Upper Fremont Glacier, but with several notable differences. The model might not work for lakes with complex geometries and multiple sediment inputs, but for lakes with simple geometries, like Hobbs, it can provide a quantitative approach for evaluating sediment focusing and estimating contaminant fallout.
Article
Trees of temperate regions usually form visible annual growth rings, which can be dated accurately. It is therefore possible to collect wood samples of different age and analyse their heavy metals content in order to get a chronological record of trace elements pollution in the tree's environment. This method of retrospective biomonitoring was called dendroanalysis. A basic assumption of dendroanalysis is the stability of the mineral distribution patterns, i.e. once the elements are stored, no significant mobility should occur. Additionally, neighbouring trees growing in the same environment should show similar radial element patterns. While some studies presented good correlations between radial distributions of heavy metals in tree rings and temporal records of pollution from industry or traffic, others failed in using dendroanalysis as a chronological record of pollution. Probably some elements can move at a certain rate in radial direction through the ray parenchyma cells. In this way the radial element distributions are subsequently changed. Growth rates of tree rings can also influence the concentrations of elements in wood. During periods of slow growth higher concentrations of elements can be found in the wood. Therefore, radial distribution patterns of heavy metals in tree rings should be used with caution as a tool for retrospective biomonitoring of environmental pollution.
Article
In a preliminary study, the uptake and the mobility of uranium (U) by black oak trees (Quercus velutina) were assessed by measuring the isotopic composition of tree rings in two mature oak trees in a heavy metal contaminated bog in Concord, MA. The bog is adjacent to a nuclear industrial facility that has been processing depleted uranium (DU) since 1959. Over the past 40 years, DU has been leaking from an onsite holding basin and cooling pond down gradient to the bog where the oaks are located. Because DU has no source outside the nuclear industry, contamination from the industrial facility is readily discernable from uptake of natural U by measuring isotopic compositions. Isotope ratio analysis confirms the occurrence of DU in bark, sapwood and heartwood tree rings dating back to 1937, pre-dating the introduction of DU at the site by at least 20 years. Isotope dilution analysis indicates high concentrations of U (>3 ppb) in sapwood that drop rapidly to relatively constant concentrations (0.3-0.4 ppb) in heartwood. These data indicate that once incorporated into tree cells, U is mobile, possibly by diffusion through the tree wood. Concentrations of U in sapwood are approximately equal to average U concentrations in groundwater onsite over the past 10 years, suggesting that oak trees can be used as present-day bioindicators of U-contaminated groundwater. We suggest that regional sampling of oak bark and sapwood is a reasonable, inexpensive alternative to drilling wells to monitor shallow groundwater U contamination.
Article
Mercury (Hg) contamination of aquatic ecosystems and subsequent methylmercury bioaccumulation are significant environmental problems of global extent. At regional to global scales, the primary mechanism of Hg contamination is atmospheric Hg transport. Thus, a better understanding of the long-term history of atmospheric Hg cycling and quantification of the sources is critical for assessing the regional and global impact of anthropogenic Hg emissions. Ice cores collected from the Upper Fremont Glacier (UFG), Wyoming, contain a high-resolution record of total atmospheric Hg deposition (ca. 1720-1993). Total Hg in 97 ice-core samples was determined with trace-metal clean handling methods and low-level analytical procedures to reconstruct the first and most comprehensive atmospheric Hg deposition record of its kind yet available from North America. The record indicates major atmospheric releases of both natural and anthropogenic Hg from regional and global sources. Integrated over the past 270-year ice-core history, anthropogenic inputs contributed 52%, volcanic events 6%, and background sources 42%. More significantly, during the last 100 years, anthropogenic sources contributed 70% of the total Hg input. Unlike the 2-7-fold increase observed from preindustrial times (before 1840) to the mid-1980s in sediment-core records, the UFG record indicates a 20-fold increase for the same period. The sediment-core records, however, are in agreement with the last 10 years of this ice-core record, indicating declines in atmospheric Hg deposition.
Article
Lead concentrations in tree rings of sycamore (Acer pseudoplatanus L.), oak (Quercus robur L.) and Scots pine (Pinus sylvestris L.) sampled at a parkland in north-west England were measured in wood formed since the mid-1800s. Concentrations of Pb in Scots pine and oak peaked in wood formed between 1900 and 1940, most likely because of Pb accumulation in heartwood, indicating that oak and Scots pine are unsuitable for monitoring temporal changes in Pb deposition at the study site. In contrast, Pb concentrations in sycamore, a species that has similar heartwood and sapwood chemistry, were relatively constant in wood formed between the mid-1800s and 1950. Lead concentrations decreased steadily in sycamore tree rings formed after the 1950s, and decreased more abruptly in wood formed after 1985. This sharp decrease in wood Pb cannot be due to decreases in soil Pb concentration. Stable Pb isotope analysis was used to further investigate Pb patterns in sycamore wood. Excess 206Pb/207Pb ratios in tree-rings of sycamore were relatively constant, approximately 1.17, in wood formed prior to the 1930s, but decreased steadily thereafter reaching a minimum value of approximately 1.16 in wood formed between 1975 and 1985 after which time 206Pb/207Pb ratios increased. This pattern is consistent with changes in Pb isotope ratios measured in peat, sediment and aerosol samples in the UK. However, the magnitude of the decrease in 206Pb/207Pb (largely due to gasoline Pb) is considerably lower than in other studies and our estimates indicate that less than 20% of the total Pb in sycamore wood measured since the mid-1800s is derived from gasoline emissions. A more likely explanation for the pattern of Pb observed in sycamore tree rings is that soil Pb accumulates within rings of the diffuse porous wood over a number of years. Such uptake patterns would result in lower Pb concentrations in the outer (more recently formed) tree rings, which coincide with recent reductions in Pb deposition in the UK. Overall, this study indicates that tree ring chemistry is unsuitable for monitoring historical changes in Pb deposition at the study site.