Hg Accumulation Rates Research Articles

Influence of global warming and human activity on mercury accumulation patterns in wetlands across the Qinghai-Tibet Plateau

Abstract Wetlands in the Qinghai-Tibet Plateau (QTP) are a unique and fragile ecosystem undergoing rapid changes. We show two unique patterns of mercury (Hg, a global pollutant) accumulation in wetland sediments. One is the ‘Surface Peak’ in monsoon-controlled regions, and the other is the ‘Subsurface Peak’ in westerly-controlled regions. The former is attributed to the combined effects of increasing anthropogenic emissions and climate-induced changes in the cryosphere and wetland hydrology in the last 100 − 150 years. The climate changes in westerly-controlled regions in the last 50 − 70 years led to a fluctuation of hydrology and an Hg peak in the sediment subsurface. The increase of legacy Hg input from soil erosion largely enhanced the Hg accumulation rate in wetlands since the 1950s, especially in the proglacial wetlands. We highlight that accelerated glacier melting and permafrost thawing caused by global warming have altered geomorphology and hydrology and affected Hg transport and accumulation in wetlands.

A 14,000‐Year Sediment Record of Mercury Accumulation and Isotopic Signatures From Lake Malaya Chabyda (Siberia)

AbstractEurasian permafrost soils contain large amounts of organic carbon (OC) and mercury (Hg), sequestered by vegetation during past and present interglacial periods. Lake sediment archives may help understand past OC and Hg dynamics and how they interact with climate‐related variables. We investigated Hg accumulation, OC dynamics, and Hg and OC stable isotopes in a 14,000‐year sediment record from Lake Malaya Chabyda (Central Yakutia, Russia). Sediment Hg was correlated to OC (p value < 0.01), with lower OC and Hg accumulation rates (OCAR, HgAR) during the cold Younger Dryas (YD, 12,900–11,700 cal BP), when the lake level was low. Elevated sediment Δ200Hg (0.05‰ ± 0.11‰), representing dominant HgII deposition, and low δ13C, indicates low lake primary productivity during the YD. During the early Holocene, Δ200Hg and Δ199Hg decreased, while δ13C, δ202Hg, OCAR, and HgAR increased, suggesting enhanced algal primary productivity in deeper, more turbid waters. From 4,100 cal BP to present, Hg/OC ratios and HgAR increased at constant OCAR, indicating an additional Hg source to the lake. Analysis of Hg isotopes suggests direct Hg0 uptake into lake waters, potentially driven by primary production and efficient Hg burial. Our observations suggest that the gradual climate warming since the Last Glacial Termination and into the early Holocene led to enhanced OC and Hg burial in northern lakes and watersheds. Late Holocene enhanced Hg burial, but not OC, is possibly related to a renewed increase in lake primary productivity. Continued global warming may lead to further Hg sequestration in northern aquatic ecosystems.

Dominant host phase of mercury within the sediments of the East China Sea inner shelf: Implications for mercury inputs

Mercury (Hg) concentrations normalized to their dominant hosts have been widely used to identify geological events such as massive volcanic eruptions. However, the modern Hg cycle has significantly changed, and the implications of host-normalized Hg concentrations for the Hg dynamics in contemporary coastal environments (e.g., sources, delivery mechanisms, and Hg fluxes) remain poorly understood. In this study, 66 surface sediment samples from the continental shelf of the East China Sea (ECS) were analyzed, and the total organic carbon (TOC), aluminum (Al), total sulfur (TS), and Hg concentrations were determined. Our results support that organic matter predominantly hosts Hg in these sediments. Factors such as the Hg source and abundances of organic fractions influence Hg distributions, with notable enrichment in the Changjiang River estuary and the inner shelf of the ECS, especially near the mud depocenter located off the Oujiang River estuary. The oxic to suboxic redox state and the lack of significant sulfide accumulation in surface sediments suggest that sulfides are not the primary Hg hosts. The Hg/TOC ratios, ranging from 3.0 to 169.6 (ppb/%), are particularly high in the Changjiang River estuary and exhibit an exponential correlation with the Hg accumulation rates (Hg-AR). These findings suggest that the host-normalized Hg concentration could serve as a valuable tool for assessing Hg dynamics where the accumulation rates are uncertain.

Quantifying soil accumulation of atmospheric mercury using fallout radionuclide chronometry

Soils are a principal global reservoir of mercury (Hg), a neurotoxic pollutant that is accumulating through anthropogenic emissions to the atmosphere and subsequent deposition to terrestrial ecosystems. The fate of Hg in global soils remains uncertain, however, particularly to what degree Hg is re-emitted back to the atmosphere as gaseous elemental mercury (GEM). Here we use fallout radionuclide (FRN) chronometry to directly measure Hg accumulation rates in soils. By comparing these rates with measured atmospheric fluxes in a mass balance approach, we show that representative Arctic, boreal, temperate, and tropical soils are quantitatively efficient at retaining anthropogenic Hg. Potential for significant GEM re-emission appears limited to a minority of coniferous soils, calling into question global models that assume strong re-emission of legacy Hg from soils. FRN chronometry poses a powerful tool to reconstruct terrestrial Hg accumulation across larger spatial scales than previously possible, while offering insights into the susceptibility of Hg mobilization from different soil environments.

Organic matter, eutrophication, and increased mass accumulation rates in the Anthropocene are the main drivers of mercury concentrations and historical enrichment in Canadian lake sediments

AbstractMercury (Hg) contamination in the environment is a persistent issue as emissions from industry are ongoing and legacy cycling is prolonged. To quantify total mercury (total Hg) and methylmercury (MeHg) concentrations in lake sediments across a wide suite of temperate to subarctic lakes, measurements and modeling were conducted for 320 sites across Canada. Total Hg varied from 7 to 567 ng g−1 dry sediment with a mean (median) value of 148 (128) ng g−1 dry sediment, with lowest concentrations observed in eutrophic lakes of the Prairies and Boreal Plains. Average total Hg concentrations have increased twofold in contemporary sediments relative to their preindustrial pair. MeHg concentrations were generally low, with a mean of 0.8 ng g−1 dry sediment and this form representing on average 0.6% of the total contemporary mercury concentrations. Water column variables related to eutrophication were among the strongest predictors of contemporary total Hg, MeHg, and total Hg enrichment in the sediments. In lakes where sediment radiometric data were collected, contemporary total Hg accumulation rates were strongly related to longitude(+) and Hg atmospheric deposition(+). Latitude(−) and Hg atmospheric deposition(+) were the best predictors of MeHg accumulation rates. Chromophoric dissolved organic matter optical properties were key predictors of both total Hg concentrations and accumulation rates. Our work highlights the paramount importance of lake trophic state and the nature of organic matter as key predictors of lake sediment mercury metrics.

Fire, volcanism and climate change: the main factors controlling mercury (Hg) accumulation rates in Tropical Lake Lantoa, Sulawesi, Indonesia (∼16,500–540 cal yr BP)

The effects of climate change on long-term mercury (Hg) cycling are still not well understood, as climate changes are usually gradual and can only be assessed using high-resolution archives. Our study site (a small, lowland tectonic lake in Sulawesi, Indonesia) provides a unique opportunity to further understanding of Hg cycling in the Southeast Asian (SEA) tropics during the transition from the Pleistocene to the Holocene, a period of significant climate variability. We present a high-resolution record of Late Glacial and Holocene Hg deposition within the sediments of tropical Lake Lantoa, Sulawesi. Using a multi-proxy framework (including pollen, charcoal, carbon:nitrogen ratio and high-resolution geochemistry records) we investigate the response of Hg accumulation rates (HgAR) in sediments to shifts in climate between ∼16,488 and 538 cal BP. This period encompasses the Bølling-Allerød (BA) warming, Younger Dryas (YD) cooling and Holocene warming events, providing new insights into the effects of global climatic transitions on HgAR in SEA sediments. The Pleistocene Termination had the highest HgAR and substantial variability (µ = 11.32, 5.38–33.91 μg m−2 yr−1), when drier conditions and high charcoal accumulation rates suggest that fire activity was the main source of Hg to the lake. The Holocene Transition was marked by a decrease in HgAR (µ = 8, 3.50–18.84 μg m−2 yr−1) as humid conditions precluded forest burning, followed by high HgAR (µ = 11.35, 3.30–158.32 μg m−2 yr−1) in the Early Holocene. Mercury accumulation rate in the Late Holocene (µ = 3.80, 1,67–43.65 μg m−2 yr−1) was the lowest in the Lake Lantoa record, marked by the lowest fire events and a stable catchment. An increase in carbon:nitrogen ratios during the Late Holocene, coupled with a decrease in HgAR, suggests that the establishment of lowland forest resulted in suppressed Hg erosion/leaching. Our results demonstrate that forest fires, vegetation change and volcanism are important drivers of Hg inputs to Lake Lantoa, a relationship which is strongly mediated by climate and lake-catchment dynamics.

Mercury deposition in the Eastern Mediterranean: Modern fluxes in the water column and Holocene accumulation rates in abyssal sediment

Modern and past mercury (Hg) fluxes in the oceanic water column and abyssal sediments are poorly quantified. Here, we investigated the particulate transfer of Hg in the water column of the ultra-oligotrophic Ionian Sea (Eastern Mediterranean) with sediment traps during a one-year period, and its accumulation in the deep central abyssal plain using sediment cores comprising the last 10 ka. The Hg concentrations in the particles collected in the sediment traps varied from 112 to 401 ng g−1 and enabled quantifying annual Hg fluxes of 2.0, 2.5, and 2.5 μg m−2 a−1, for traps deployed at 250, 1440, and 2820 m deep, respectively. Hg collected in the upper trap originates from atmospheric deposition, including Saharan dust, which is scavenged by the biological pump. Higher Hg fluxes found at mid-depth and near-bottom than in the upper water layer are attributed to lateral advection under the mixed layer of Hg-rich resuspended sediments from the Adriatic continental margin. In the abyssal sediment, Hg concentrations range from 15 to 134 ng g−1 with the highest levels in the Sapropel S1. Methylmercury concentrations varied from 0.06 to 0.24 ng g−1 following the distribution of total Hg, with evidence of its specific accumulation at the oxidized front of the sapropel. We estimated that <1.8% of the total Hg in the sedimentary column was diagenetically reallocated. The reconstruction of historical Hg accumulation rates (HgAR) during the Holocene shows low pre-anthropogenic values (∼0.3 μg m2 a−1 before 4 ka BP), increasing up to ∼0.9 μg m2 a−1 during the late Iron Age and the Roman period (1.5–2.5 ka BP), and up to 2.9 μg m2 a−1 during the Industrial Era. During the Sapropel S1 period (∼6–10 ka BP), HgARs rose to 6.4 μg m−2 a−1 likely due to the intensity of the Hg removal by the biological pump, the organic matter preservation, along with high inputs of Hg-rich terrigenous matter and a possible restricted recycling in the atmosphere. Hg accumulation in the Ionian Sea deep sediment is found ∼3-fold lower than those in the western Mediterranean abyssal plain.

In Vivo Mercury (De)Methylation Metabolism in Cephalopods under Different pCO2 Scenarios.

This work quantified the accumulation efficiencies of Hg in cuttlefish, depending on both organic (MeHg) and inorganic (Hg(II)) forms, under increased pCO2 (1600 μatm). Cuttlefish were fed with live shrimps injected with two Hg stable isotopic tracers (Me202Hg and 199Hg(II)), which allowed for the simultaneous quantification of internal Hg accumulation, Hg(II) methylation, and MeHg demethylation rates in different organs. Results showed that pCO2 had no impact on Hg bioaccumulation and organotropism, and both Hg and pCO2 did not influence the microbiota diversity of gut and digestive gland. However, the results also demonstrated that the digestive gland is a key organ for in vivo MeHg demethylation. Consequently, cuttlefish exposed to environmental levels of MeHg could exhibit in vivo MeHg demethylation. We hypothesize that in vivo MeHg demethylation could be due to biologically induced reactions or to abiotic reactions. This has important implications as to how some marine organisms may respond to future ocean change and global mercury contamination.

Mercury bioaccumulation in thresher sharks from the eastern tropical Pacific: Influences of body size, maturation stage, and feeding habitat

Sharks, as top order predators, provide a guidance on how contaminants such as mercury bioaccumulate in marine environments. This study assessed the bioaccumulation of mercury (total mercury, THg) in the muscle, liver, red blood cells (RBC), and plasma of pelagic and bigeye thresher sharks (Alopias pelagicus and A. superciliosus) from eastern tropical Pacific. Additionally, the concentration of methylmercury (MeHg) in muscle was also determined to assess risks for human consumption. For both species, muscle THg concentrations (4.05 ± 2.15 and 4.12 ± 1.84 μg g−1 dry weight for pelagic and bigeye thresher shark) were higher than that in other tissues. THg concentrations for all tissues were significantly correlated with precaudal length, with higher accumulation rates after maturity in pelagic than bigeye thresher sharks, suggesting an associated dietary shift at maturation. Correlations among tissues in both species suggested similar transportation and distribution patterns in internal tissues. The δ13C values in muscle, RBC and plasma suggested that habitat shifts influenced Hg accumulation, whereas trophic position, estimated by δ15N values, had limited effects on patterns of Hg bioaccumulation. Diet shifts towards prey more cephalopods that content higher Hg than small fishes (large fishes: 1.77 μg g−1; cephalopods: 0.66 μg g−1 and small fishes 0.48 μg g−1, dry weight) increased Hg accumulation rates in adult pelagic thresher sharks. Concentrations of MeHg in the muscle of both thresher shark (3.42 ± 1.68 μg g−1 in A. pelagicus and 3.78 ± 2.13 μg g−1 in A. superciliosus) exceeded the recommended levels for human consumption. This research provides insight into the factors influencing mercury bioaccumulation in thresher sharks, which are essential for the management and conservation of these species.

Prehistoric and technogenic loads of Hg in raised and blanket peats from the lower Amur River basin, eastern Asia.

Prehistoric and modern records of Hg in peats from the lower Amur River basin, eastern Asia, were investigated. The Hg accumulation rates (AR) of two 14C-dated cores from Holocene raised and blanket peats were determined. The low and monotonous Hg AR values of the raised peats determined in the lower part of the core (mean 1.9 μg Hg per m2 per year) represent prehistoric Hg inputs from the atmosphere. A sharp increase in Hg AR was determined upward of 52 cm depths (∼1600 year BP) (max 17.0 Hg μg per m2 per year), which apparently reflects the modern increase in global Hg emissions. The Hg loads in the blanket peats were apparently defined by a combined input from the atmosphere and the watershed followed by diagenetic redistribution. One major peak (max 12.0 μg Hg per m2 per year) coincided with increased Corg contents in herbaceous and sphagnum peat formed in the warming and wet climate of the early Holocene. Another peak (11.8 μg Hg per m2 per year) correlated both with the accumulation of Fe/Mn compounds and the warm and wet climate of the Holocene Climate Optimum. Minima Hg AR values coincided with the weakening of the East Asia Monsoon and cold and dry climate periods. In the uppermost layers, the Hg AR increased again up to 7.3 μg Hg per m2 per year, most likely due to anthropogenic pollution.

The timing and magnitude of anthropogenic mercury pollution: A 200-year record from multi-lake sediment cores in northeast China

The recent substantial expansion of human activities in northeast (NE) China has resulted in increased emission of environmental pollutants. Longer-term records of such environmental pollutants provide a benchmark against which it is possible to evaluate the nature, extent and timing of anthropogenic environmental changes. Based on measurements of mercury (Hg) concentrations and accumulation rates in 11 lake sediment cores from the Songnen Plain in NE China, we here present a reconstruction of the historical deposition of Hg as an indicator of the changing scale of human impact. The results demonstrate an increasing trend of Hg concentration, concurrent with elevated anthropogenic emissions, beginning from the early 1900s, accelerating through the mid-1950s and slightly decreasing from the late 1990s onwards. The increase in anthropogenic Hg coincides with the reform and opening up of China, which precipitated social and economic transformation, and rapid industrial and economic growth. Measurements of the Hg enrichment factor in all the cores enables identification of the anthropogenic contribution to Hg accumulation. The geoaccumulation index indicates that the lakes are in general moderately polluted by Hg. The historical trend of Hg accumulation rate parallels the temporal progression of biomass burning and fossil fuel consumption in the region. The findings elucidate the extent of anthropogenic pollution in the Anthropocene and underline the importance of identifying Hg sources to reduce emissions and guide the implementation of effective mitigation strategies.

Wildfires trigger multi-decadal increases in sedimentation rate and metal loading to subarctic montane lakes

We evaluated how two large wildfires affected the sedimentation rate and accumulation of lead (Pb), mercury (Hg), and cadmium (Cd) in sediment of four subarctic montane lakes in the Yukon, Canada. The wildfires occurred 60 and 20 years (1958, 1998) before sediment collection in 2018. Site-specific fire exposure was inferred from the charcoal accumulation histories in the lake sediments and the burned catchment area was determined from historical fire maps. The two major wildfires caused a two to five-fold increase in sedimentation rates and a two to eight-fold increase in sediment metal accumulation rates in Little Fox Lake. The mass accumulation rates of metals in Little Fox Lake sediment increased by a maximum of 2.7–4.7 mg Pb m−2 yr−1, 19–29 μg Hg m−2 yr−1 and 37–114 μg Cd m−2 yr−1 following wildfires. Modelling using elemental ratios of lithogenic sources suggested a large proportion of the Pb and Hg accumulating in post-fire sediment was from remobilized legacy anthropogenic pollution. In contrast, Cd fluxes were consistent with variation in catchment weathering. Impacts of wildfires were visible but more muted in the sediment of Little Braeburn Lake, whereas Fox Lake and Grayling Lake sediments showed little to no wildfire impact and served as a reference for external (long-range) metal deposition. Major changes to lake sediment geochemistry in Little Fox Lake were caused by the lack of vegetation and soil recovery in the catchment following the severe 1998 fire. Wildfire impacts were persistent in the lake more than 20 years after the last fire, with no sign of a return to pre-fire Pb, Hg, and Cd accumulation rates. This study shows that wildfires in northern montane catchments can significantly increase the rate of metal accumulation in affected lakes, thereby impeding recovery from reductions in anthropogenic air emissions of these metals.

Climatic Controls on a Holocene Mercury Stable Isotope Sediment Record of Lake Titicaca

Mercury (Hg) records in sediment archives inform past patterns of Hg deposition and the anthropogenic contribution to global Hg cycling. Natural climate variations complicate the interpretation of past Hg accumulation rates (HgARs), warranting additional research. Here, we investigated Hg stable isotopes in a ca. 8k year-long sediment core of Lake Titicaca and combined isotopic data with organic biomarkers and biogeochemical measurements. A wet period in the early Holocene (8000–7300 BP) induced strong watershed erosion, leading to a high HgAR (20.2 ± 6.9 μg m–2 year–1), which exceeded the 20th century HgAR (8.4 ± 1.0 μg m–2 year–1). Geogenic Hg input dominated during the early Holocene (fgeog = 79%) and played a minor role during the mid- to late Holocene (4500 BP to present; fgeog = 20%) when atmospheric Hg deposition dominated. Sediment Δ200Hg values and the absence of terrestrial lignin biomarkers suggest that direct lake uptake of atmospheric Hg(0), and subsequent algal scavenging of lake Hg, represented an important atmospheric deposition pathway (42%) during the mid- to late Holocene. During wet episodes of the late Holocene (2400 BP to present), atmospheric Hg(II) deposition was the dominant source of lake sediment Hg (up to 82%). Sediment Δ199Hg values suggest that photochemical reduction and re-emission of Hg(0) occurred from the lake surface. Hg stable isotopes show promise as proxies for understanding the history of Hg sources and transformations and help to disentangle anthropogenic and climate factors influencing HgAR observed in sediment archives.

Foraging plasticity diversifies mercury exposure sources and bioaccumulation patterns in the world's largest predatory fish

Large marine predators exhibit high concentrations of mercury (Hg) as neurotoxic methylmercury, and the potential impacts of global change on Hg contamination in these species remain highly debated. Current contaminant model predictions do not account for intraspecific variability in Hg exposure and may fail to reflect the diversity of future Hg levels among conspecific populations or individuals, especially for top predators displaying a wide range of ecological traits. Here, we used Hg isotopic compositions to show that Hg exposure sources varied significantly between and within three populations of white sharks (Carcharodon carcharias) with contrasting ecology: the north-eastern Pacific, eastern Australasian, and south-western Australasian populations. Through Δ200Hg signatures in shark tissues, we found that atmospheric Hg deposition pathways to the marine environment differed between coastal and offshore habitats. Discrepancies in δ202Hg and Δ199Hg signatures among white sharks provided evidence for intraspecific exposure to distinct sources of marine methylmercury, attributed to population and ontogenetic shifts in foraging habitat and prey composition. We finally observed a strong divergence in Hg accumulation rates between populations, leading to three times higher Hg concentrations in large Australasian sharks compared to north-eastern Pacific sharks, and likely due to different trophic strategies adopted by adult sharks across populations. This study illustrates the variety of Hg exposure sources and bioaccumulation patterns that can be found within a single species and suggests that intraspecific variability needs to be considered when assessing future trajectories of Hg levels in marine predators.

Mercury Accumulation in Marine Sediments – A Comparison of an Upwelling Area and Two Large River Mouths

Understanding marine mercury (Hg) biogeochemistry is crucial, as the consumption of Hg-enriched ocean fish is the most important pathway of Hg uptake in humans. Although ocean sediments are seen as the ultimate Hg sink, marine sediment studies on Hg accumulation are still rare. In this context, studying Hg behavior in the marine environment, especially in upwelling environments, is of particular interest due to its importance in these great upwelling regions for the global fishery. There are contradictory statements about the fate of Hg in upwelling regions. Some studies have suggested high biotic reduction of oxidized Hg and gaseous elemental mercury evasion to the atmosphere. More recent work has suggested that in upwelling regions, where productivity is high, evasion of gaseous elemental mercury is diminished due to scavenging and sedimentation of Hg by organic particles. In this study, we compared Hg concentrations and accumulation rates in the past ∼4,300 and 19,400 years derived from sediment cores collected in the Peruvian upwelling region (Peru Margin) and compared them with those of two other cores collected from the sediment fan of the Amazon and a core from the Congo Basin, which is influenced by both seasonal coastal upwelling and discharge from the river. Median Hg concentrations were higher at the Peru Margin (90.7 μg kg–1) and in the Congo Basin (93.4 μg kg–1) than in the Amazon Fan (35.8 μg kg–1). The average Hg accumulation rates in sediments from the Peru Margin (178 μg m–2 yr–1) were factors of ∼4 and ∼39 times higher than those from the Congo Basin (46.7 μg m–2 yr–1) and Amazon Fan (4.52 μg m–2 yr–1), respectively. Principal component analysis (PCA) of the geochemical data set reveals that Amazon Fan sediments are strongly influenced by the deposition of terrestrial material, which is of less importance in the Congo Basin and of minor importance in Peru Margin sediments. Accordingly, Hg export to sediments in upwelling areas largely surpasses that in fans of large rivers that drain large terrestrial catchments. The high Hg accumulation rates in the sediments from the upwelling area and the minor influence of terrestrial Hg fluxes there suggest that atmospheric-derived Hg in upwelling areas is effectively exported to the sediments through scavenging by organic particles.

Mercury accumulation in the sediment of the Western Mediterranean abyssal plain: A reliable archive of the late Holocene

Temporal reconstruction of Hg deposition from sediment archives is relatively straightforward in organic-rich or high sedimentation rate environments, such as lakes and ocean margins. To retrieve long-term records at regional or global scales, deep-sea sediments are more appropriate, but such records are scarce and their reliability has been questioned because of possible post-depositional Hg diagenetic remobilization. Here, we investigated the accumulation of Hg in the Balearic Abyssal Plain (2850 m deep) of the Western Mediterranean through a comprehensive characterization of the chemical and isotopic composition (organic carbon, nitrogen, sulfur, major and redox-sensitive elements) of sediment trap material and sediment cores. The analysis of material collected in the sediment traps, deployed at 250, 1440, and 2820 m, indicates that Hg is (i) partially re-emitted to the atmosphere and mobilized in the twilight zone and that (ii) the Hg downward flux depends on the primary production in surface waters, suggesting that organic matter (OM) acts as the main Hg-carrier phase. As the Hg concentrations of material collected in the traps vary little with depth but the Hg:Corg ratio of the settling particulate matter decreases with depth, Hg must be re-adsorbed onto the more refractory fraction of the settling OM. Results of selective chemical extractions of the sediment indicate that Hg is very weakly coupled to the iron cycle but strongly associated with sulfur, supporting the assumption that its vertical distribution was only weakly altered by diagenetic remobilization. In addition, the distributions of S and δ34S in the sedimentary column exclude the possibility that local volcanism impacted on Hg enrichment of the sediments. Accordingly, a reconstruction of Hg accumulation rates (Hg-AR) during the Late Holocene is readily achieved. Biological mixing and smoothing of the sediment record, as revealed by the distribution of radionuclides in surface sediments, was considered in the interpretation of the Hg-AR record. The first anthropogenic Hg signal recorded in the studied cores corresponds to the Iron Age and the Roman Empire period, as Hg-ARs rose from the baseline (0.7 ± 0.2 µg m−2 yr−1) to an average value of 2.2 ± 0.5 µg m−2 yr−1. The Hg-ARs return to baseline values at the decline of the Roman Empire, display a small increase during the Medieval Period (1.5 ± 0.5 µg m−2 yr−1), increase abruptly at the onset of the Industrial Era, leading to a ~ 10-fold increase in Hg deposition in the last 120 years (8.9 ± 1.4 µg m−2 yr−1), and retreat progressively over the past 50 years.

Bog and lake sediment archives reveal a lagged response of subarctic lakes to diminishing atmospheric Hg and Pb deposition

We used a flux deconstruction approach on peat and sediment archives of four bogs and five lakes from two subarctic taiga ecoregions of the Northwest Territories (Canada) to distinguish the atmospheric and catchment-based responses to changing metal pollution emissions over the last 2000 years. Bogs tracked the atmospheric signal, whereas lake sediments provided a mixed atmospheric and catchment-based response. Anthropogenic mercury (Hg) and lead (Pb) contamination was identified in lake sediment and bog records from the mid-1800s onward and increased rapidly after ca. 1900. Long-range transport of Hg and Pb was likely the dominant source of the post-1900 enrichment in lake sediments, with minor contributions from local mining, mostly between 1950 and 1970. Bogs and sediment records of small lakes on the Taiga Plains showed that atmospheric deposition peaked in the late 1990s for Pb and in the 1970s for Hg. In contrast, the Pb and Hg accumulation rates in Taiga Shield lakes have continued to increase since the peak atmospheric deposition period due to on-going catchment transport, and may reflect recent climate change favoring late-fall and early-winter precipitation as rain rather than snow. The divergent trends in metal accumulation rates between ecoregions and environmental archives demonstrate that interactions of climate and catchment characteristics will be key to future contamination trajectories for subarctic lakes following reductions of anthropogenic metal emissions in North America.

Why productive lakes are larger mercury sedimentary sinks than oligotrophic brown water lakes

AbstractMercury accumulation in lake sediments is a widespread environmental problem due to the biomagnification of Hg in the aquatic food chain. Soil Hg concentrations, catchment vegetation, erosion, and lake productivity are major factors controlling the accumulation of Hg in lakes. However, their influence on the Hg mass balance in lakes with different catchment characteristics and trophic state is poorly understood. In this multilake study, we decipher the effects of catchment vegetation (coniferous vs. deciduous forest), soil Hg content, and trophic state on Hg sedimentation at six lakes in Germany. We investigated Hg concentrations in leaves, soils, and the lake's water phase. Soils under coniferous stands show slightly higher Hg concentrations than under deciduous forest. Hg concentrations in the water phase were higher in the oligotrophic brown water lakes (8.1 ± 5.6 ng L−1 vs. 3.0 ± 1.9 ng L−1). Lower Hg concentrations in sediment trap material indicate dilution by algae organic matter in the mesotrophic lakes (0.12–0.17 μg g−1 vs. 0.57–0.89 μg g−1). However, Hg accumulation rates in sediment traps were up to 14‐fold higher in the mesotrophic lakes (113–443 μg m−2 yr−1) than in the brown water lakes (32–144 μg m−2 yr−1), which could not be explained by higher Hg fluxes to the productive lakes. Hg mass balance calculation reveals that water phase Hg scavenging by algae is the major reason for the intense Hg export to the sediments of productive lakes which makes them significantly larger sedimentary sinks than oligotrophic brown water lakes.

The effect of EDTA and citric acid on biochemical processes and changes in phenolic compounds profile of okra (Abelmoschus esculentus L.) under mercury stress

The present study aimed to explore the effect of synthetic and naturally occurring chelators, EDTA and citric acid (CA), respectively, on changes in physiological and biochemical factors including cell death, level of mercury ions accumulation, malondialdehyde (MDA) content, total phenol and total flavonoids, anthocyanins and DPPH free radical scavenging activity, in the leaves of okra (Abelmoschus esculentus L.) plants exposed to mercury stress. In addition, polyphenolic compounds profile was assessed by high-performance liquid chromatography. The okras were planted in completely controlled hydroponic conditions (Hoagland solution). After they reached the four-leaf stage, they were treated simultaneously with different concentrations of HgCl2, EDTA and CA chelators, and their combination for one month. At the stage of maturity, the physiological and biochemical factors of the plant leaves were measured. The results showed that with the application of higher concentration of HgCl2, cell death, level of shoot and root Hg2+ content and root MDA, total phenols and total flavonoids, anthocyanin content, and DPPH free radical scavenging activity were increased. Also, the results indicated that okra plants have high biomass and a high rate of Hg mobilization and accumulation in the shoot versus the roots (TF=2.152 for the plants treated with 60 mg L−1 Hg2+), hence, can be considered as Hg hyperaccumulator plant for the phytoremediation of Hg-polluted soils and waters. In the Hg-treated plants changes in their phenolic profile were induced, and the increase of chlorogenic acid, rosmaric acid, apigenin, quercetin and rutin content was observed. The application of EDTA and CA improved the toxic effects of Hg2+, by modifying phenolic compounds, chelating Hg2+, and its proper compartmentation, while EDTA outperformed CA in this respect. Based on the results, it could be concluded that due to the high biomass and growth of okra in the presence of Hg2+, this plant is suitable for phytoremediation of soil and water contaminated with mercury. In addition, EDTA and CA can play a significant role in removing this toxic metal through transferring it from the culture medium to the plant.

Unexpected Heavy Metal Pollution in Wetland Soils Along the Yarlung Tsangbo River in Tibet, China: Assessment and Source Apportionment

The Qinghai-Tibetan Plateau (QTP), as the third pole, has recently experienced unexpected heavy metal pollution. However, few were available on heavy metal pollution and source apportionment in regions near South Asia, where the economy is currently experiencing rapid development. This paper evaluated heavy metal pollution in wetland soils along the Yarlung Tsangbo River (YTR), the largest river in the QTP. Results indicated minor contamination levels of Cu, Pb, Zn, Hg, As, Mn, and Cr. However, Cd pollution was serious and was the main pollutant. Nemero synthesis index values (PN) were relatively high and ranged from 4.54 to 10.47 caused by high loading from Cd and Zn. Roughly 92% Pb, 89% Cr, and 89% Mn were from natural sources while 86% Cd and 61% Zn were from anthropogenic sources in wetland soils along the YTR. Differences in heavy metal contents and sedimentation rates were accentuated by the metal accumulation rates of each site. Cd, Zn, and Hg accumulation rates were estimated to 0.966 mg m−2 year−1, 0.187 g m−2 year−1, and 0.061 mg m−2 year−1 in the 1800s and increased to 5.351 mg m−2 year−1, 1.144 g m−2 year−1, and 0.263 mg m−2 year−1 in the 2000s, respectively. This implied unexpected heavy metal pollution in wetland soils along the YTR basin and more attention should be paid to avoid the risk of heavy metal pollution here.