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

Reliability assessment of using different heavy metals in remote sedimentary records to reconstruct historical pollution

Interpreting heavy metal variations in sedimentary records is an important approach to reconstruct historical pollution. However, few studies have investigated the reliability of using different heavy metals in sedimentary records for reconstructing historical pollution. This study retrieved sediment cores from two adjacent remote lakes in North China and investigated their temporal changes in excessive metal fluxes. Combined with a novel index of the ratio of atmospheric emissions of anthropogenic metals to background metal concentrations in lake sediments, the reliability and influencing factors of using eight metals (Cr, Co, Ni, Cu, As, Zn, Cd, and Pb) in sediments to reconstruct historical pollution were studied. (1) Vertical As variations in the sediments were controlled mainly by early diagenesis effects, although anthropogenic pollution might also have had an impact. Consequently, As profiles in lake sediment records are usually difficult to reflect historical pollution. (2) Among the eight metals, Zn, Cd, and Pb in both lakes have been affected considerably by anthropogenic pollution and insignificantly by natural factors. Their excessive flux variations in the two records were highly consistent with the historical trend of economic/industrial development. Relatively, these three metals can reliably reflect historical pollution. (3) Compared with Zn, Cd, and Pb, Cr, Co, Ni, and Cu in the two lakes have been relatively less affected by anthropogenic pollution. During the high anthropogenic metal emission period (post-∼1980), they could roughly reflect historical pollution in the region; however, during the low emission period (before ∼1980), they appeared to exhibit no signs of pollution. This suggests the complexity of using heavy metals in remote sediment records to reconstruct historical pollution. The importance of this study lies in its potential to provide guideline for the use of aquatic sediment records from relatively remote areas in reconstructing the history of metal pollution.

Tree Rings Mercury Controlled by Atmospheric Gaseous Elemental Mercury and Tree Physiology

Tree rings are an emerging atmospheric mercury (Hg) archive. Questions have arisen, though, regarding their mechanistic controls and reliability. Here, we report contrasting tree-ring Hg records in three collocated conifer species: Norway spruce (Picea abies), Scots pine (Pinus sylvestris), and European larch (Larix decidua), which are from a remote boreal forest. Centennial atmospheric Hg trends at the site, derived from varved lake sediments, peats, and atmospheric monitoring, indicated a steady rise from the 1800s, peaking in the 1970s, and then declining. Prior to ca. 2005, larch and spruce tree rings reproduced the peak in the atmospheric Hg trend, while pine tree rings peaked in the 1930s, likely due to the prolonged sapwood period and ambiguity in the heartwood-sapwood boundary of pine. Since ca. 2005, tree rings from all species showed increasing Hg concentrations in the physiologically active outer rings despite declining atmospheric Hg concentrations. The good agreement between Hg and nitrogen concentrations in active tree-ring cells indicates a similar transport mechanism and cautions against their applicability as atmospheric Hg archives. Our results suggest that tree-ring Hg records are controlled by atmospheric Hg and tree physiology. We provide recommendations for using tree-ring Hg archives that take tree physiology into account.

Three thousand years of Hg pollution recorded in mangrove wetland sediments from South China

Mercury (Hg) is known to affect aquatic, terrestrial ecosystems as well as human health, through biomagnification. Mangrove wetlands are potential Hg sinks because of their low tidal velocity, fast sedimentation rate, strong reducing condition and high organic matter content. The spatial and temporal distribution of Hg has been a hot topic of recent studies in mangrove wetlands. In this study, we investigated Hg concentration, accumulation rate and isotopes to reconstruct the Hg pollution history and to differentiate its potential sources in the Gaoqiao mangrove wetland (Guangdong province), which is part of the largest mangrove area in China. We reconstructed a first, continuous, high-resolution Hg pollution history over the last 3000 years in South China. Our findings show that mangrove wetland sediments are more enriched in Hg than the adjacent grasslands. The increased Hg concentration and δ202Hg in recent sediments mirror the enhanced anthropogenic impacts; Hg concentrations in areas with high levels of anthropogenic disturbance are up to 5× higher than the average background value (9.9 ± 1.2 μg kg-1). Compared to mangroves in coastal areas of South China and around the world, the Hg concentration in Gaoqiao is much lower. The significant increase of Hg since the 1950s and the major Hg peak since the 1980s were the evidence of the human activities influences and indicated the possible start date of Anthropocene. After 2007 CE, a decline in Hg pollution occurs due to the effective implementation of the mangrove protection policy. Three potential sources were identified by the Hg isotope traces including urban gaseous Hg, industrial Hg, and regional soil and leaf litter Hg input.

Natural and anthropogenic impacts on mercury accumulation in Xiaohai Lagoon, South China over the last 1200 years

Anthropogenic use and release of mercury (Hg) have profoundly affected the global Hg cycle since preindustrial times. However, it is often difficult to quantify the relative contributions of natural and anthropogenic factors to environmental Hg accumulation. Here, we have presented a 1200-year record of Hg deposition in a sediment core from the Xiaohai Lagoon (South China), in combination with multiple environmental indicators (e.g., geochemical elements, grain size and total organic carbon, etc.). Using principal component analysis (PCA) and stepwise regression analysis (SRA), we aimed to explore the latent processes governing the accumulation of Hg over time and to quantitatively assess the natural and anthropogenic impacts on Hg deposition over the last millennium in Xiaohai Lagoon. Our results have demonstrated that between ∼ 870 and ∼ 1860 CE, natural factors were the main drivers controlling Hg concentrations in the lagoon. These were directly driven by higher soil erosion and increased inputs of fine-grained matter. However, from 1860 to 2013 CE, enhanced anthropogenic activities played a significant role in Hg accumulation in Xiaohai Lagoon. Anthropogenic Hg fluxes increased significantly from ∼ 1860 CE, peaked several times during the 1860s to the 1950s, accelerated from the late 1950s to the early 2000s, and then declined gradually owing to the stringent environmental protection strategies and efficient pollutant control technologies. Our results have suggested that the increased anthropogenic Hg inputs between the 1860s and mid-1970s were mostly attributed to wars, the "Westernization Movement", and global industrial activity, with a surge mainly after the 1980s dominated by industrial activities in China and numerous developing countries in Southeast Asia. This study has shown the natural and anthropogenic influences associated with mercury pollution through quantitative analysis and can deepen our understanding of the processes and mechanisms of mercury deposition in natural environments under the influence of human activities.

Comparison of spatiotemporal burial and contamination of heavy metals in core sediments of two plateau lakes with contrasting environments: implication for anthropogenic-driven processes

Investigating the impacts of climatic factors and human activities on sedimentary records of heavy metal (HM) contamination in lakes is essential for decision-making in global environmental monitoring and assessment. Spatiotemporal distributions of grain size (GS) and HM (Al, Cr, Mn, Ni, Cu, Zn, As, and Pb) concentrations have been conducted in core sediments that are collected from two adjacent plateau fault-bound lakes in southwest China with contrasting environments, i.e., deep oligotrophic Lake Fuxian (FX) and shallow hypertrophic Lake Xingyun (XY). Results showed that the average value of d50 in FX (4.61 μm) was lower than that in XY (8.35 μm), but the average concentrations of HMs (except Cr and Mn) in XY were higher than those in FX. Heavy metal burial rates (HMBR) were mainly controlled by sediment accumulation rates (SARs) rather than HM concentrations. The correlation coefficients between GS and HM concentrations became strong as the increasing water depths were associated with a stable sedimentary environment. Time-integrated enrichment factors (EF) and source identification of HMs between FX and XY represented that Cr, Ni, and Cu originated from natural sources but Mn, Zn, As, and Pb from anthropogenic sources, respectively. Regardless of FX and XY, the transition times of HMs from natural to anthropogenic sources occurred in the mid-1960s. Comparison of qualification impacts of climatic factors and human-induced factors on increased anthropogenic HMBR by the partial least squares path modeling (PLS-PM) implied that socio-economic activities, such as population density (PD) and gross domestic product (GDP), provided higher contributors to increased anthropogenic HMBR in XY (0.23/0.71) than FX (0.11/0.18). The comparative results of this study provided new insights into environmental monitoring and management of HM contamination for adjacent lakes with contrasting environments.

Vertical distribution, environmental occurrence, and risk assessment of organic pollutants in lacustrine sediments in southeast China

To clarify the impact of human activities on the natural environment, as well as the current ecological risks to the environment surrounding Zhushan Bay in Taihu Lake, the characteristics of deposited organic materials, including elements and 16 polycyclic aromatic hydrocarbons (∑₁₆PAHs), in a sediment core from Taihu Lake were determined. The nitrogen (N), carbon (C), hydrogen (H), and sulfur (S) contents ranged from 0.08 to 0.3%, 0.83 to 3.6%, 0.63 to 1.12%, and 0.02 to 0.24%, respectively. The most abundant element in the core was C followed by H, S, and N, while elemental C and the C/H ratio displayed a decreasing trend with depth. The ∑₁₆PAH concentration was in the range of 1807.48-4674.83 ng g^-1, showing a downward trend with depth, with some fluctuations. Three-ring PAHs dominated in surface sediment, while 5-ring PAHs dominated at a depth of 55-93 cm. Six-ring PAHs appeared in the 1830s and gradually increased over time before slowly decreasing from 2005 onward due to the establishment of environmental protection measures. The ratio of PAH monomers indicated that PAHs in samples from a depth of 0 to 55 cm were mainly derived from the combustion of liquid fossil fuels, while the PAHs in the deeper samples mainly originated from petroleum. The results of a principal component analysis (PCA) showed that the PAHs in the sediment core of Taihu Lake were mainly derived from the combustion of fossil fuels, such as diesel, petroleum, gasoline, and coal. The contributions of biomass combustion, liquid fossil fuel combustion, coal combustion, and unknown source were 8.99%, 52.68%, 1.65%, and 36.68%, respectively. The results of a toxicity analysis indicated that most of the PAH monomers had little impact on the ecology, and the annual increase of a small number of monomers might have toxic effects on the biological community, resulting in a serious ecological risks, that requires the imposition of control measures.

Anthropogenic activity and millennial climate variability affect Holocene mercury deposition of an alpine wetland near the largest mercury mine in China

Mercury (Hg) is a potentially toxic element that can be transported globally through the atmosphere, once deposited in the environment, has strong bioaccumulation and extreme toxicity in food webs, especially in wetland ecosystems. Anthropogenic Hg emissions have enhanced Hg deposition by 3-5 times since the industrial revolution, and the mining and smelting of Hg ore are important emission sources. However, the dynamics in Hg deposition around the largest Hg mine in China before the industrial revolution and their driving forces remain poorly explored. Here we reconstruct the atmospheric Hg depositional fluxes (named here Hg influx (Hg_influx)) during the Holocene using a 450-cm alpine wetland sediment core taken from the Jiulongchi wetland, which is only 65 km to the Wanshan Mercury Mine. Our record shows an abrupt rapid increase in Hg concentration since 2500 cal yr BP, suggesting that Hg mining in southwest China may have started before the establishment of the Qin dynasty. Two major Hg_influx peaks were found during the periods 10,000-6000 and 6000 - 3800 cal yr BP, with an increase in Hg deposition by a factor of 4-8. These two peaks are also found in other terrestrial archives from several sites across the Northern Hemisphere. We speculate that critical millennial-scale climate changes, i.e., the Holocene Climatic Optimum (HCO) and the Mid-Holocene Transition (MHT), were the potential triggers of these two Hg_influx peaks. This study highlights the importance of climatic variability and local Hg mining in controlling atmospheric Hg deposition during the Holocene.