•The sediments of Huguangyan Lake truly recorded the atmospheric Hg deposition.•The Hga reflected global mercury pollution history before 1950.•The Hga clearly recorded the great increase of Asian Hg emission after 1950.•Huguangyan Lake recorded air Hg pollution more sensitively than alpine lakes.Mercury is a highly toxic metal that can cause harm to environment and human health. As atmospheric deposition is the main source of total Hg input to aquatic system in remote and pristine regions, almost all the studies on atmospheric Hg pollution history concentrated in these areas, while the studies in non-remote areas are much limited, especially for the long history records. In this study, Huguangyan Maar Lake, an undisturbed lake system at low altitude in China, was selected to reconstruct the atmospheric mercury pollution history. Variation patterns of TOC, Hg and non-residual Sr in the sediment core indicated that, compared to the direct atmospheric Hg deposition, the effect of either Hg scavenging from water column by algae or the catchment inputs of previously deposited Hg on the Hg accumulation in the lake sediment was limited. The sediment Hg content in Huguangyan Lake was mainly controlled by the atmospheric Hg deposition, and thus accurately reflected the atmospheric Hg pollution history. The Hga (Hg content from atmospheric deposition) in Huguangyan Lake presented a comparable variation pattern to that in remote sites. It had the same variation trend as the global atmospheric Hg before 1950 CE, which could be attributed to the Industrial Revolution. After that, it was mainly controlled by Hg emissions from Asian countries. The variation of Hga also indicated that atmospheric Hg deposition accelerated significantly since 2000 CE. This study, along with other investigations in remote sites in China, showed that the sediment Hg in Huguangyan Lake responded to the atmospheric Hg pollution more sensitively than in the alpine regions. It should be noted that, the more intensive acceleration of Hg deposition in Huguangyan Lake may imply that the South of China suffered from much more serious atmospheric Hg pollution than previous studies revealed.Download high-res image (149KB)Download full-size image

Extractable polycyclic aromatic hydrocarbons (EPAHs) and bound PAHs (BPAHs) were measured in a sediment core using conventional Soxhlet extraction and a more astringent extraction method, with the objectives of determining the influence of BPAHs on the historical reconstruction of PAHs and exploring the formation of BPAHs and long-term behaviors of PAHs in sediment. The results indicated that the formation of BPAHs was clearly sediment-depth and molecular-size dependent. BPAHs represents an important portion of PAHs in sediment and cannot be extracted by conventional Soxhlet extraction. This suggests that the previously developed vertical profile of PAHs is not the real chronology of PAHs and the plausible interpretation derived from the sedimentary records of PAHs needs reexamination. Based on the previous findings, a biphase model was proposed and the formation of BPAHs was predicted. Due to the different nature of geosorbents in sediment, redistribution of PAHs among these geosorbents logically leads to the formation of BPAHs and is kinetically favorable for smaller molecular PAHs. This is consistent with the obtained results. Many factors may influence the formation of BPAHs, such as the physicochemical structure of sediment and environmental conditions. There is still a long way to reveal the thermodynamical characteristics in action during the formation of BPAHs.

The Rb/Sr ratio of lake sediments has been demonstrated to be a potential indicator of chemical weathering by increasing work. However, Rb and Sr in lake sediments are derived from both chemical weathering and physical erosion. Rb and Sr of different forms in lake sediments may record different environmental processes and information. In this study, the variation patterns of Rb and Sr of different forms in sediments of Daihai Lake were investigated. The results show that Rb and Sr of different forms display noticeably different variation patterns due to their different sources and associated environmental processes. Using the Rb/Sr ratios of bulk lake sediments to reflect chemical weathering is not accurate. The non-residual Sr of the sediments without detrital carbonates, representing the Sr leached from the catchment, can be used as an index of chemical weathering because the weathering of Sr minerals is very sensitive to climate change, and the non-residual Sr content of the sediment is determined more by Sr2 + influx than by the physicochemical conditions of lake water. The correspondence between the non-residual Sr and TIC/TOC in Daihai Lake also indicates that the non-residual Sr of the sediments is a good indicator of chemical weathering in the catchment.

Silicon is essential for the growth of diatoms, which utilize dissolved silicic acid in lake water and form opaline silica (SiO2·nH2O). During the uptake of dissolved silicic acid, there is a preferential incorporation of light silicon isotope (28Si) into biogenic silica, resulting in the enrichment of heavy silicon isotope (30Si) in dissolved silicic acid. The silicon isotope composition of diatom silica (δ30Sidiatom) may thus record changes in the percentage utilization of dissolved silicic acid by diatoms, which can be then related to other aspects of climate/environment. With the aim of exploring the potential of δ30Sidiatom as an indicator of lacustrine environment, here we made the first measurements of δ30Sidiatom in the sediment core from Lake Huguangyan, a closed crater lake in China. The result shows that δ30Sidiatom varies from −0.6‰ to 1.1‰ and displays broad similarities to variations in contents of biogenic silica and organic carbon throughout the sediment core. δ30Sidiatom is a reliable paleotemperature proxy in Lake Huguangyan, which is supported by good correlation between δ30Sidiatom and available temperature records. Heavier δ30Sidiatom indicates greater dissolved silicic acid utilization at higher temperature while lighter δ30Sidiatom reflects decreased utilization at lower temperature. The most negative δ30Sidiatom values in the sediment core occur between AD 1580 and 1920, which suggests AD 1580–1920 was the coldest period in Lake Huguangyan over the past 2000 years, thus providing evidence for the existence of the LIA in tropical South China.There are few means by which to reconstruct the history of temperature changes in tropical terrestrial region. δ30Sidiatom, in this study, has proven to be a new promising paleotemperature proxy in lacustrine sediments, and may play important role in reconstructing past temperature changes at low latitude in the future. Detailed investigations on the silicon isotopes of diatoms in more lakes would be desirable in further research.

Although temperature decreased in a similar trend in many regions around the word during the Little Ice Age (LIA), the reconstructed humidity is remarkably different from region to region. The precipitation history during the LIA is poorly understood as compared to the temperature history in tropical South China. In this study, a sediment core with a length of 117.5 cm was recovered in the central part of Huguangyan Lake in tropical South China. Total organic carbon (TOC), total nitrogen (TN), inorganic carbon (IC) and non-residual strontium (Sr) were analyzed at approximately 1 cm intervals to study the regional precipitation changes during the LIA. Generally, Sr-containing minerals are sensitive to chemical weathering which is dominated by the precipitation in tropical South China. Thus the non-residual Sr in Huguangyan sediments can be used as an indicator of precipitation changes, which is also verified by the downcore variations of TOC, TN and IC in Huguangyan Lake. The non-residual Sr correlated positively with TOC and TN but negatively with IC in the sediment profile. TOC, TN, IC and the non-residual Sr jointly demonstrated a wet period from AD 1500 to 1750, which corresponds to the LIA. Coincidently, both the total solar irradiance (TSI) and Northern Hemisphere temperature have the lowest values between AD 1500 and 1750 over the past millennium. Therefore, the wet LIA in tropical South China was most likely caused by the low solar irradiation. During the LIA, the low solar irradiation likely resulted in the decrease of the Northern Hemisphere temperature, which weakened the intensity of the East Asian summer monsoon (EASM) and synchronously moved the north edge of the Asian summer monsoon southward, leading to an increase in the precipitation in tropical South China.