Due to its high altitude, the Tibetan Plateau (TP) is known as the world’s roof and is considered the third pole (
Journal of Geographical Sciences, Volume. 30, Issue 9, 1481(2020)
Magnetic characteristics of lake sediments in Qiangyong Co Lake, southern Tibetan Plateau and their application to the evaluation of mercury deposition
Heavy metals, one of the most toxic classes of pollutants, are resistant to degradation and harmful to the biological environment. The lakes that have developed on the Tibetan Plateau are ideal regions to investigate historic heavy metal pollution, particularly through the use of the reliable210Pb dating technique. Environmental magnetism has been successfully applied to estimate heavy metal pollution in different environmental systems due to its characteristics of simple processing steps, good sensitivity, and non-destructibility. However, it has not yet been applied to assess heavy metal pollution in lake sediments on the Tibetan Plateau. A series of environmental magnetic investigations of Qiangyong Co Lake sediments (southern Tibetan Plateau) was therefore conducted to explore the relationship between magnetic minerals and mercury (Hg) concentrations. The results showed that the magnetic mineral species in lake sediments remained stable, with similar levels of four different components from 1899 to 2011. However, the proportion of component 1 (C1, hematite) increased continuously with the corresponding decrease in the proportion of C2 (goethite), while the proportions of C3 and C4 (magnetite) did not change significantly. As a result, the bulk magnetic signals (e.g., SIRM and χlf) were unsuitable for the evaluation of the Hg concentration; however, the proportion of hematite had a strong positive correlation with the Hg concentration. It is possible that the Qiangyong Glacier (the main water supply for Qiangyong Co Lake) has experienced faster melting with global and local warming, and the Hg trapped in cryoconite and ice was released. Hematite, with a large specific surface area, has a strong capacity for absorbing Hg, and both materials are ultimately transported to Qiangyong Co Lake. The proportion of hematite in a sample is therefore a suitable semi-quantitative proxy that can be used to evaluate the Hg concentration in Qiangyong Co Lake sediments. This study confirmed that the variation of magnetic minerals can provide a new method to estimate the variation of Hg concentrations and to study the process of Hg deposition in lakes in the southern Tibetan Plateau on the basis of a detailed environmental magnetic analysis.
1 Introduction
Due to its high altitude, the Tibetan Plateau (TP) is known as the world’s roof and is considered the third pole (
Due to their high levels of toxicity, long half-life, and refractory properties, heavy metal pollutants are considered to be one of the most harmful pollutants, and can pose a tremendous hazard to the ecological environment (
Based on 210Pb dating, continuous lake sediments from the TP can be used to investigate the heavy metal pollution history of lakes over the last hundred years (
2 Geological setting, sampling, and experiments
Qiangyong Co Lake (28.883°N, 90.217°E, ~4870 a.s.l), a proglacial lake of the southern TP, is mainly supplied by glacial meltwater from the Qiangyong Glacier in the summer. There are significant seasonal changes in the hydrologic conditions (
Figure 1.
A sediment core was drilled with a gravity corer (6 cm in diameter) in 2011 by a scientific expedition team from the Institute of Tibetan Plateau Research, Chinese Academy of Sciences (CAS). In the laboratory, the core was split, and one half was sampled with a stainless-steel knife at 5 mm intervals. Based on the 210Pb dating results, the variation of the Hg concentrations in the top sediment (0-9.5 cm) reflected the continuous increase in anthropogenic Hg emissions in South Asia from 1899 to 2011 (
After freeze-drying and weighing, samples were placed into cubic plastic boxes (2.0× 2.0×2.0 cm3). First, low and high field susceptibility (χlf and χhf) were measured using an MFK1-Kappabridge magnetic susceptibility meter at frequencies of 976 and 15616 Hz, respectively. Then, anhysteretic remnant magnetization (ARM) was produced and acquired in a 100 mT peak alternating field under a bias field of 0.05 mT. Isothermal remnant magnetization (IRM) was acquired in direct current (DC) fields of 1 T (IRM1 T) and -300 mT (IRM-300 mT), with a 2G Enterprise pulse magnetization meter. The IRM1 T was referred to as saturation IRM (SIRM) in this study. Both ARM and IRM were measured with a 2G-760 magnetometer. Magnetic hysteresis loops, IRM curves, and the backfield demagnetization curves were measured in a maximum field of 1 T using a Model VSM 3900 Magnetometer. Coercivity (Bc), remnant magnetization (Mrs), saturation magnetization (Ms), and the remnant coercivity (Bcr) were acquired. The first three parameters were obtained after correction for paramagnetic contribution.
To identify the magnetic assemblage of Qiangyong Co Lake sediments, selected samples were analyzed in a series of experiments. (1) Temperature-dependence magnetic susceptibility (χ-T) curves were constructed based on measurements in an argon atmosphere using an MFK1-Kappabridge magnetic susceptibility meter from room temperature to 700℃, with an interval of 5℃. (2) Zero-field-cooling (ZFC) curves were constructed based on measurements with the MPMS XL-5 magnetic measurement system. After first being cooled to 20 K in a zero-field, the IRM of samples was acquired in a 2.5 T field, with measurements made as the temperature was increased from 20 to 300 K at a rate of 5 K/min in a zero-field. (3) First-order reversal curves (FORCs) were measured on a VSM 3900 magnetometer, and FORC diagrams were constructed from 108 FORCs, with a smoothing factor of 7. (4) Diffuse reflectance spectroscopy (DRS) measurements were made from 350 to 2500 nm using a Varian Cary 5000 spectrophotometer.
3 Results
3.1 Depth plot of variations in rock magnetic proxies
Based on the 210Pb chronology results and the variation of the Hg concentration (
Figure 2.
When evaluating the magnetic content χlf is one of the most widely used magnetic properties (
ARM is mainly related to the SD particle content (
The S-ratio (S0.3, S0.3 = IRM-300 mT/SIRM) is commonly used to determine the relative contribution of “soft” magnetic minerals (e.g., magnetite) compared to “hard” magnetic minerals (e.g., hematite and goethite) (
The grain size dependent parameters ARM/SIRM, ARM/χlf and SIRM/χlf exhibited significant variations (Figures 2f-2h). Both ARM/χlf and SIRM/χlf tended to decrease from 1899 to 1951, implying the reduction of finer magnetic particles, but ARM/SIRM changed only slightly. During the period 1951-1988, all grain size dependent parameters varied slightly. Since 1988, both ARM/SIRM and SIRM/χlf had a tendency to increase, implying an increase in the amount of finer magnetic particles. The overall pattern of variation among the three parameters was similar, but not completely equivalent at the different stages. In comparison, SIRM/χlf was more suitable for representing the variation of magnetic grain size.
3.2 Magnetic assemblage
Day-plot diagrams of the Mrs/Ms and Bcr/Bcr values are widely used to visualize the domain state of the ferromagnetic minerals (
Figure 3.
Characterized by a wasp-waisted hysteresis, the loops displayed a similar behavior from 1899 to 2011 that did not close until 800 mT (
A FORC diagram is very useful for identifying the domain state of magnetic materials and the interaction field among magnetic particles (
Figure 4.
The characters of the χ-T curves for selected samples were similar (Figures 4k-4m). The heating curves of χ-T exhibited a weak increase between ~250 and 300℃, which was probably related to the neoformation of maghemite (
No Verwey transition was found in the raw data of the ZFC curves, but it was found in the first-order derivative data of the ZFC curves (
Figure 5.
Combined with the results of all experiments, the magnetic minerals of Qiangyong Co Lake sediments are contributed by four components from 1899 to 2011, implying the sources of magnetic minerals for the Qiangyong Lake sediments have not changed since 1899. From the FORC diagrams, χ-T, ZFC curves, and DRS data, it was possible to identify C3 and C4 as magnetite, C2 as goethite, and C1 as finer hematite based on the Day-plot analysis. This kind of hematite has also been found in paleosols (
4 Discussion
Both χlf and SIRM can be used as proxy indicators for heavy metals. For example, χlf was found to correlate strongly with the Hg concentration in samples from Baiyangdian Lake, northern China (
Figure 6.
In Qiangyong Co Lake, the proportions of C3 and C4 changed slightly, which was related to the increased Hg content. However, there was a strong positive (or negative) relationship between the proportion of C1 (or C2) and the Hg concentration (Figures 6d-6e, 7a-7b and 7h), with the exception of 1922 and 1971. A closer relationship was found between the ratio of C1 to C2 (C1/C2) and the Hg concentration (
The pathways by which Hg and magnetic minerals entered Qiangyong Co Lake were similar. Hg enters the lake system by dry or wet deposition, while magnetic minerals are transported to the lake system by dust, rainfall, and runoff. The most likely mechanism for the strong positive correlation between the Hg content and the proportion of C1 is that hematite, with a large surface area, can absorb Hg and both will then settle into Qiangyong Co Lake via the same transport process. In contrast, C2 avoids Hg adsorption and its proportional content declined with the increase in C1 content since 1899. This could explain the strong negative correlation between Hg and C2, and why there was significant stronger linear relationship between the Hg concentration and C1/C2.
The main water sources for Qiangyong Co Lake are glaciers and snow meltwater rivers on Qiangyong Glacier. Glacier and snow runoff are therefore the main carriers of magnetic minerals into Qiangyong Co Lake. Of the four components, C1 (hematite) accounted for the largest percentage since 1899. Therefore, C1 should correspond to the magnetic minerals transported by glacier and snow runoff. The annual average temperature of both Nagarze station and the whole TP displayed a warming tendency from 1951 to 2011 (
Unstable local atmospheric circulations can induce dust storms when the temperature increases (
Figure 7.
5 Conclusion
The magnetic mineral species in Qiangyong Co Lake sediments did not change from 1899 to 2011, and were composed of four components: C1 (hematite), C2 (goethite), and C3 and C4 (magnetite). The percentage content of each component varied, with the proportion of C3 and C4 changing slightly, the proportion of C1 increasing, and the proportion of C2 decreasing. Therefore, SIRM and χlf were unsuitable for evaluating the Hg concentration. There was a strong positive correlation between the proportion of C1 and the Hg concentration. The proportion of hematite can therefore be used as a semi-quantitative proxy to evaluate the Hg content for Qiangyong Co Lake sediments. The most likely mechanism is that a large amount of Hg that had deposited and was sealed on the Qiangyong Glacier was released again with the rapid melting of the glacier as the climate has warmed since 1899. In this process, hematite with a large surface area absorbed Hg, which was then transported to Qiangyong Co Lake by glacier meltwater.
This study confirmed that magnetic parameters (i.e., the proportion of hematite) can be used to evaluate the variation of heavy metal (Hg) concentrations in the TP lakes. Such evaluations should be conducted with detailed and careful environmental magnetic experiments. Future studies will be conducted on the other proglacial lakes of the southern TP, and then extended to the whole TP to confirm our findings. A determination of the hematite and Hg transport processes is vital, and therefore cryoconite and Qiangyong glacier meltwater, surface soils around Qiangyong Co Lake and runoff area, local rainfall, and dust will be sampled to conduct magnetic and Hg analyses.
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Xing GAO, Shichang KANG, Qingsong LIU, Pengfei CHEN, Zongqi DUAN. Magnetic characteristics of lake sediments in Qiangyong Co Lake, southern Tibetan Plateau and their application to the evaluation of mercury deposition[J]. Journal of Geographical Sciences, 2020, 30(9): 1481
Category: Research Articles
Received: Nov. 28, 2019
Accepted: May. 21, 2020
Published Online: Apr. 21, 2021
The Author Email: DUAN Zongqi (duanzq@igsnrr.ac.cn)