| 其他摘要 | Lake is the important component of continental aquatic ecosystem. With the development of society, the lake ecosystem has been deteriorating owing to expansion of human activities. So, there were a variety of environmental problems occurred such as secondary pollution, eutrophication, heavy metal contamination and so on. Iron is one of the essential micronutrients for organisms and it will have influences on algal productivities and species compositions in the lake under some circumstances. Moreover, iron is highly sensitive to the redox state, and the change of oxidation state of iron will affect the distribution of other heavy metals. Recently, numerous studies have demonstrated that Fe stable isotope fractionation is potentially a useful tool for investigating and quantifying biological and abiological processes involving Fe in the environment. It has been extensively used in studying ocean and river ecosystems, whereas relatively little work has been done on the lake ecosystem. Hongfeng Lake is a mesotrophic reservoir in Wujiang watershed while Aha Lake is a highly mineralized reservior in the southwest of Guiyang, Guizhou Province. They two lakes have different characteristics, both of which are natural laboratories for biogeochemical cycling studies. In this study, a variety of geochemisty approaches such as sulfur isotope, iron isotope and trace elements have been used in investigating the possible sources of sulfur, the characteristics of the seasonal and depth profiles, iron isotope behaviors and the influences. Accordingly, the mechanism of iron isotope fractionation has been improved, and this study has also provided valuable scientific data for further studies on the biogeochemical cycling of iron and sulfur in lake ecosystem. The main conclusions are listed as follows:
Sulfur isotope geochemistry of two lakes
(1) The δ34S value and the sulfate concentration display large ranges for two lake watersheds. Within the inflowing rivers, there are light isotope compositions (-8.10‰～-14.92‰) for those which have been contaminated by acid coal mining drainages and the dump filtrates, while those are with relatively high δ34S values because of the contamination by sewage drainages. For these two lakes, Aha lake was more seriously contaminated by coal mining drainage than Hongfeng lake, resulting in a high concentration of sulfate (2.30 mmol.L-1) and a light sulfur isotope composition (-8.10‰) for Aha lake and a low concentration of sulfate (0.96 mmol.L-1) and a heavy sulfur isotope composition (-6.80‰) for Hongfeng lake.
(2) The results show that coal mining drainage, dump filtrates and rainfall are the main sources of SO42- for Aha watershed, and the major inputs of SO42- to Hongfeng lake are the oxidation of pyrite and the rainwater while sulfide oxidation and gypsum dissolution have only limited relative contributions. It shows that rainwater shows a higher contribution to Hongfeng lake.
(3) It is clearly observed that there are different characteristics for depth profiles within different seasons. δ34S-SO42- values display homogenous vertical distributions in water columns in winter and spring due to water circulation and overturn mixing, while in summer and autumn, δ34S-SO42- values of lake water display significant positive shift in epilimnion and hypolimnion layers mainly due to physical stratification of lake water. During summer stratification, it is suggested that the retention of rainwater in the epilimnion makes a positive shift of δ34S values and a negative shift of sulfate concentration, while it maybe caused by biogeochemical processes such as sulfate reduction by bacteria in the hypolimnion.
Iron isotope geochemistry of two lakes
(1) The iron isotope compositions for samples studied are variable, with a wide range from -2.03‰ to +0.12‰. δ56Fe values for SPM sampled in the lake display statistically negative shift compared with IRMM-014, ranging from -1.36‰ to -0.03‰ while those for rivers are within the similar range, from -0.88‰ to +0.07‰. The sediments display variations in δ56Fe from -1.75‰ to -0.59‰, while those for pore water samples are from -2.03‰ to +0.12‰. Besides, the average iron isotope composition of aerosol samples and algae are +0.06±0.02‰ and +0.08‰, which is very similar to that of igneous rocks, 0.09±0.05‰. In contrast, samples for Hongfeng Lake are less variable in δ56Fe than those for Aha Lake, ranging from -0.92‰ to +0.36‰. The δ56Fe for lake SPM and river SPM are with the similar ranges, which are -0.85‰~+0.14‰ and -0.89‰~+0.10‰, respectively. The iron isotope compostions for sediments in Hongfeng lake is significantly heavy than those for Aha Lake, and the corresponding pore water samples are all light than the sediments. The algae and crucian were also analysed and their δ56Fe values are +0.36‰ and -0.92‰, respectively.
(2) It is suggested that the complex biogeochemical processes play important roles in changing the δ56Fe values for SPM in lake except the various inputs. The iron isotope compositions change with the seasonal variation for SPM in two different lakes, with its own characteristics for each lake. During summer stratification for Aha Lake, they are the organically bonded iron particles that make the iron isotope compositions of SPM in the epilimnion light, with an average value of -0.29‰, while the aerosol and algae have only limited contributions. An iron cycle named “Ferrous Wheel” plays an important role in redistribution of iron isotopes for SPM near the redox boundary. Due to the random transportation and diffusion, the profiles of δ56Fe value for the SPM near the redox regions develop to approximately a Gaussian shape, with a minimum value just below the redox interface. Besides, the formation of ferrous sulfides may be another reason for the light iron isotope compositions for the SPM near the water-sediment interface. The values for SPM are less variable in winter due to the weakening of iron cycle during overturn mixing. However, it is different in Hongfeng Lake. The excellular Fe adsorption may be the dominate factor in the epliminion and the δ56Fe values increase with the increase of the concentration of Chl-a. It may also be the “Ferrous Wheel” iron cycle that has influence on the iron isotope variation in the bottom. It is -0.18‰ for HW location and about -0.46‰ for DB location, with a smaller range than Aha Lake, probably owing to the influence by organic matters. In winter, the depths profiles for HW are similar with those in summer. They are the different factors which have major influences in δ56Fe variations for SPM in the upper water column and lower water column, respectively. No significant variations have been observed for SPM in the upper water column, and there are good positive correlations of δ56Fe values with Fe, Al, Mn, Zn, Co concent. However, it is more variable for SPM in δ56Fe in lower water column in winter than that in summer, with the minimum value of -0.85‰ at 20 m depth. And there are good negative correlations of δ56Fe values with Fe, Al, Zn, Co concent. As to the detailed mechanisms, it needs to be further studied. |
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