| 其他摘要 | The characteristics of the chemical weathering in the basin and the circles of the elements in the Earth's surface system are able to be investigated by the research on the geochemical characteristics of rivers; meanwhile, the chemical weathering in the basin is an important part of the environmental effects of the long-term carbon cycle in the world. Domestic scholars have conducted a substantial amount of research on the basin areas of the carbonate rock but the silicate rock areas are relatively lack of attention. According to the relevant reports, the composition of ions in domestic river water is mainly affected by the carbonate karst weathering and the erosion of evaporates rather than the aluminum silicate weathering. It is well known that the weathering products of silicate rock are in charge of the geochemical composition of surface water to some extent, therefore, the relationship between the chemical weathering and the characteristics of hydro-geochemistry in silicate rock area and a variety of the factors which control the geochemical composition of river water could be investigated by the research on the geochemical changes of the river water’s composition in silicate rock area. The Ganjiang River is the main tributary of the Yangtze River. It plays an important role in the national economy, ecological environment protection and social life in Jiangxi Province. In addition, it is also an typical area that silicate rocks are widely distributed. The material transported by water and suspended particles in the Ganjiang River contains plenty of information about environmental changes. Therefore, the river in the Ganjiang River is worth studying.
In this paper, the chemical characteristics, the main ion sources and the major chemical weathering reactions were discussed by means of the physical and chemical parameters, chemical mass balance and isotope geochemistry about the Ganjiang River. The rate of chemical erosion and its consumption of atmospheric CO2 were calculated so as to discuss the relationship among the chemical characteristics of river water, human activities, climate, topography and lithology. The main conclusions are as follows.
The chemical weathering of carbonate and silicate rocks is in the charge of the humid subtropical monsoon climate and human activity. In the samples collected both in the dry season and the wet season, the contents of Na+ and Ca2+ are higher than the other cations, meanwhile, K+ is the lowest and Mg2+ is moderate. Cl- and HCO3- are the main anions, meanwhile, NO3- is the lowest and SO42- is moderate. The contents of soluble silicon are kept in a relatively stable level. This chemical composition is recognized as the typical characteristics of the impact on river water from silicate rock weathering.
The Ganjiang River basin affected by climate, lithology, topography and the impact from human activities, ion components of the river water come from the rock /soil weathering. The rate of the carbonate weathering is higher than that of silicate rock, therefore, the weathering of the Ganjiang River basin ought to be dominated by the carbonate weathering. In addition, human production activity can not be ignored.
The obvious correlations between some cation and some anion can be found, such as HCO3- and Ca2+, HCO3- and Mg2+, SO42- and Mg2+, Na+ and Cl-. On the contrary, the other ion partnerships have little correlations. The numbers of Si/(Na*+K) are smaller because the secondary minerals riched in cations are generated in the supergene environment. Organism and other factors have little affected on the concentration of silicon other than lithology.
Apart from the dissolution of rock weathering, the precipitation of the Ganjiang River basin might affect the surface runoff and river flow directly. The order of river flow’s impact on ion concentration is HCO3->Ca2+>Mg2+>SO42->Na+>Cl-. Both lithology and climate have affected on the characteristics of the Ganjiang River water.
High strontium isotope ratio(87Sr/86Sr) in the Gangjian River basin means that the source of Sr has originated from the silicate rock weathering, moreover, the concentration of Sr decreases in dry season. The characteristics of strontium isotope is a mixed type of atmospheric precipitation and rock weathering with two endmember, carbonate and silicate.
The analysis shows that carbonate weathering has most important effect on the composition of river water, silicate weathering is the secondary and human activity is not significant, furthermore, the quantitative evaluation of those factors’ contribution to dissolved substance in river water has been completed. The results as follows: 520.2×103mol/km2 in dry season and 383.4×103mol/km2 in wet season atmospheric CO2 are depleted, and carbonate weathering and dissolving as well as silicate weathering cause the higher consumption of CO2 which deviates from the average of the world. After discounting the effect of atmospheric CO2 and human activity, the average annual rate of chemical weathering is 30.3t/km2•a in the Ganjiang River. Geological and chemical factors are most important than the others in the progress of the formation of the Ganjiang River water. It is worth mentioning that the Ganjiang River is a carbon sequestration to a large extent. |
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