Temporal and spatial variations of soil CO2 and δ13${\mathrm{C}}_{{\mathrm{CO}}_2} $ from different land uses in typical dolomite areas of Southwest China
Shuanghedong national geopark is a prominent example of dolomite on a global scale. Soil CO2 in the dolomite areas is an important driving force of karstification and has great significance for the global carbon cycle. However, the mechanism of soil carbon cycle in dolomite areas has yet to be clarified. In order to investigate the spatial and temporal variations and influencing factors of soil CO2 and δ13$ {\rm{C}}_{{\rm{CO}}_2} $ in different land uses in typical dolomite areas and to clarify the mechanism of soil carbon cycle in these areas, this study selected Shuanghedong National Geopark, a typical dolomite area in Southwest China, as the study area. Soil CO2 and δ13$ {\rm{C}}_{{\rm{CO}}_2} $ were sampled from six typical land use types in Shuanghedong national geopark. These samples were monitored outdoors and subjected to laboratory experiments over the course of one year. Additionally, the data were systematically analyzed with mathematical and statistical methods. The study results indicate as follows, (1) Soil CO2 concentrations of different land use types were observed in the following order: cropland>irrigation grassland>abandoned land>scrubland>forested land>fallow land. Soil temperature and water content, overlying vegetation, organic carbon, and human activities were important factors influencing the changes of soil CO2 in different land use types. (2) The values of soil δ13$ {\rm{C}}_{{\rm{CO}}_2} $ of different land use types were ranked as follows, scrubland>fallow land>cropland>irrigation grassland>abandoned land>forested land. Differences in diffusion rates of soil 12C and 13C, decompositional conversion of organic carbon and isotopic fractionation, plant photosynthesis and carbonate rock dissolution led to changes of soil δ13$ {\rm{C}}_{{\rm{CO}}_2} $ in different land use types. (3) In terms of temporal changes, soil CO2 concentrations of different land use types were high in the rainy season and low in the dry season, while concentrations of soil δ13$ {\rm{C}}_{{\rm{CO}}_2} $ exhibited opposite seasonal patterns, showing small values in the rainy season and large in the dry season. Oxidative decomposition of soil organic matter, metabolic activities of microorganisms, respiration of plant roots and dissolution of carbonate rocks are the main sources of soil CO2 in dolomite areas. Therefore, oxidation and decomposition of organic matter, dissolution of carbonate rocks, photosynthesis of plants, and metabolism of microorganisms store part of CO2 in soil to form carbon sinks. The other part of soil CO2 in the dolomite area are primarily attributed to the higher partial pressure of soil CO2 compared to that in the atmosphere. This difference in pressure facilitates diffusion, allowing CO2 to escape into the atmosphere as a carbon source. In addition, the study also indicates that different land use types, soil temperature, water content, overlying vegetation, human activities, and organic carbon all influence the intensity of carbon sources and carbon sinks in the soil carbon cycle in dolomite areas to varying degrees. The research findings have deepened the understanding of soil CO2 in dolomite areas of northern Guizhou, revealed the spatiotemporal differences and influencing factors of soil CO2 and δ13$ {\rm{C}}_{{\rm{CO}}_2} $ in different land use types, clarified the mechanism of soil carbon cycle in typical dolomite areas, and provided a reference for the study of soil carbon cycle in these areas.
Study on hydrochemical characteristics and water inflow sources of typical karst water-filled mining areas under the influence of extended mining
Mineral resources are an important pillar of China's vigorous economic development. With the rapid increase in the demand for metal minerals, the original metal mining areas continue to be mined. However, the specific geological conditions indicate that a significant number of ore bodies in China are closely associated with the limestone strata. As a result, the karst aquifer has emerged as one of the major hidden dangers threatening the deep mining of these ore deposits. Therefore, it is an important topic to study the prevention and control of pit water in mines of karst areas, but there is a lack of comprehensive case studies that have revealed the changes in water sources in pits due to deep mining on the basis of hydrochemical data from water inflow points at different mining levels. The Makeng mining area is a typical karst water-filled mining area in East China. Its hydrogeological conditions and pit water-filling conditions are complex, presenting significant challenges for water prevention and control efforts. In this study, 59 sets of hydrochemical data were selected from the water inflow points of horizontal tunnels at four mining levels (+420 m, +300 m, +200 m and +100 m) in the Makeng mining area from 2006 to 2016. Combined with the evolution law of the groundwater flow system in the mining area, various hydrochemical methods such as Piper three-line diagram, ion ratio, Gibbs diagram and chlor-alkali index were used to analyze the source of water inflow. The mixing ratios of different water sources were calculated, and a conceptual model of evolution of water sources at the water inflow points was generalized. Besides, the hydrochemical causes and recharge sources of water inflow points at different mining levels in the mining area were discussed. The results show that the decrease of groundwater levels gradually slowed down while and water inflow continuously increased during the mining process; therefore the recharge sources of pit water inflow points constantly changed. Secondly, the long-term water drainage and depressurization changed the direction of groundwater runoff in the mining area. Additionally, the water temperature, pH and TDS increased with the increase of mining depth, and the hydrochemical type gradually changed from HCO3-Ca type to HCO3·SO4-Ca type and HCO3-Na·Ca type. The dissolution of carbonate salt is the main controlling factor for the formation of groundwater in the mining area. Besides, controlled by faults and other geological structures, some water inflow points were influenced by different water sources in some degree. Moreover, different mining levels were also affected by pyrite oxidation, cation exchange, mixing action, etc. The sources of water inflow points at +420 m and +300 m were from karst water and P1j1 sandstone water, while the main sources at mining levels of +200 m and +100 m were also from karst water. The water inflow point at +100 m was recharged by low-temperature hot water along the water diversion fault from deep granite. The Ca2+ and Cl− equilibrium models were used to calculate the respective ratios of mixing karst water with sandstone fracture water and of mixing karst water with hot water from deep granite. The recharge rates of fracture water at +420 m and +300 m were about 22.7%–82.8%, and the recharge rates of low-temperature hot water from deep granite at +100 m were 35.1%–38.3%. In the future, the monitoring of water quality in water inflow points at +100 m or above should be strengthened. If necessary, F1 and other faults exposed underground should be grouted so as to prevent the low-temperature hot water from the contact zone between the deep granite and the quartz sandstone of Lindi formation (C1l) from entering the pit along the water diversion structure to recharge karst water. In this paper, the hydrochemical formation of water inflow source in Makeng mining area under the influence of extended mining has been discussed, and the dynamic characteristics of water inflow sources have been preliminarily analyzed, which can provide a certain reference for groundwater management and prevention and control of pit water in the same type of karst water-filled mining area.
Changes of air environmental factors in Zengpiyan cave site and its influence on the cultural layers
The Zengpiyan cave site is a significant Neolithic cave site in Southern China, yielding complete human skeletal fossils as well as artifacts related to the diet and tools of ancient human inhabitants. This cave site provides essential evidence for understanding human activities in Southern China between 12,000 and 7,000 years ago. The cave system at the site consists of three distinct areas: the main cave, the water cave, and the low cave. The main cave retains an intact original cultural layer, featuring clear stratigraphic sequences. Cultural layers, representing the area where artifacts are preserved within the cave site, embody the most important cultural content of the site. Previous research on karst caves has shown that temperature, relative humidity, and carbon dioxide (CO2) concentrations inside the cave exhibit markedly different temporal and spatial variations compared to the outside environment. These factors influence the formation and degradation of carbonate deposits such as stalactites. To understand the temporal and spatial variations of air environmental factors within the Zengpiyan cave site and their influence on the weathering of cultural layers, this study acts as a crucial supplement and extension to the environmental research on the Zengpiyan cave site, with a particular emphasis on the microenvironment of the cultural layers of the cave. The study specifically examines environmental factors such as cave temperature, relative humidity, and CO2 concentrations. A hydrological year-long observation was conducted in Zengpiyan cave with the use of various air monitoring devices, along with the sampling and analysis of cultural layer deposits from different excavation units. The results reveal that the air environmental factors inside the cave exhibit three distinct seasonal stages: in winter, both temperature and humidity are low with large fluctuations, and CO2 does not accumulate at the cave floor. In contrast, summer, brings high temperature and humidity with small and stable variations, resulting in CO2 accumulation at the cave floor. Spring and autumn serve as transitional phases between winter and summer. Spatially, temperature and humidity show a gradual variation with the increase of cave depth. CO2 concentrations exhibited a banded distribution with high-concentrations in the eastern section of the cave, suggesting that the water cave plays a significantly role in evevating the CO2 levels within the main cave. The primary source of CO2 in the cave is the overlying soil, with rainfall events identified as critical triggers for peak CO2 levels within the cave. A comprehensive analysis suggests that ventilation effects are the main factors influencing the temporal and spatial variations of air within the cave. Additionally, cave morphology and tourism activities are also recognized as significant contributors to the air environment within the cave. Observing and combining X-ray diffraction analysis, it was found that there are significant differences in the degree of weathering of cultural layers near the entrance of the cave, water cave connections, cave middle sections, cave bottoms, and the upper and lower parts of the exploration area, which are related to the spatiotemporal changes of air environmental factors. For example, there are more dissolution cavities in the upper cultural layer at the cave floor, mainly due to the accumulation of water vapor in the upper part, which condenses and dissolves, transporting easily soluble components away from the cultural layer. In contrast, the cultural layer at the cave entrance is characterized by loose and powdery cementation, which is associated with the significant temperature fluctuations and frequent cycles of drying and wetting at the entrance. This study has documented the temporal and spatial variations of the main air environmental factors at the Zengpiyan cave site, which is characterized by a shallow, wide-open entrance. It has also explored the potential influencing factors and their impact on the weathering of cultural layers. These findings provide a scientific basis for the improved preservation and utilization of cave sites.
Research on hydrogeomorphologic characteristics and evolution of the watershed on the Gaopo karst tableland in Huaxi of Guizhou
In order to explore the hydrogeomorphologic characteristics and formation mechanism of the watershed on the Gaopo karst tableland in Huaxi, Guizhou, this study investigates the spatial distribution of geological structure, lithology and geomorphology, in conjunction with the hydrological characteristics of the study area. It employs the theory of karst hydrogeomorphology to discuss the development of these characteristics. The results indicate: (1) The karst landforms of the Gaopo karst tableland, including poljes, edge poljes, peak-forest valleys, island peak clusters and karst caves, are well developed and exhibit layered structural characteristics. (2) The peak-forest valley is mainly developed in the watershed area between the Dumu river and the Baijin river. (3) The landform transitions from the watershed to the drainage base, displaying a spatial distribution pattern of peak-forest valley→peak-cluster depression→valley (or isolated peak). The analysis shows that fractures, joints and faults are the dominant factors controlling the northeastward and southeastward trend of the valley in the study area. Spatial distributions of carbonate rocks and non-carbonate strata play a certain decisive role in karst geomorphology. On the Gaopo karst tableland in Huaxi, there is an erosion base at Grade-5 in the valley and at the transition end of the broad anticline, but not in the watershed area. This phenomenon is caused by the fact that the crust began to rise again before the river tractive erosion in a certain period spread to the study area. In the broad anticline area, the tractive erosion path of the previous period spread to the center area, so only a Grade-4 erosion base was formed in the study area. Based on the layered geomorphic structure, it can be seen that from the center to the edge of the tableland and then to the valley, there is a turning point in the erosion base at the edge of tableland. In addition, the erosion base of each grade from the anticlinal transition end to the anticlinal center (from the edge to the center of tableland) is relatively stable with small hydraulic gradients, but the hydraulic gradients in erosion bases from the transition end to the synclinal valley are much larger than those on the anticline. Therefore, in the process of drainage from the center to the edge of tableland and then to the valley, there is also a turning point for the groundwater level as well as the surface water level of Gaopo karst tableland at its edge. As a result, the flowing rates of both the surface runoff and groundwater runoff on the box anticlinal karst tableland are slow, or the runoffs are transformed into karst lakes and pools inside the box anticlinal karst tableland, whose side erosion is strong, forming karst landforms of peak-forest plain, peak-forest valley, etc. However, with the gradual strengthening of traceable erosion, the turning point also gradually retreated to the center of the tableland, and finally the groundwater level of the tableland disappeared. With the retreat of the turning point, the groundwater level gradually decreased, the saturation zone gradually thickened, and the movement of surface water and groundwater shifted from the horizontal to the vertical. As a result, a large number of depressions, funnels, sinkholes and vertical shafts were developed on the surface. Therefore, it is believed that the formation and evolution of karst geomorphology in the watershed area of the Gaopo karst tableland in Huaxi are closely linked to the development of platform hydrological structure. Throughout geological history, the platform water system has been affected by the tractive erosion caused by surrounding rivers, leading to the disintegration of the surface hydrological network from the edges toward the center of the platform. In this process, the karst geomorphology of the Gaopo karst tableland experienced four stages: stone clint-lake, peak-cluster depression, peak-forest valley and the degradation and extinction of platform geomorphology system.
Articles in press have been peer-reviewed and accepted, which are not yet assigned to volumes /issues, but are citable by Digital Object Identifier (DOI).
Founded in 1982 bimonthly
ISSN: 1001-4810
CN 45-1157/P
Editor-in-chief: Jiang Zhongcheng
Sponsor: Chinese Academy of Geological Sciences
Sponsored by: Institute of Karst Geology, Chinese Academy of Geological Sciences
Cosponsor: International Research Center on Karst under the Auspices of UNESCOKarst Geology Committee of Geological Society of ChinaSpeleology Committee of Geological Society of China
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