The Permian global mass extinction events and the eruption of the Emeishan flood basalts in the Upper Yangtze region should display certain responses during the evolution of carbon isotope. In this paper, the Permian carbon isotopic evolution in the Upper Yangtze region is examined through systematic stratotype section sampling and determination of 13C in the northern Upper-Yangtze regions and Southern China. Additionally, the carbon isotopic evolution response characteristics of the geological events in the region are evaluated, comparing the sea-level changes in the Upper Yangtze region and the global sea-level change curves. Results of this study indicated that the carbon isotopic curves of the Permian in the Upper Yangtze region are characterized by higher background carbon-isotope baseline values, with three distinct negative excursions, which are located at the Middle–Late Permian boundary and the late period and end of the Late Permian. The three distinct negative excursions provide an insightful record of the global Permian mass extinction events and the eruption of the Emeishan flood basalts in the Upper Yangtze region. The first negative excursion at the Middle–Late Permian boundary reflected the eruption of the Emeishan flood basalts, a decrease in sea level, and biological extinction events of different genera in varying degrees. The second negative excursion in the Late Permian included a decrease in sea level and large-scale biological replacement events. The third negative excursion of the carbon isotope at the end of the Permian corresponded unusually to a rise rather than a decrease in sea level, and it revealed the largest biological mass extinction event in history. 相似文献
Seepage analysis and assessment of the effect of seepage control at reservoir sites are essential parts of dam design and operations, and of considerable significance for the safe and economic design of the masses and hydraulic structures associated with reservoir sites. In this study, a systematic process is provided for the optimal design and assessment of seepage control of reservoir dams under karst development conditions. A reservoir dam planned for construction in the middle-upper reaches of the Huayang River in China is selected as a case example for illustration. A three-dimensional equivalent continuum seepage finite-element numerical model is applied to investigate the effectiveness of the proposed and optimized seepage-control schemes, and it is calibrated by an inversion analysis of the initial seepage field based on the flexible tolerance method and field penetration test data. By analyzing the sensitivity of seepage discharge to the length and depth of the grouting curtain, a safe and economic seepage-control optimization scheme is suggested. Additionally, the sensitivity of seepage discharge to the hydraulic conductivity of the limestone layer is analyzed, and the results show that the hydraulic conductivity of the limestone layer has a significant impact on the seepage discharge of the reservoir site. The methodology and results derived from this study can provide technical support and reference for the optimal design and assessment of seepage control for reservoir dam engineering under karst conditions.
