Earth's critical zone and karst critical zone: Structure, characteristic and bottom boundary

Abstract

Critical zone science has become an important research area in Surface-Earth system science, representing a future new concept and developing trend in Earth system science. The karst critical zone, a unique type, covers approximately 15.2% of the global ice-free continental land, which is a typical case and represents the Earth's critical zone. However, there is still a lack of a unified understanding of the scientific connotation of the karst critical zone (KCZ) and a lack of related discussions on the structure, characteristics and lower boundary of the KCZ at present. On the basis of reviewing the history of the scientific development of the Earth's critical zone and summarizing its main characteristics, this study sorts out the background and development processes of the concept and summarizes the characteristics of the KCZ. The bottom boundary of the KCZ, which is still controversial, is analyzed and discussed, and the further development direction of the KCZ is also analyzed. These results show that the KCZ is a unique type of Earth's critical zone shaped by material circulation and energy flow at the interface of the lithosphere, hydrosphere, atmosphere, biosphere as well as pedosphere in soluble rock areas. The spatial span of the KCZ ranges from the vegetation canopy to the lower boundary of the karst aquifer in the soluble rock area, including the vegetation layer, soil layer, epikarst, vadose zone and saturated zone, which are sensitive to environmental changes and have surface and subsurface double geological structures and special geochemical processes characterized by coupling cycles of carbon, water and calcium. By comparison, the KCZ has eight following characteristics: active involvement of carbonate rock in the material cycle, sensitive response to external environmental changes, multilayer hydrogeological structures, strong horizontal spatial heterogeneity, large underground space network, large-span biota structure, unique ecohydrological processes with carbonate rock water-vegetation interaction as well as varying lateral boundary controlled by subsurface divide change. The bottom boundary of the KCZ is the depth at which the groundwater recharged by precipitation has no influence on the dissolution of carbonate minerals (calcite, dolomite, etc.) and has no ability to further dissolve carbonate minerals (calcite, dolomite, etc.) in a certain range below the surface. According to the development trend of the disciplines and national strategic demands, further attention should be paid to the following four directions: the structure, formation and evolution mechanisms of different types of the KCZ; the ecological service function and regional sustainable development in degraded karst ecological zones; the impacts of engineering activities on the structure, attributes and evolution process of the KCZ; and the coupling process between climate change and the change in the structure, function and attributes of the KCZ.

Keywords

• earth's critical zone
• karst critical zone
• structure
• characteristic
• lower boundary