Abstract:

Vertical cycle karst zone has been studied for more than 100 years, however karst subzones in the zone have never been divided and affected depth of CO₂ from rainwater in the zone has never been studied. On the basis of field observation, survey and chemical analysis, the difference of karst processes indicated by CaCO₃and pH values in fine and loose sedimentary strata as well as limestone strata, and the vertical cycle zone ascertained by predecessors can be divided into three subzones, that is, the upper first subzone, characterized by unsaturated water solution and strong dissolution processes, the middle second subzone, characterized by supersaturated water solution and precipitation, and the lower third subzone, characterized by unstable water solution and weak dissolution or weak precipitation. The three subzones can indicate the vertical co₂ cycle. In fine and loose sediment strata, the bottom of the first subzone is the lower boundary strongly influenced by co₂ from rainwater, soil and air; all co₂ from rainwater, soil and air is almost exhausted in the second subzone. In the early developmental period of karst process in limestone strata, karst funnels and vertical caves do not form, vertical seeping of rainwater and soil water is very slow, and co₂ from soil, rainwater and air almost can reach the third subzone, but in the middle and late developmental periods, karst funnels and vertical caves occur, co₂ from soil, rainwater and air can reach deep seasonal change zone and horizontal cycle zone and quicken development of karst morphology. Deep karst morphology near groundwater level under vertical cycle zone develops better in the middle and late periods of karst process.

Key words: vertical cycle zone, karst subzone division, three subzones, dividing signs, co₂ cycle