Abstract
The geochemical cycle of CO2 in a carbonate rock area was discussed by using for an example obtained for Akiyoshi-dai Plateau (Yamaguchi Prefecture), one of the big gest karst plateaus in Japan. The calcium concentration of the baseflow of the groundwater in the area showed seasonal fluctuations, and followed changes in CO2 concentration in the soil. Soil CO2 measured in the meadows which cover most of the area varied from a minimum of 0.08% at a soil temperature of 3.8℃, and a maximum of 1.2% at 20.8℃. The calcium concentration in the groundwater is controlled by waterlimestone dissolution equilibrium, under open system conditions depending on the meadow's soil CO2 concentration. At the runoff peak of groundwater issuing from Akiyoshi-do Cave, which has the biggest drainage basin in Akiyoshi-dai Plateau, 18.5 km2, the calcium concentration increases due to the flushing out of water with a long residence time in the deeper phreatic zone. During 1983-1986, a yeary average of 2,100 tons of limestone was dissolved in 2.1×107m3 of groundwater issuing from Akiyoshi-do Cave, its catchment basin including 16.5km2 of a limestone area: the mean solutional denudation rate is 51mm/ka. The amounts of CO2 untilized on chemical weathering in carbonate rock areas in the world, corresponding to the same amounts of chemically weathered carbonate rocks in mol, were estimated by using the limestone denudation rate of 50mm/ka and found to be 8x1011kg/y. The role of chemical weathering of carbonate rocks cannot be ignored in the geochemical cycle of CO2.
