SPECIAL SPELEOTHEMS IN CEMENT-GROUTING TUNNELS AND THEIR IMPLICATIONSOF THE ATMOSPHERIC CO2 SINK

Based on the analyses and comparisons of water chemistry, stable carbo n isotopes and deposition rates of speleothems, the authors found that there are two kinds of speleothems in the tunnels at the Wujiangdu Da m site, Guizhou, China, namely the CO2-outgassing type and the CO2-abs orbing type. The former is natural, as observed in general karst caves , and the product of karst processes under natural conditions. The lat ter, however, is special, resulting from the carbonation of a cement-g routing curtain and concrete. Due to the quick absorption of CO2 from the surrounding atmosphere, evidenced by the low CO2 content in the ai r and the high deposition rate of speleothems (as high as 10 cm/a) in the tunnels, the contribution of the carbonation process to the sink o f CO2 in the atmosphere is important tin the order of magnitude of 10( 8) tons c/a) and should be taken into consideration in the study of th e global carbon cycle because of the use of cement on a worldwide scal e.