Special speleothems in cement-grouting tunnels and their implications of the atmospheric CO2 sink

 Based on the analyses and comparisons of water chemistry, stable carbon isotopes and deposition rates of speleothems, the authors found that there are two kinds of speleothems in the tunnels at the Wujiangdu Dam site, Guizhou, China, namely the CO₂-outgassing type and the CO₂-absorbing type. The former is natural, as observed in general karst caves, and the product of karst processes under natural conditions. The latter, however, is special, resulting from the carbonation of a cement-grouting curtain and concrete. Due to the quick absorption of CO₂ from the surrounding atmosphere, evidenced by the low CO₂ content in the air 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 of CO₂ in the atmosphere is important (in the order of magnitude of 10⁸ tons c/a) and should be taken into consideration in the study of the global carbon cycle because of the use of cement on a worldwide scale. Received: 21 July 1997 · Accepted: 13 January 1997