Deep source CO 2 in natural waters and its role in extensive tufa deposition in the Huanglong Ravines, Sichuan, China

Abstract

The aqueous geochemistry of Huanglong Ravine, Sichuan, China, where tufa is being deposited extensively, is discussed using chemical composition data together with the carbon and oxygen stable isotope ratios. The water quality of tufa-depositing surface streams in this area is basically controlled by the mixing of two source waters—fault-bounded spring water and snow and/or glacier melt water. The spring water contains high concentrations of CO 2 supplied via faults: the resulting high concentration of dissolved CO 2 gives rise to much greater dissolution of carbonate rocks than is usual in karst springs and also to the dissolution of silicate rocks. Upstream spring waters in the Ravine are close to equilibrium with respect to calcite. Concentrations of dissolved inorganic carbon (DIC) and their δ 13C values result from the interaction of about 0.02 mol l −1 CO 2(aq) of δ 13C −3‰ with carbonate rocks of δ 13C +3‰ under closed system conditions. About 70% of the CO 2 is estimated to be supplied from the upper mantle. The data for all the springs fall in the kaolinite stability field, in agreement with observed dissolutional aggressiveness towards Na- and Ca-feldspar, and that the dissolution rates of these minerals are too slow to move the composition of these waters into the feldspar stability field. The DIC species in the surface streams are in isotopic equilibrium; variations in δ 13C DIC found at different sites are mainly due to differences in the degree of CO 2 degassing from the water. There is a linear relationship between δ 18O values and the altitude at which samples were collected; the oxygen isotope composition of surface stream water in this area is controlled by evaporation. Water chemistry in daytime and nighttime and daily change of pH in the surface water flowing on the tufa deposition indicate that biological activities contribute to the carbonate precipitation, although their extent is not great. The daily precipitation rate in this area is estimated to be 4300 kg as CaCO 3 day −1 over a surface area of 0.9 km 2, i.e, 0.1 cm year −1.