Geochemical and stable isotope studies on natural water in the Taroko Gorge karst area, Taiwan—chemical weathering of carbonate rocks by deep source CO 2 and sulfuric acid

Abstract

The geochemical cycle of CO 2 and the hydrological cycle in the Taroko Gorge karst area, Taiwan, are discussed using chemical composition data of the natural water, together with stable isotope ratios of carbon and oxygen of the water and soil CO 2. The soil CO 2 concentration at a depth of 1 m showed seasonal variations with a maximum of 5.2% (v/v) and the minimum of 0.6% (v/v) at three different altitudes ranging from 50 to 2000 m asl. The soil temperature is the most important factor controlling the soil CO 2 concentration in this area. The solubility of carbonates in groundwater can be explained by evolution under closed system conditions depending on the soil CO 2 concentration measured. Unexpectedly, high δ 13C values of HCO 3 − in groundwater were observed at some springs. There was also an equivalent relationship between the sum of the concentration of Mg 2+ and Ca 2+ ions and that of HCO 3 − and SO 4 2− ions in their waters. These two facts suggest that some deep source CO 2 and sulfuric acid derived from weathering of pyrite may take part in the chemical weathering of carbonate rocks (mainly calcareous schist) as well as the soil CO 2. After the correction for the contribution of sulfuric acid, the δ 13C values of HCO 3 − of groundwater issuing from some springs were still high, suggesting the presence of a deep source CO 2 flux. Altitude isotope effects were observed for surface water and rainwater in this area and could be expressed as δ 18O (‰)=−0.0024 h−3.67 ( h: altitude, m asl). The estimated altitudes of catchment basins of springs using this equation were over 1000–2000 m higher than those of springs. The rain falling high in the mountains is supplied with soil CO 2, dissolves carbonate rocks under closed system conditions, penetrates deep underground, and comes out again from springs together with the deep source CO 2 along fault crushing belts.