Geochemical and stable isotope studies on natural water in the Taroko Gorge karst area, Taiwan—chemical weathering of carbonate rocks by deep source CO2 and sulfuric acid

The geochemical cycle of CO2 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 CO2. The soil CO2 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 CO2 concentration in this area. The solubility of carbonates in groundwater can be explained by evolution under closed system conditions depending on the soil CO2 concentration measured. Unexpectedly, high δ13C values of HCO3− in groundwater were observed at some springs. There was also an equivalent relationship between the sum of the concentration of Mg2+ and Ca2+ ions and that of HCO3− and SO42− ions in their waters. These two facts suggest that some deep source CO2 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 CO2. After the correction for the contribution of sulfuric acid, the δ13C values of HCO3− of groundwater issuing from some springs were still high, suggesting the presence of a deep source CO2 flux. Altitude isotope effects were observed for surface water and rainwater in this area and could be expressed as δ18O (‰)=−0.0024h−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 CO2, dissolves carbonate rocks under closed system conditions, penetrates deep underground, and comes out again from springs together with the deep source CO2 along fault crushing belts.