Compositions and flux of soil gas in Liu-Huang-Ku hydrothermal area, northern Taiwan

Soil CO2 flux has been considered a useful proxy to remotely monitor volcanic activity in a hydrothermal area within a safe distance. The Liu-Huang-Ku (LHK) area, which is close to the Taipei basin and many existing active fumaroles and hot springs, was chosen for the first systematic soil gas study in a hydrothermal area of Taiwan in 2004 and 2006. The soil CO2 flux was measured by the closed-chamber method with a non-dispersive infrared detector. Soil CO2 emission rate was estimated 19.8 ± 0.2 t day− 1 and 582 t km− 2 day− 1 in 2004, and 22.4 ± 0.2 t day− 1 and 659 t km− 2 day− 1 in 2006, respectively. These values are close to those reported for other active high CO2 flux hydrothermal areas of the world. It implies that magmatic gases are actively degassing from subsurface in northern Taiwan. Helium and carbon isotopic data reveal that soil gas in LHK is mainly derived from the mantle and is mixed with a small amount of crustal and air components. The abundance of soil CO2 shows a positive correlation with the 3He/4He ratios and total sulfur contents, which are considered as magmatic-sensitive indicators for the fumarolic samples. Therefore, soil CO2 could be used as a good proxy for future monitoring of magmatic activity in this area.