East Asian monsoonal variation recorded in the soda straw in a limestone cave (South Korea)
우경식; 홍기훈; 석봉출; 박병권
Limestone caves in the Korean peninsula have been affected by monsoonal climatic changes (humid in summer and dry in winter), thus the speleothems in these caves should reflect the same climatic variations. Monsoonal climatic variations can be detected from the soda straw in the Seopdong Cave, which grew for five years (July, 1999 ~ July, 2004). This soda straw is ca. 20 cm long, and this implies that the growth rate is 4 cm/yr. Excess 210Pb variation coincides well with the amount of precipitation during its growth, and the values are high during the summers (rainy seasons) and low during the dry winters (dry seasons). This indicates that radioactive-decayed 210Pb incorporated into rainwater reached the cave very rapidly because the transit time of rainwater into the cave was very short. d18O and d13C compositions of the soda straw are -8.6 ~-7.7‰ and -7.8 ~-5.8‰, respectively. Carbon isotope contents are relatively more depleted from 2000 to 2001, and this was probably due to the lower amount of precipitation during this interval. It is believed that smaller rainfall is reflected by more depleted d13C values, because degassing rate of CO2 decreases with decreasing rate of cave water supplied from overlying limestone. On the contrary, the periods of higher precipitation (1999, 2002~2004) show more enriched d13C values. The overall trend suggests that d13C of soda straw should be controlled by the degassing rate of CO2. Higher degassing rate can be confirmed by higher partial pressure of CO2 (1500 ppm) of cave atmosphere during summer, and lower pressure (430 ppm) during winter.