Seasonal variation of stable isotopic compositions recorded in a laminated tufa, SW Japan

High-resolution stable isotopic analyses of an annually laminated tufa from Shirokawa, SW Japan, show cyclic variations that correspond to mm-scale summer (dense) and winter (porous) laminae. Both δ18O and δ13C values are high in winter and low in summer, and correlate well with each other (r=0.88). Because the δ18O content of stream water has remained relatively stable over time, the δ18O curve reflects seasonal changes in water temperature. Cyclic variation in δ13C suggests that it is controlled by seasonal changes in the δ13C of dissolved inorganic carbon in groundwater, which is probably caused by groundwater degassing. This degassing effect is normally strongest during winter when subsurface air is warmer (less dense) than the atmosphere. This decreases PCO2 in cave air, and the associated degassing of 13C-depleted CO2 from the groundwater increases the δ13C value of dissolved inorganic carbon in winter. Calcite was deposited in isotopic equilibrium with stream water. Extreme climatic events, such as drought, can be recorded as anomalies in both the δ18O and δ13C curves. This study demonstrates that annually laminated tufa can be used for high-resolution analysis of terrestrial paleoclimate.