Seasonal temperature changes in Shennonggong and Xianglong caves and their potential impact on oxygen isotope composition of stalagmite carbonate

Consecutive 19-month temperature measurements in Shennonggong and Xianglong caves were conducted synchronously. The results indicate that the temperature was not stable in both caves, but oscillated on both hourly and seasonal time scales. Temperature data from inside and outside the caves show that in Shennonggong cave the temperature fluctuated more significantly during the summer season than in winter, revealing the impact of the amount of precipitation on the cave temperature. On the other hand, in Xianglong cave the amplitude of temperature oscillations is smaller during summer than in winter, implying that the chamber structures limit ventilation during the summer as it is hard for cold cave air in to flow out the high opening of the cave. The temperature inside the cave corresponds well with temperature outside the cave, with summer-winter temperature differences of 4.8 ℃ (4.6 ℃ at second site) and 3.2 ℃ in Shennonggong and Xianglong caves, respectively. The cave temperature response time to outside temperature changes varied seasonally at both cave sites. The lags in the decrease of cave temperature at the end of autumn were longer than those of the temperature increase at the end of spring in both caves. The different lags are likely due to the relatively high temperature in the deeper surrounding rock,the seasonal changes of the thermal conductivities of the air and the surrouding rock, as well as effects of seasonally changes in precipitation. It is noted that modern observations have shown the speleothem calcite may deposit only during the summer or winter (e.g., summer deposition mode or winter deposition mode). Thus, it may be deduced that the shift in speleothem growth seasons (deposition mode) may induce significant calcite δ18O changes in caves with large seasonal temperature changes.