Seasonal and annual changes in soil/cave air pCO2 and the δ13CDIC of cave drip water in response to changes in temperature and rainfall

Abstract

This study analyzes cave pCO2 and the δ13CDIC of drip water in response to surface environmental changes in the Furong Cave, Chongqing, southwestern China, between 2009 and 2016. Several indices were continuously monitored, including air temperature, rainfall, soil pCO2 outside the Furong Cave, as well as cave air pCO2 and δ13CDIC of drip water inside the Furong Cave. The results revealed that (1) the overlying soil pCO2 at the Furong Cave is directly controlled by the surface temperature and rainfall. Soil pCO2 is higher in summer and autumn and lower in winter and spring. On an interannual time scale, soil pCO2 shows a trend similar to annual rainfall. (2) Cave pCO2 and soil pCO2 both show characteristics of significant seasonal variation, which is similar to the seasonal variation in rainfall in Chongqing. Rainfall significantly affects cave pCO2. (3) The δ13CDIC values of the drip water at Furong Cave are generally lower in summer and autumn and higher in winter and spring. They are mainly affected by seasonal variation in rainfall and the consequent soil CO2 yield, which is also related to the increase in CO2 degassing of the drip water caused by cave pCO2 decreases in winter and spring. (4) The annual rainfall decreased in 2010–2011, and the δ13CDIC of the drip water was generally high. The annual rainfall gradually increased from 2012 to 2016, and the δ13CDIC of the cave drip water showed a consistent reduction. The δ13CDIC of the drip water at the Furong Cave may be used as an index of changes in surface rainfall which can reflect drought and flood events.