Understanding of δ13C behavior and its significance in the Furong Cave system through a 10-year cave monitoring study

Abstract:

The debate about how speleothem δ¹³C to reflect paleoclimate and paleovegetation changes calls for understanding of δ¹³C behavior in a cave system. We have conducted a monitoring study in Furong Cave, Chongqing, China since 2005 involving multiple sites in the overlying soil and inside the cave. The measured proxies include pCO₂ of the atmosphere, soil and cave air; dripping rate, pH, electronic conductivity, Ca²⁺, HCO₃^-, d¹³C_DIC of the dripwaters; and deposition rate, d¹³C of seasonally selected carbonate deposits. The many observations from this study are: (1) Soil pCO₂ is close to the atmospheric pCO₂ in winter and reaches >10000 ppm during the summer due to high productivity under warm and wet climates. (2) The pCO₂ in cave air is slightly higher than Soil pCO₂ and atmospheric pCO₂ in winter (400~900 ppm), but elevates up to ~2000 ppm around October due to seepage of soil CO₂ into the cave. The cave air pCO₂ exhibits double peaks following the seasonal rainfall (1^st peak in April-June controlled by Indian monsoon and 2^nd peak in July-September influenced by the North Western Pacific monsoon). The cave air pCO₂ has about 2-3 month lag to the soil pCO₂. (3) Under the influence of soil CO₂ and rainfall, the d¹³C_DIC of drpiwaters are depleted in the summer and enriched during the winter. During 2009-2011, southwestern China experienced extremely drought due to summer monsoon failure. The d¹³C_DIC of dripwater in Furong Cave was strongly enriched because of low soil CO₂ productivity, less transportation of soil CO₂ into the cave, and high CO₂ ratio from bedrock input. Dripping rate does not affect the d¹³C_DIC significantly. (4) The d¹³C of speleothems is controlled by the open/closed system of the seepage pathway on the first order. Monitoring site MP1 owns a relatively open system shown by fast dripping rates, low Ca and HCO₃^- contents, and low carbonate deposition rate. The d¹³C of modern deposits from this site not only shows the seasonal variation following that of soil pCO₂ with heavier values in the winter and lighter values in the summer, but also heavier than these in the other sites. This open system has more efficiency to exchange CO₂ with the atmosphere. Such an exchange will lead to d¹³C enrichment and D¹⁴C close to that of the atmosphere. Sites MP4 and MP5 in Furong Cave have more closed pathway systems, showing opposite to MP1.