Significance of active speleothem δ18O at annual-decadal timescale——A case study from monitoring in Furong Cave

Abstract

Stalagmites are the product of the combined effects of the surface environment and the cave environment, and their stable carbon and oxygen isotope ratios (δ18O and δ13C) have been widely used as proxies for palaeoclimatic and palaeoenvironmental reconstructions. Modern cave monitoring is essential for the accurate interpretation of multiple-proxies in stalagmites. However, the interpretation of the climatic and environmental signals indicated by the carbon and oxygen stable isotopes in stalagmites remains controversial. Based on the ongoing monitoring work of Furong Cave in Southwest China over 12 years (2008-2019 AD) involving a combination of field monitoring and experimental data, the following conclusions were drawn. (1) On the interannual scale, the isotopic fractionation of oxygen in active speleothems (AS) exhibits equilibrium fractionation. On the seasonal scale, the crystallization of a mixture of calcite and aragonite minerals is one of the reasons for the deviation of oxygen isotopic composition from the value expected under equilibrium fractionation. (2) There was a significantly positive correlation between δ18O and δ13C values in the AS, which may have been controlled by climate change rather than dynamic fractionation. (3) Mainly because of the mixing effect in the 300–500 m of bedrock overlying Furong Cave, the δ18O value of the drip water reflected the annual average δ18O value of precipitation. (4) The AS δ18O (δ18OAs) values over the 12-year monitoring records did not show significant seasonal variations but exhibited multiyear trend, which might reflect changes in the external atmospheric circulation and “amount effect” on decadal and longer timescales. Consequently, speleothem δ18O values in Furong Cave can be used for isotope-based palaeoclimate reconstruction.