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

Highlights: • A 12 years monitoring work in a karst cave in Southwest China. • Deposition of active speleothem was under isotope equilibrium fractionation. • Speleothem δ¹⁸O can be used to reconstruct change of atmospheric circulation. • A positive correlation between δ¹⁸O and δ¹³C can be controlled by climate change. 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 (δ¹⁸O_As) 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.