Significance of active speleothem delta O-18 at annual-decadal timescale-A case study from monitoring in Furong Cave

摘要

Stalagmites are the product of the combined effects of the surface environment and the cave environment, and their stable carbon and oxygen isotope ratios (delta O-18 and delta C-13) 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 delta O-18 and delta C-13 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 delta O-18 value of the drip water reflected the annual average delta O-18 value of precipitation. (4) The AS delta O-18 (delta O-18(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 delta O-18 values in Furong Cave can be used for isotope-based palaeoclimate reconstruction.