Daily response of drip water isotopes to precipitation in Liangfeng Cave, Guizhou Province, SW China

摘要

Oxygen isotope (δ~(18)O) is one of the most commonly used palaeoclimate proxies, and monitoring their modern evolutionary processes is very significant for palaeoclimate reconstruction. In this study, drip water samples are daily collected from two drip sites in Liangfeng Cave, Guizhou Province, SW China, between June 2008 and June 2010. The stable hydrogen and oxygen isotopes of these samples and the contemporary precipitation samples are measured. The relationships between the isotopes and the local air temperature, precipitation and relative humidity are analysed. The results show that the hydrogen and oxygen isotopic compositions of precipitation have obvious seasonal variations in the study area: lower in the rainy season and higher in the dry season. The local meteoric water line (LMWL) is δD = 8.64 δ~(18)O + 17.44. Precipitation is the only source of cave drip water, and its oxygen isotope signals are reflected in the two drip waters (SD: slower drip rate and previously called 1#, and FD: faster drip rate and previously called 5#). However, the amplitude of the drip water oxygen isotope variation is much smaller than that of the precipitation, i.e., homogenization occurs to some extent. However, there are significantly different responses to precipitation between the two drip sites. The response time of SD to precipitation is much longer than that of FD. The amplitude of oxygen isotope variation in SD, where the isotopic data deviate from the LMWL to a higher degree, is much smaller than that in FD. A comparison of these isotopic differences with previous research in the same cave indicates that the isotopic differences between the two drip waters result from different flow paths. The oxygen isotope signal in cave drip water perhaps mainly reflects summer monsoon information in the study area. The speleothems fed by drip waters (e.g. FD) with shorter response time to precipitation may be more suitable for high-resolution palaeoclimate research. Moreover, the d-excess from speleothem fluid inclusion has the potential to be used as an indicator of relative humidity of local air.