Individual event, seasonal and interannual variations in delta O-18 values of drip water in Maomaotou Big Cave, Guilin, South China, and their implications for palaeoclimatic reconstructions

摘要

Accurately interpreting the stable oxygen isotopic composition (delta O-18) of cave drip water is critical to the retrieval of palaeoclimatic information based on the delta O-18 records preserved in speleothem calcite. Substantial cave monitoring data sites of interest can make essential contributions. In this study, six speleothem drip water sites and one pool water site were selected for a five-year (2013-2017) monitoring of O and H stable isotope ratios in Maomaotou Big Cave, Guilin, South China. The results reveal that the drip water delta O-18 generally inherits the signals in the precipitation but may lag behind by times ranging from 1 to 6 months. Variations in drip water delta O-18 are determined by climatic variables operating over several time scales. At the individual storm-event scale, drip water delta O-18 is a function of rainfall amount, moisture source delta O-18 and hydrological processes within the vadose zone. Moisture from nearby Beibu Bay in the northwest of the South China Sea often causes the rainfall and thus drip water to be enriched in(18)O at Maomaotou Cave, for example. Seasonally or annually, the higher amounts of precipitation associated with the occurrence of El Nino events (e.g. in 2015/2016) result in more depleted drip water delta O-18. On the interannual scale, variations of drip water delta O-18 are linked to changes in the intensity of the East Asian summer monsoon (EASM), the Southern Oscillation Index (SOI) and the West Pacific Subtropical High (WPSH). An enhancedEASMcoupled with La Nina-like conditions likely results in depleted precipitation delta O-18, as does a strongerWPSHthat leads to more rainfall. This study illustrates the importance of understanding these complex and varying external environmental conditions that contribute to delta O-18 changes in drip water, when interpreting the palaeoclimatic information contained in speleothems.