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

Accurately interpreting the stable oxygen isotopic composition (δ 18 O) of cave drip water is critical to the retrieval of palaeoclimatic information based on the δ 18 O 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 δ 18 O generally inherits the signals in the precipitation but may lag behind by times ranging from 1 to 6 months. Variations in drip water δ 18 O are determined by climatic variables operating over several time scales. At the individual storm‐event scale, drip water δ 18 O is a function of rainfall amount, moisture source δ 18 O 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 Niño events (e.g. in 2015/2016) result in more depleted drip water δ 18 O. On the interannual scale, variations of drip water δ 18 O 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 enhanced EASM coupled with La Niña‐like conditions likely results in depleted precipitation δ 18 O, as does a stronger WPSH that leads to more rainfall. This study illustrates the importance of understanding these complex and varying external environmental conditions that contribute to δ 18 O changes in drip water, when interpreting the palaeoclimatic information contained in speleothems.