Geochemical characteristics of cave drip water respond to ENSO based on a 6-year monitoring work in Yangkou Cave, Southwest China

The scientific explanation of speleothem delta O-18 in Chinese monsoon region is a greatly debated issue. Modern cave monitoring combined with instrument observation maybe is an essential solution to deal with this issue. During the period from 2011 to 2016, we monitored local precipitation, soil water in three soil profiles, and six drip water sites in Yangkou Cave, which is located in Chongqing City, Southwest China. This article presents measurements about delta O-18, SD and Mg/Ca ratios of drip water and compared these geochemical proxies with contemporaneous atmospheric circulations. The main conclusions are: (1) As water migrates from precipitation to soil water to cave drip water, the amplitudes of seasonal variations in SD and delta O-18 decreased gradually. Due to the existence of complex hydrogeological conditions, the range of variation and the seasonal characteristics of SD and delta O-18 differ among the drip sites where samples were collected, but the interannual variability is nearly the same. The drip water Mg/Ca ratios are mainly regulated by changes in hydrological conditions in the epikarst zone, with higher values during winter months than that during summer months. (2) When an El Nino event occurs, the Western Pacific Subtropical High (WPSH) is migrated westward, and the production of near source water vapor from the western Pacific and the South China Sea increases, leading to higher delta O-18 values in the precipitation and the cave drip water. The drip water Mg/Ca ratios were significantly lower with increased summer precipitation. On the other hand, during La Nina events, the WPSH is migrated eastward, and inputs of water vapor that has traveled greater distances (from the Indian Ocean) become comparatively important, resulting in lower delta O-18 values in the precipitation and the cave drip water. The drip water Mg/Ca ratios were higher with decreased summer precipitation. In summary, the interannual variability of delta O-18 in the drip waters of Yangkou Cave reflects changes in water vapor sources caused by atmospheric circulation patterns. Mg/Ca ratios respond to changes of precipitation and CO2 in soil and can be used to reconstruct abnormal drought or flood events.