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Precisely dated high-resolution speleothems may record past typhoon events, however, the state of the art cave monitoring is a prerequisite to identify suitable stalagmites for the reconstruction of such events. With this motivation, we examined the isotopic composition (δ 18 O and d-excess values) of rainfall, outside river, cave drip water, and an underground river in the Xianyun cave system, located in southeastern China. Monthly to bimonthly monitoring of environmental and isotopic conditions was conducted for 1 year, from December 2018 to December 2019, including a typhoon event (August 24, 2019 to August 26, 2019), called Bailu. The δ 18 O of rainfall samples over the cave and outside river water ranged from −9.7‰ to −1.9‰ and −8.2‰ to −6.3‰, respectively, while the δ 18 O of Typhoon Bailu rainfall and instantaneous outside river water ranged from −19.6‰ to −6.3‰ and −10.4‰ to −7.7‰, respectively. Typhoon Bailu-induced rainfall showed distinctly negative δ 18 O values as compared to those of the monthly and bimonthly rainfall, exhibiting a three-stage inverted U-shaped variation characteristic. Four drip water monitoring sites inside the cave revealed low variations during the studied period with average values of −7.8‰, −8.0‰, −8.0‰, and −8.1‰. However, during the typhoon, the drip water δ 18 O values exhibited similar characteristic as outside rainfall but with just 0.2‰ negative deviation owing to precipitation amount and drip water source reservoir. The integration of rainfall amount with drip water source reservoir determines the degree to which a typhoon isotopic signature gets diluted during epikarst infiltration. This study provides the first instrumental evidence of typhoon signal in karst system in southeastern China. Our results imply that the δ 18 O of drip water in Xianyun cave can instantaneously respond to typhoon rainfall. However, the 0.2‰ shift in drip water δ 18 O is difficult to be recorded by speleothems. We suggest multiyear monitoring to ascertain fully if the stalagmites could be used as paleotyphoon proxy.

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Typhoons produce a large amount of rainfall with a highly depleted isotopic composition. However, the mechanisms of isotope fractionation during typhoons remain poorly understood. Here, we present the hourly resolution isotope composition of rainfall during Typhoon Lupit (2021) in Fuzhou, on the southeast coast of China, to better understand these processes. Our results showed that the rainfall of Typhoon Lupit had significant fluctuations in δ 18 O values, ranging from À20.1‰ to À6.3‰, and exhibited a three-stage variation with distinctly negative δ 18 O values in the second stage. In the first stage of Lupit, water vapour originated primarily from the nearby sea area, which did not undergo sustained convective activity, resulting in relatively enriched δ 18 O values. In the second stage, the δ 18 O values were extremely negative mainly because of the accumulated convective activity and continuous precipitation as they travelled towards the centre of the typhoon. In the third stage, the mixing of more localized and nearby sea water vapour caused an increase in the δ 18 O values. In summary, the three-stage characteristics of Typhoon Lupit were largely caused by the integration of accumulated convective activity and moisture sources.Our findings reveal how upstream rainout affects the isotopic behaviour of rainfall and may have implications for understanding the dynamics of typhoon rainfall isotopes.

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