An integrated study of constraining the initial ²³⁰Th of a stalagmite and its implications

High-precision U–Th age is the key to promoting speleothem as an important archive for paleoclimate study. Notably, obtaining reliable U–Th age primarily relies on the accurate initial ²³⁰Th correction. However, correcting the U–Th age of stalagmite with the average ²³²Th/²³⁸U value in the crust is sometimes not enough to eliminate the effect of the initial ²³⁰Th, using other dating methods (e.g., ¹⁴C) and referring to the characteristics of speleothem δ¹⁸O values are useful to guide its age correction and establish the chronology. This paper presents a comprehensive study of the chronology and isotopic record of a stalagmite XBL-5 from Xiaobailong Cave, Yunnan, China, using a combination of stalagmite U–Th dating, ¹⁴C dating, and δ¹⁸O wiggle match dating. The low uranium content of this stalagmite and the apparent inversion of U–Th dating prevent us from establishing the proxy profiles with reliable chronology. We determined the actual age range based on ¹⁴C ages assuming various dead carbon proportion (DCP) and uncorrected U–Th ages, along with the characteristic of stalagmite δ¹⁸O variations, and then established a credible chronology of this stalagmite through wiggle match of δ¹⁸O records. Taking the tuned ages as a reference, the initial ²³⁰Th/²³²Th values were reciprocally calculated. The initial ²³⁰Th/²³²Th atomic ratio of the studied samples was back-calculated to be 75.7 ± 11.4 × 10⁻⁶, an order of magnitude higher than the commonly used correction value. The reconstructed XBL-5 δ¹⁸O record spans the Younger Dryas (YD) period, indicating that the precipitation δ¹⁸O generally increased within the YD chronozone with significant centennial scale oscillations in regions affected by the Indian summer monsoon, in contrast to the speleothem records from East Asian summer monsoon domains which show no or even weakly decreased long-term trend in precipitation δ¹⁸O. This study highlights the importance of initial thorium correction in stalagmite chronology study and provides a paradigm for sequence reconstruction of similar samples.