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 initial230Th, using other dating methods (e.g., 14C) and referring to the characteristics of speleothem 818O 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 818O 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 818O variations, and then established a credible chronology of this stalagmite through wiggle match of 818O 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 x 10-6, an order of magnitude higher than the commonly used correction value. The reconstructed XBL-5 818O record spans the Younger Dryas (YD) period, indicating that the precipitation 818O 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 longterm trend in precipitation 818O. This study highlights the importance of initial thorium correction in stalagmite chronology study and provides a paradigm for sequence reconstruction of similar samples.