Variations of monsoonal rain and vegetation during the past millennium in Tiangui Mountain, North China reflected by stalagmite δ¹⁸O and δ¹³C records from Zhenzhu Cave

An 11-cm long stalagmite (ZZ12) collected from Zhenzhu cave, Tiangui Mountain, northwest of the North China Plain (NCP) has been measured by ²³⁰Th/U, AMS ¹⁴C and ²¹⁰Pb dating methods. Only AMS ¹⁴C dating yielded the chronology successfully, while ²³⁰Th/U did not due to low uranium content and the young age of the stalagmite. The top layers in the stalagmite contain “nuclear bomb carbon” signal, and the AMS ¹⁴C dates of charcoal and carbonate samples at the bottom are similar. Based on a series of AMS ¹⁴C dates which contain minimal dead carbon influence (DCI) in the stalagmite, the age model of the stalagmite has been constructed, indicating that the stalagmite was 1970 years old. A total of 470 subsamples from the upper 51 mm (since 1180 CE) of the stalagmite have been analyzed for δ¹⁸O and δ¹³C. The δ¹⁸O record of stalagmite ZZ12 reflects changes in the summer monsoonal rainfall in the NCP, being comparable with the local and north-central China dry-wet index. The driest periods were found in 14th-15th centuries in the Little Ice Age (LIA) and since 1970s, perhaps due to weakening of East Asian summer monsoon. The δ¹³C of ZZ12 mainly recorded vegetation changes in Tiangui Mountain and revealed deforestation in two major periods: 1500–1600 CE and 1830–1940 CE caused by local human activities. Spectral analysis of the δ¹⁸O record suggests that the Pacific Decadal Oscillation (PDO) and Atlantic Multidecadal Oscillation (AMO) may both play important roles in changes of multi-decadal monsoonal rainfall in the NCP, with more rainfall responding to the negative PDO, La Niño-like, and positive AMO phases.