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

Abstract

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.