The variation of summer monsoon precipitation in central China since the last deglaciation

Two stalagmites (C996-1 and C996-2) collected from the Jiuxian Cave in the Shaanxi Province in central China have been studied with U-series dating and stable isotope analysis. Thirty-eight ²³⁰Th dating results showed that the stalagmite C996-1 was continuously deposited through the last 8.5 ka BP (thousand years before present, present=1950 AD), and C996-2 was deposited through the last 19 ka BP excluding two growth hiatuses between 8.3 and 5.7 ka BP, and 15.4 and 11.9 ka BP. With a relatively stable boundary condition, we interpret the δ¹⁸O of speleothem calcite as most indicative of the amount of summer monsoon precipitation, although temperature and other factors might have some minor impact. The δ¹⁸O records show notable changes within the last glacial maximum (LGM), resembling other East Asian monsoon records such as those from the Hulu and Sanbao Caves, suggesting that significant monsoonal climate changes occurred in eastern Asia as far north as the Qinling Mountains during the LGM interval. A comparison of our records to precisely dated contemporaneous speleothem records from other caves shows that the increasing trend of δ¹⁸O during the Holocene commenced as early as ∼ 7.5 ka BP in the low-latitude monsoonal area, i.e. the Hoti Cave, while at higher latitudes this shift occurred later, such as ∼ 7.0 ka BP in the Dongge Cave, ∼ 5.3 ka BP in the Heshang Cave, ∼ 4.7 ka BP in the Sanbao Cave and ∼ 4.5 ka BP in the Jiuxian Cave. These results imply an asynchronous change of the summer monsoon precipitation occurred in East Asia during the Holocene. The asynchrony may be related to the responses of a coupled tropical and subtropical monsoon system to changes of the insolation and the differences in thermal forcing, which result from the complex geographical configuration. The variation of sea surface temperature (SST) in the western tropical Pacific may also have important impacts on the summer monsoon precipitation changes in central and northern China because it affects the Northwestern Pacific Subtropical High, a monsoon-front regulator. More robust tests are needed to confirm this phenomenon and to evaluate the contribution of different factors in detail.