Millennial-scale variability in the Asian monsoon: Evidence from oxygen isotope records from stalagmites in Southeastern China

摘要

Two stalagmites from Xiangshui (X3) and Yaoba Don (YB1) Caves in southeastern China provide high-resolution 6180 time-series that exhibit prominent millennial-scale fluctuations in the intensity and character of the Asian monsoon for the period from 20,000 to 50,000 yr. B.P. Timing of these fluctuations, established by U-series disequilibrium (Th-230/U-234). correlates with Dansgaard-Oeschger events (2-13) and Heinrich events (H2-H5) recorded in the GISP2 ice core, indicating a climatic link between Asian monsoon circulation and air temperatures over the North Atlantic for much of the last glaciation. Although the exact mechanisms linking climatic fluctuations in the North Atlantic to those in eastern China have yet to be identified, climatic signals associated with changes in global ice volume and air temperatures over Greenland may be transferred to Asia by atmospheric mechanisms that affect the strength of the Siberian high-pressure cell and the amount of snow cover on the Tibetan Plateau, which force the intensity of the Asian monsoon. Another mechanism linking climate of the North Atlantic to the Asian monsoon relates to variation in oceanic circulation. Millennial-scale fluctuations in thermohaline circulation in the North Atlantic may have affected ocean currents in the tropical western Pacific Ocean, which is the moisture source for the East Asian monsoon. Despite the similarity of these paleoclimatic records and the implication of global teleconnections, the magnitude and timing of millennial-scale events at different locations in China reveal regional variations in climatic conditions. Comparisons of the delta O-18 curves from Xiangshui and Yaoba Don Cave stalagmites with those from Qixin Cave and Hulu Cave show general concordance between millennial-scale events, albeit with some notable differences among all the records. The well-studied Hulu Cave records show delta O-18 values that are lower than those of Xiangshui and Yaoba Don Caves, reflecting geographical differences. Hulu Cave is located near the eastern coast at a relatively low elevation dominated by the East Asian monsoon. By comparison, Xiangshui Cave and Yaoba Don Cave are further inland on the eastern slope of the Yunnan-Guizhou plateau, which receives precipitation from both the East Asian monsoon and quasi-statio nary frontal systems. Rainfall contributed by the East Asian summer monsoon is relatively diminished in this region by these geographic and atmospheric circulation conditions, resulting in higher delta O-18 values.