The Asian Monsoon is recognized as a crucial part of the global climate system and has attracted tremendous attention from worldwide palaeoclimate researchers. The Asian Monsoon regime is responsible for severe drought and flooding in large areas from the western Arabian Sea to East Asia and even influences northern Australia through the interaction between the East Asian Monsoon and the Australian Monsoon. Information about the Asian Monsoon evolution over the Last Glacial-Interglacial cycle has been recovered from both terrestrial and maritime geology archives including loess-paleosol sequences, lake and ocean sediments. The difficulty in decoding these climate archives is the relatively high uncertainty of dating and the low resolution of climate records. Thanks to the state-of-the-art precise Uranium-Thorium dating technique, speleothem, a specimen formed in caves, can provide accurate chronology constraints of climate events and its growth characteristic (annual growth bands) further improves the resolution of climate information.
Speleothems from China have been targeted to reconstruct the history of the East Asian Monsoon; however, most studied stalagmites came from coastal areas and some time slots during the Last Glacial-Interglacial cycle are still obscure or missing among current stalagmite-derived records. The primary aim of this PhD research is to employ the advanced research facilities in Radiogenic Isotope Facility, Centre for Microscopy and Microanalysis and Stable Isotope Laboratory, School of Earth Sciences, the University of Queensland, to acquire well-dated, high-resolution multi-proxy monsoon climate records from central and southwest China to complement and consummate the existing records. A number of stalagmites have been collected from two caves and three among these stalagmites have selected for high-resolution analyses of U-Th dating, oxygen and carbon isotopes and elemental ratios. The results are reported in detail in three chapters in this thesis.
A stalagmite (SL5) from Shilong Cave, central China was dated from 85 to 57 thousand years before present (ky BP). Although the whole stalagmite was punctuated by three hiatuses, stable isotopes and element ratios record monsoon activities during the stalagmite growth. Chinese Interstadial (CIS) A17-19 of δ18O records has been identified in SL5 records and the differing details in SL5 δ18O records should be attributed to the adjacent Indian Monsoon. The reverse correlation of δ13C and δ18O spanning most of the stalagmite has also been reported here. Excluding the unlikely explanation of vegetation alternation and duration of percolation, corrosion of wall rock is considered as the main control for δ13C after CIS A19. Mg/Ca, Sr/Ca, Ba/Ca and U/Ca are also utilized to supply climatic implication. When combined with δ18O and δ13C records, these element data can be used as auxiliary proxies for climate change.
A well-dated Holocene record derived from a 25cm long stalagmite (SL2) also from Jiangjia Cave was investigated to reconstruct the Asian Monsoon and its tele-connection with climate regimes archived in GISP2, sea surface temperatures (SST) and the Pacific decadal oscillations (PDO) index. SL2 multiple-proxy records indicate a climate-stable Holocene in the past 2800 years. The δ18O values increased throughout the growth of SL2, implying the weakening tendency of the Asian Monsoon. However, the record of δ18O in SL2 is also full of small scale fluctuations of ~1‰ and several drier and wetter events have been identified in the record. The high synchrony between SST and SL2 record demonstrates a strong connection between monsoon intensity and SST variations. Two dry events centred at 400 and 230 yr BP with synchronous drifts displayed in SL2 δ18O, SST and PDO index also suggest that El Niño is one possible cause for such change.
Another chapter reports a stalagmite (BG3) from Baigu Cave, southwest China, which has grown continuously from 30 to 11.5 ky BP. Southwest China is now currently under the influence of the Southwest Indian Monsoon. BG3 records provide a solid and continuous addition to published climatic records spanning the same period and also a good opportunity to compare the Indian Monsoon and East Asian Monsoon. Stable isotopes and elemental ratios of BG3 have been analysed to reconstruct monsoonal climate evolution. From 30 to 17 ky BP, monsoonal precipitation was generally at a stable level with a few fluctuations at the millennium scale. Since 17 ky BP, the Asian Monsoon has experienced several weak and strong periods indicated by stalagmite δ18O. Heinrich Events and the Younger Dryas are evident in BG3 record and their timing and duration are generally synchronous with records of GISP2 and Hulu Cave stalagmite record, indicating that climate events occurred on a global scale and there was no major difference in the strength of the East Asian Monsoon and Indian Monsoon for this time period. Identical climate change in middle and high latitude areas also suggests that monsoonal
precipitation is connected to global or at least northern hemisphere climate change.