| Summary: | University of Minnesota Ph.D. dissertation. January 2010. Major: Geology. Advisor:R. Lawrence Edwards. 1 computer file (PDF); xii, 221 pages, appendices 1-3. Dongge Cave, located in southern China, is ideally suited for studies of past climate. When calcite is deposited under equilibrium conditions, oxygen isotope ratios recorded by Dongge Cave stalagmites may be considered a proxy for the intensity of Asian Monsoon precipitation. Ages of five stalagmites from Dongge Cave were obtained using uranium-thorium dating methods, providing a continuous record of Asian Monsoon variability between 50 and 250 ky BP, covering much of the last three glacial-interglacial cycles. Confirming previous indications, the Asian Monsoon record closely follows Northern Hemisphere summer insolation over orbital timescales, and is punctuated by millennial-scale strong monsoon events that correspond with abrupt Greenland warming events during the last glacial period. Conventional theory suggests that glacial-interglacial climate shifts are most directly related to variations in high northern latitude summer insolation. A critical relationship between Asian Monsoon intensity and atmospheric methane concentration at the last three glacial terminations was established that enables the correlation of the Asian Monsoon oxygen isotope record to ice core records in both hemispheres. This correlation strategy sheds light on the timing and sequence of events surrounding last three glacial terminations, demonstrating that these climate shifts occurred in two phases, with much of the marine termination, the full rise in atmospheric carbon dioxide and Antarctic temperature, and initial rise in atmospheric methane occurring prior to monsoon strengthening and presumed Greenland temperature rise. The two-phased pattern indicates that Northern Hemisphere summer insolation plays a key role initiating climate changes at major deglaciations, but feedbacks internal to the climate system must also be involved. Similar correlation strategies were utilized to study abrupt, millennial-scale climate variability during the last two glacial periods. A link between speleothem records of Asian Monsoon intensity and Greenland temperature is clearly demonstrated across the last glacial period. The North Atlantic/Greenland climate signal is propagated to the low-latitude Asian Monsoon region through oceanic and atmospheric processes related to the position of the Intertropical Convergence Zone. While Greenland ice core records are not available during the penultimate glacial, the larger, longer duriation Asian Monsoon events can be linked to Antarctic temperature maximum, continuing the trend seen during the last glacial cycle and suggesting that the thermal bipolar seesaw mechanism persisted into the older climate cycle. Thus, mechanisms of millennial climate variability appear to be similar during the last two glacial periods.
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