Doctor of Philosophy (PhD)
Geography and Anthropology
Using quantitative reconstructions of vegetation and climate based on 234 surface samples and four fossil pollen records, a systematic study of millennial-scale variations and centennial-scale events in the Southwest monsoon over the last 14 000 years in the Tibetan Plateau was conducted. The SW monsoon stayed weak between 14 000 and 11 000 cal. yr BP. A marked drop in July temperature during 12 800 –11 500 cal. yr BP may indicate the occurrence of the Younger Dryas cold event. The SW monsoon started to intensify at 11 000 cal. yr BP. However, it did not increase monotonically, but abruptly in three steps to reach its maximum. The three transitions from weak to strong monsoon occurred at 11 000, 10 000 and 8000 cal. yr BP, respectively. The last transition marked the onset of a 1200-yr period of monsoon maximum. After the monsoon maximum, the SW monsoon decreased to the present level through two strong-to-weak transitions, one starting at 6800 cal. yr BP, and the other at 3100 cal. yr BP. The millennial-scale variation seems to suggest that the monsoon system switched from one mode to another during different periods, probably triggered by variations in insolation and glacial boundary conditions. The pollen records have also revealed a clear pattern of abrupt centennial-scale monsoon weakening events. These events exhibit a distinct duration at centennial-scale and a pacing at millennial-scale. There are ten events, occurring at about 1100, 2100, 3000, 4500, 5800, 6500, 7700, 8200, 9200, and 10 100 cal. yr BP with a significant periodicity of 830-900 years. The mechanisms responsible for these events probably include external forcing such as solar activities and tidal cycles, and internal forcing such as non-linear feedbacks and threshold behavior in the climate system. A long pollen record from the Zoige Basin in the northeastern Tibetan Plateau provides information on vegetational and climatic changes during the last two glacial-interglacial cycles. Subalpine spruce-fir forests were widespread during the interglacial and interstadial times, suggesting warm and wet climatic conditions. Alpine periglacial or dry desert existed under cold and dry climatic conditions during the penultimate and the last glacial maxima. Alpine sedge meadows dominated during the stadial epochs.
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Shen, Caiming, "Millennial-scale variations and centennial-scale events in the Southwest Asian monsoon: pollen evidence from Tibet" (2003). LSU Doctoral Dissertations. 3440.