A multi-proxy speleothem record of Asian monsoon variability during the late Pleistocene from Wanxiang Cave, Gansu Province, China

Speleothems collected from Wanxiang Cave, China (33.32° N, 105.00° E) provide high-resolution records of Asian Monsoon variability during the Pleistocene. Previous records have exhibited a general pattern of increased Asian monsoon intensity during interglacial or interstadial periods, and decreased intensity during glacial or stadial periods. The goal of my dissertation was to provide a more detailed and higher-resolution record of monsoon intensity spanning the past 315,000 years, and relate this record to other high-resolution records from ice cores and marine sediment cores.; The dissertation is subdivided into four chapters. In chapter one, I present a detailed study of the modern stable isotope systematics of precipitation in the Asian monsoon region, and the implications for stable isotope based paleoclimate reconstructions from this region.; In chapter two, I present a thorough study of the stable isotope systematics of the modern carbonate-water system in Wanxiang Cave to assess the potential for obtaining high-resolution paleoclimate records from fossil speleothem calcite. I show that modern speleothem calcite is forming in isotopic equilibrium and reflects only the temperature in the cave and the composition of the cave drip water, which is closely related to mean annual precipitation.; In chapter three, I present a simple model for interpreting oxygen isotopic variations along speleothem growth axes in terms of paleomonsoon intensity. I apply this model to two well-dated speleothems that formed over the last three glacial-interglacial cycles. The delta18O of speleothem calcite that formed during stadial periods is more positive than the delta 18O of calcite that formed during interstadial periods, suggesting that summer monsoon intensity was increased during globally warmer periods and decreased during globally cooler periods.; In chapter four, I apply this model to four well-dated speleothem samples that formed during the last deglaciation (MIS 2) and the penultimate deglaciation (MIS 6). Oxygen and carbon isotopic variations during the last deglaciation provide evidence for decreased summer monsoon intensity during Heinrich event 1, increased monsoon intensity during the Bolling-Allerod interstadial, and decreased monsoon intensity at the onset of the Younger Dryas. During both MIS 2 and MIS 6, the oxygen and carbon isotopic records are characterized by numerous centennial to millennial scale fluctuations that occur synchronously with global climate changes. This type of variability may be related to changes in the strength of the oceanic thermohaline circulation and suggest a strong teleconnection between the Asian monsoon system and the North Atlantic.