Radiometric dating of the late Quaternary summer monsoon on the Loess Plateau, China
T. Stevens, H. Lu, 2010. "Radiometric dating of the late Quaternary summer monsoon on the Loess Plateau, China", Monsoon Evolution and Tectonic–Climate Linkage in Asia, P. D. Clift, R. Tada, H. Zheng
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Recent advances in radiometric dating have enabled independent investigation into monsoon variations. In this study, summer monsoon pedogenesis proxies (CaCO3 and magnetic susceptibility) have been analysed by optically stimulated luminescence (OSL) age for five loess-palaeosol sections over the Chinese Loess Plateau. The use of CaCO3 is complicated by the multiple influences on its variation. However, changes in magnetic susceptibility can be used as a proxy for summer monsoon induced pedogenesis. The data suggest that the summer monsoon in north-central China is not prone to high frequency shifts, although abrupt transitions occur. The overall patterns show general decreasing trends from c. 50 to 18 ka. However, between 9 and 6 ka, magnetic susceptibility increases abruptly and dramatically at the sites. These findings suggest that the Holocene ‘optimum’ in the region may be a more recent phenomenon than previously suggested, and that this summer monsoon intensity increase significantly post-dates the insolation peak occurring at 11.5 ka. An apparent close correspondence to ice volume is suggested to be a consequence of forcing via atmospheric circulation. Independently dated records that employ high sampling resolution can be used to test this hypothesis, together with suggestions over the apparent lag between insolation forcing and monsoon response.
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The Earth’s climate varies through geological time as a result of external, orbital processes, as well as the positions of continents, growth of mountains and the opening and closure of oceanic gateways. Climate modelling suggests that the intensity of the Asian monsoon should correlate, at least in part, with the uplift history of the Tibetan Plateau and the Himalaya, as well as the evolution of gateways and the retreat of shallow seas in Central Asia. Long-term reconstructions of both mountain building and monsoon activity are key to testing the proposed links. This collection of papers presents a series of new studies documenting the variations of the Asian monsoon on orbital and tectonic timescales, together with the impact this has had on environmental conditions. The issue of which proxies are best suited to measuring monsoons is addressed, as is the effect that the monsoon has had on erosion and the formation of the stratigraphic record both on and offshore.