Evolution of the Indian summer monsoon: synthesis of continental records
Prasanta Sanyal, R. Sinha, 2010. "Evolution of the Indian summer monsoon: synthesis of continental records", Monsoon Evolution and Tectonic–Climate Linkage in Asia, P. D. Clift, R. Tada, H. Zheng
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Fluvial sediments of the Siwalik succession in the Himalayan Foreland Basin form the most important continental archive for reconstructing monsoonal fluctuations during the Late Miocene to Late Pleistocene. A number of proxy records suggest multiple phases of monsoonal intensification with peaks at 10.5, 5.5 and 3 Ma after which the strength of the monsoon decreased to modern day values with minor fluctuations. Detailed evaluation of Late Quaternary interfluve stratigraphic development in the Ganga plains shows that interfluve areas near the major rivers aggraded periodically between 27 and 90 ka. They subsequently degraded or accumulated sediment only locally, probably reflecting decreased monsoonal precipitation around the Last Glacial Maximum. Increased precipitation during the 15 to 5 ka period of monsoon recovery probably increased discharge and promoted incision and widespread badland formation. In western India, the fluvial records back to c. 128 ka suggest a stronger monsoon around 80 ka followed by periods of weakened monsoon around 70 to 30 ka and progressive desiccation in the glacial period. Holocene lacustrine records from western Rajasthan suggest maximum lake levels at c. 6 ka and complete desiccation between c. 3 and 4 ka.
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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.