Himalayan River Terraces as A Landscape Response to Quaternary Summer Monsoon Variability
Abstract
In order to interpret marine sedimentary archives as records of the erosional response to Asian monsoon variability, we must first recognize how transport processes affect the storage and release of sediment to the ocean. River terraces, such as found in the Greater Himalaya, provide a pivotal role in the source-to-sink story, because this is where sediment storage occurs and is likely modulated. We investigate the role that climate plays in controlling erosion and sediment flux to the Indus delta and fan by looking at the Indus River system, which is dominated by the strong forcing of the Asian monsoon, as well as winter Westerly winds. Paleoceanographic, speleothem, and lacustrine records indicate that summer monsoon intensity was strong from 29 to 37 ka, decreased after that time until ~16 ka, reached maximum intensity from 8 to 10 ka, and then weakened until ~3 ka. Some lacustrine records, however, indicate a more complex pattern of monsoon variability in the Greater Himalaya, which contrasts with monsoonal forcing in central India. This disagreement suggests that floodplains of major river systems may not experience the same climatic conditions as their mountain sources, resulting in contrasting landscape responses to climate change. High altitude river valleys, at least north ofthe Greater Himalaya, appear to be sensitive to monsoon strength because they lie on the periphery of the present rainfall maximum, in the Himalayan rain shadow. These steep river valleys may be affected by landslide damming during periods of increase moisture transport and strong monsoonal precipitation, where damming provides sediment storage through valley-filling and later sediment release through gradual incision or dam-bursting. The Zanskar River, a major tributary to the upper Indus River, provides a record of the erosional response of mountain river valleys to these extreme phases through river terracing. New OSL ages from alluvial terraces indicate reworking of sediment and valley-filling, with river incision by at least 32 ka, ~25 ka, ~8.2 ka, and 6.3 ka. Drier phases are suggested at ~6 ka and 3 ka from overlying loess-type eolian deposits. Initial sediment budget estimates indicate that sediment reworking may dominate the sediment supply during times of strong monsoon, with Himalayan river terraces contributing almost half of the total sediment fluxed to the ocean over the last 10 kyr. Understanding how climate controls the flux of sediment to the ocean is essential to our knowledge of source-to-sink transport processes and how continental erosion and weathering records are tied to marine deposits.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFMEP23B..04J
- Keywords:
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- 1625 GLOBAL CHANGE Geomorphology and weathering;
- 1105 GEOCHRONOLOGY Quaternary geochronology;
- 1824 HYDROLOGY Geomorphology: general