HIMALAYAN RIVER TERRACES AS A LANDSCAPE RESPONSE TO QUATERNARY SUMMER MONSOON VARIABILITY

In order to interpret marine 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 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.

Paleoclimatological records indicate that summer monsoon intensity was strong from 29-37 ka, decreased until ~16 ka, reached maximum intensity from 8-10 ka, and weakened until ~3 ka. Some lake 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, especially those north of the Greater Himalaya in the rain shadow, appear sensitive to monsoon forcing and form river terraces during especially dry or wet phases. The Zanskar River, a major tributary to the Indus River, provides a record of the erosional response of mountain river valleys to these extreme phases. New OSL ages indicate valley-filling and river incision by at least 32 ka, ~25 ka, 8.2 ka, and 6.3 ka, with dry phases at 6.1 ka and 3.1 ka. Initial estimates indicate that sediment reworking may dominate the sediment supply to the Indus River during times of strong monsoon. 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.