HIMALAYAN RIVER TERRACES AS EROSIONAL ARCHIVES

In order to interpret marine sedimentary archives, such as deltas and submarine fans, as records of evolving continental environmental conditions we must first understand how sediment is transported from the headwater catchments to the ocean. Recognizing what processes control sediment storage and release is essential for high-resolution paleo-environmental reconstructions because long transport times and large-scale reworking can complicate the link between erosional processes and the sediment deposited at any given time. Quantifying the lag times is essential if we are to compare paleoclimate and environmental records. River terraces, such as in the Greater Himalaya, provide a pivotal role in this source-to-sink problem, because this is where sediment storage occurs and is likely modulated by periods of strong monsoon. High river valleys north of the Greater Himalaya appear to be sensitive to monsoon strength because they lie on the periphery of the Himalayan rain shadow. During periods of strong monsoon, enhanced precipitation triggers landslide-damming of the narrow river valleys and generally increases sediment supply resulting in sediment back-filling. New OSL ages now help constrain the timing of landslide and valley-filling events, as well the timing of sediment release to the trunk Indus because of terrace incision and/or dam bursting. A sequence of river terraces of a major tributary to the Indus, the Zanskar River, indicates valley-filling at least at ~47 ka and possibly more recently. These data suggest that formation of landslide-dammed lakes and terraces on the Zanskar occurred during an abrupt strengthening of the summer monsoon in the Late Pleistocene and possibly the Early Holocene. Initial estimates indicate that such reworking may dominate the sediment supply to the Indus River during times of strong monsoon. Understanding how climate controls flux of sediment to the ocean is essential to our knowledge of source-to-sink transport processes and the interpretation of continental erosion and weathering records based on marine deposits.