Late Quaternary Climate Forcing of Rapid Sedimentation and Erosion Processes in the NW Himalaya (Invited)

The intensity of the Asian summer-monsoon circulation varies over decadal to millennial time scales and is reflected in changes in surface processes, terrestrial environments, and marine sediment records. The impact of climatic forcing on the landscape’s response and associated erosion processes varies, depending on magnitude and size of the climatic events, as well as on the tectonic and geomorphologic preconditioning of the landscape. Here, we present new surface-exposure ages combined with previously published data to quantify erosion and process rates and their changes through the time. The NW Himalaya is located at the end of the monsoonal conveyer belt that transports moisture from the Bay of Bengal to the Sutlej Valley and farther west. Along the Himalaya, orographic barriers force out rainfall with peak amounts located consistently at a 3-km-radius relief of ~1 km. The tail and northward end of the rainfall distribution decays rapidly with only little or no moisture reaching the southern Tibetan Plateau. However, during a generally stronger monsoon circulation in the early Holocene called the Intensified Monsoon Phase rainfall reached the today arid regions of the northern Himalaya and southern Tibetan Plateau as documented in numerous terrestrial sediment archives. During this time period in the NW Himalaya, the presently arid, high-elevation areas have been impacted by flooding and heavy landsliding ultimately leading to a significant increase in sediment-flux rates. Some of the sediment material has been derived from glaciers. We show that the transiently-stored sediments in these valleys have been rapidly removed during the early Holocene at the onset of the Intensified Monsoon Phase. The limiting factor for sediment removal and transport on millennial timescales are large bedrock landslides that impounded the river network and formed intramontane basins lasting for several thousands of years. We suggest a feedback process between sediment removal and landsliding, where large landsliding predominantly occurs when the transiently-stored valley fills have been eroded, leading to bedrock erosion, lateral scouring of rivers, and oversteepening of hillslopes.