Spatiotemporal characteristics of erosion processes along a steep climatic gradient in the NW Himalaya: Insights from satellite-rainfall data, mass-transport measurements, and surface-exposure dating

The intensity of the Indian summer-monsoon circulation varies at decadal to millennial time scales and affects the terrestrial and marine sediment records. The impact of climatic forcing on erosional and sedimentary processes and their manifestation in the landscape varies, depending on magnitude and size of the climatic events, as well as on the tectonic and geomorphic preconditioning of the landscape. Significant mass-transport processes during the large Pakistan flood 2010 and previous abnormal years have emphasized the importance of extreme events on shaping landscapes. Here, we analyze gauge and satellite-derived rainfall data combined with new cosmogenic nuclide surface-exposure ages and basin-wide erosion rates to elucidate the impact of climatic forcing on Earth-surface processes at different time scales. The NW Himalaya is located at the western end of the monsoonal conveyer belt that transports moisture from the Bay of Bengal to the Sutlej Valley and farther west. Orographic rainfall processes at the southern Himalayan mountain front create a steep south-to-north rainfall gradient. The tail and northward end of the rainfall distribution decays rapidly with only little or no moisture reaching the southern Tibetan Plateau. However, during recent, abnormally strong monsoon years rainfall did reach the presently arid to semi-arid regions of the northern Himalaya and southern Tibetan Plateau. Gauge and satellite-derived rainfall data show that erosion in these environments does not follow the mean annual rainfall distribution. Instead, we document the northward penetration of rainfall into the orogen and highlight the importance of extreme-event rainfall (>90th percentile) on sediment-transport processes by analyzing basin-wide erosion rates tied into a specific stream-power model. Similarly, but on longer time scales (> 100 yr), rainfall propagated farther into the orogen during a generally stronger monsoon circulation in the early Holocene. During these times, the presently arid, high-elevation areas were impacted by flooding and large-volume mass-movement processes, ultimately leading to a significant increase in sediment-flux rates. Some of the sediments were derived from glacial environments, and we provide new ages spanning the past 100ka to constrain climatic oscillations and associated sediment dynamics such as the formation and destruction of fluvial terrace systems. In addition, we measured bedrock-incision rates for different time intervals in the past, resulting in values between 2 and 10 mm/yr. The limiting factor for sediment removal and bedrock incision on millennial timescales are large bedrock landslides that impounded the river network and formed transient intermontane basins lasting for several millennia. 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.