INVITED: OROGRAPHIC FORCING, HOLOCENE MONSOON DYNAMICS, AND FLUVIAL TERRACE FORMATION IN THE NW HIMALAYA

The interplay between topography and Indian summer monsoon circulation profoundly controls precipitation distribution, sediment transport, and river discharge along the Southern Himalayan Mountain Front (SHF). The Higher Himalayas form a major orographic barrier that separates humid sectors to the south and arid regions to the north. During the Indian summer monsoon, vortices transport moisture from the Bay of Bengal to the northwest along the SHF, and cause heavy precipitation when colliding with the mountain front. In the eastern and central parts of the Himalaya, newly processed precipitation measurements derived from high-resolution (~5x5 km2) passive microwave analysis (TRMM) show a strong gradient, with high values at medium elevations and extensive penetration of moisture along major river valleys into the orogen. The end of the monsoonal conveyer belt is near the Sutlej Valley in the NW Himalaya, where precipitation is lower and rainfall maxima move to lower elevations. This region thus comprises a climatic transition zone that is very sensitive to changes in Indian summer monsoon strength. Here, we characterize variations in present-day and Holocene monsoonal precipitation distribution. Aluminium-26 and Beryllium-10 surface exposure dating on cut-and-fill river-terrace surfaces from the lower Sutlej Valley documents the close link between Indian Summer Monsoon (ISM) oscillations and fluvial incision. During the early Holocene ISM optimum, precipitation was enhanced and moisture penetrated farther into the orogen, thereby creating amplified sediment flux. This choked locally the downstream valleys with debris of up to 120 m above present grade. Contrary to common concepts linking fluvial incision of terrace systems to increased precipitation and runoff, we demonstrate that less moisture during centennial-long weak phases centered at 9.7, 8.2, 6.3, 3.1, and 2.3 kyr BP of the ISM system resulted in lower sediment supply, allowing (sediment-) underloaded rivers to incise episodically.