The Himalayas were formed by the collision of the northward-moving Indian continent with the Eurasian plate, an event initiated around 60 million year ago. However, the past 10 Million years to the present is an important period in Earth history because of the uplift of the Himalayas Mountain, forming the more than 8000 m high mountain range in South Asia. The uplift of the Himalaya is thought to have caused changes in world climate. A global cooling of world climate is the best example of the effects produced, due to absorption of carbon dioxide by chemical weathering induced by uplift leading to seasonal rainfall (Indian summer Monsoon).
So when did the Indian summer Monsoon begin? This question has been addressed by many researchers, but it remains a matter of debate. Several methods have been used to reconstruct past climate, including carbon and oxygen isotopes analysis, plant and pollen fossils, and the characteristics of Siwalik Group sediment accumulation in the front of the Himalayas. Carbon isotope analyses show that major climatic changes occurred 7-8 million years ago (Quade et al., 1995). However, other studies suggest that climatic changes occurred about 10 million years ago (Tanaka, 1997), and some oxygen isotopic studies have concluded that the Indian monsoon began at around 10.7 Million years ago. Recently, the age of Indian monsoon inititation has been put back to around 15 Million year ago, predating the formation of the Himalaya (e.g. Clift, 2008). Previous studies have not discussed the possible causes of the age gap of estimates of the timing of Summer Monsoon onset. The effect of local climate changes by changing local topography is a possible cause. Study of sediment records whose sedimentation patterns were not affected by local climate and topography are thus needed to interpret past climate.
The Siwalik Group lying in the southern flank of the Himalaya is expected to constitute a good record of the regional changes in climate and tectonics. My target is the 6 km thick Karnali River section, where the large Paleo-Karnali River is thought to have flowed.