Chronology of Himalayan valley fills: a key to assessing the fluvial geomorphic response to climate change

Fluvial landforms reflect a balance between tectonics, climate, and their interaction through erosion and sediment deposition. The occurrence of thick valley fills straddling the major Himalayan rivers testify an imbalance between sediment supply and river transport capacity. Whether the aggradation is related to enhanced sediment supply or reduced stream capacity is a matter of debate. The changes in runoff can qualitatively be determined from the paleoclimatic records but the changes in hillslope sediment supply are more difficult to measure and often remain speculative. In-situ produced cosmogenic nuclide inventories in fluvial sediments provide an estimate of catchment-averaged erosion rates. When applied to chronologically-constrained valley fill sediments, this method has the potential to provide paleo-erosion rate and, by implication, sediment discharge from the catchment hillslopes. The paleoerosion rate data in conjunction with the chronology of valley aggradation and paleoclimatic proxy records would allow assessing the impact of monsoon rainfall change on both the hillslope erosion rates and transport capacity of streams. We have applied this approach to the ~90-m thick Beas River valley fills that are exposed near the town of Kullu in Himachal Pradesh, northwestern India. Here, we present preliminary sediment depositional ages determined using the OSL and IRSL methods. Our new luminescence ages suggest that the aggradation of exposed deposits occurred between ̴ 155± 36.99 ka and 58.61± 12.98 ka. These ages when compared with other Himalayan River valleys, indicate a much older and prolonged phase of aggradation. We speculate that the observed discrepancy in depositional ages indicates that either the deposition began significantly earlier in the Beas river valley pointing towards the diachronous valley filling within the Himalaya or the river has incised at a comparably faster rate, resulting in the excavation of older valley fill deposits. We also observed a potential linkage between the terrace formation and monsoon variability where the existing aggradation phase correlates well with the higher rainfall trend when compared with the existing paleoclimate records. The results from our study are in well agreement with already existing depositional age models from other river valleys of Himalayas.