Plio-Pleistocene and modern erosion rates in the Himalaya from paired cosmogenic radionuclides

To understand sediment transport from the Himalayan source regions to the foreland basin over multi-millennial timescales, it is essential to quantify both hillslope erosion rates and sediment storage times within the mountain belt. In this study, we estimate paleo-erosion rates of the Himalaya using paired cosmogenic radionuclide (¹⁰Be and ²⁶Al) concentrations from 62 alluvial sediment samples collected from drill cores and riverbeds across four major Himalayan-River basins. By correcting the nuclide concentrations for sediment storage time, we show that neglecting storage-effects can lead to a systematic overestimation of erosion rates.

Paleo-erosion rates of Himalayan hillslopes derived from all four river basins show a consistent, near-linear increase through time from the late-Pliocene to the present. This trend is observed from the western Himalaya (modern riverbed samples from north of the Main Frontal Thrust and the paleo-Sutlej) through the central Himalaya (paleo-Yamuna and Ganges) to the central-eastern Himalaya (Kosi). In the Sutlej basin, erosion rates increase from ~0.07 mm/y at 2.95 Ma to 1.79 mm/y at present. The paleo-Yamuna records a rise from ~0.2 mm/y at 3.3 Ma to 1.9 mm/y at 0.6 Ma. Similarly, the Ganges basin shows an increase from ~0.24 mm/y at 3.7 Ma to 2.04 mm/y at 0.9 Ma, while the Kosi basin exhibits the strongest acceleration, from ~0.12 mm/y at 4.42 Ma to 4.37 mm/y in modern samples.

We attribute the temporally increasing trend of the paleo-erosion rates on glacial erosion. Expansion of glaciers in the high altitudes of the Himalayan region occurred after ~2.7 Ma, due to the growth of the northern hemispheric icesheet. Additionally, the increased seasonality of the south-east Asian monsoon and initiation of the Pleistocene glacial-interglacial cycle possibly have led to higher glaciation, which in turn resulted in higher erosion rate in the Himalaya after ~2 Ma.