Abstract:
The Glaciations impacted erosion during the Late Cenozoic but no consensus has emerged whether they led to increased erosion rates globally. In the Himalayas, recent work used past sediment concentrations of the terrestrial cosmogenic nuclide (TCN) 10Be and demonstrated that erosion rates have not permanently increased in the Himalayas. However, for the Quaternary, the published sedimentary records suffer from provenance uncertainties which prevent to elaborate on the causes of steady erosion rates. Here, we document the new, 4,000-m thick Valmiki Section (VS) to address this question. In the remote Valmiki Tiger Reserve, the VS consists of Siwalik sediment deposited in the Himalayan foreland basin by the Narayani River, a major river of Central Himalayas. To quantify past Himalayan erosion rates from TCN 10Be measurements, we determine: (1) the magnetostratigraphic deposition age model, (2) provenance using major elements and Sr-Nd isotopes, and (3) the recent cosmic exposure related to Siwalik exhumation using TCN 36Cl measurement in feldspar. The VS records Himalayan erosion from 7.5 to 1.25 Ma. Our 10Be results confirm steady erosion rates, close to modern values, 1.4-2.3 mm/y, with a brief increase by 35% at 2 Ma, possibly due to sustained glacial erosion of the high peaks as suggested by the geochemical signature. The Narayani Catchment may be more sensitive to the onset of the Glaciations because of larger glacial cover (presently ~10%) than elsewhere in the Himalayas. Despite this sensitivity, our results support that over long timescales, rather than climate, tectonics control Himalayan erosion.