¹⁰Be-derived Himalayan denudation rates and sediment budgets in the Ganga basin

Abstract

The Himalayas represent the archetype of mountain building due to active continental collision and are considered in many studies as the locus of intense interactions between climate, denudation and tectonics. Estimates of modern denudation rates across the entire range remain, however, relatively sparse. In this study, in situ-produced cosmogenic ¹⁰Be concentrations were measured in detritic quartz in order to determine basin-scale denudation rates for the central part of the Himalayan range. River sand was sampled over several years in the main trans-Himalayan rivers, from the Himalayan front to the Ganga outlet in Bangladesh. The calculated ¹⁰Be denudation rates of the trans-Himalayan river basins range from 0.5 to 2.4 mm yr−1 (average 1.3 mm yr⁻¹) and vary by up to a factor of 3 between sampling years. These denudation rates strongly contrast with the 0.007 mm yr⁻¹ denudation rate of southern tributary basins draining the Indian craton. This work also shows that in the Ganga basin, no systematic evolution of average ¹⁰Be concentrations is observed during floodplain transfer, implying that distal samples can be used to estimate the integrated denudation rate of the whole central Himalayan range. Samples from the Ganga in Bangladesh display remarkably low variability in ¹⁰Be concentration, implying an average Himalayan denudation rate of 1.0–1.1 mm yr⁻¹. However, within the floodplain, several samples suggest a recent perturbation of sediment transport dynamics with a recent increase in the relative sediment contribution from southern tributaries. The Himalayan sediment flux, deduced from the ¹⁰Be denudation rate of the range, is 610±230 Mt yr⁻¹. This flux is consistent, within uncertainty, with sediment fluxes derived from sediment gauging. The similarity of the two flux estimates suggests that Himalayan erosion fluxes have remained stable over the last centuries, even if the large uncertainties associated with each method hamper more precise assessments.

Schematic downstream evolution of ¹⁰Be concentration in the Gangetic plain. Samples from the Chambal River (BR937) and the Ganga at Varanasi (BR8135) have been omitted from the Gangetic floodplain because of their higher ¹⁰Be concentrations.

¹⁰Be denudation rates and sediment fluxes of the Himalayan range. Sample order is identical to Fig. 1. (Fc) : Floodplain corrected denudation rates, calculated using production rates from upstream sediment-contributing areas >300 m. (Sc) : Floodplain corrected denudation rates for the Himalayan range only, i.e. corrected for southern tributary contributions. (1) : Concentration data from Vance et al. (2003). Uncertainties include the 1σ analytical uncertainty and uncertainties in production rates and the scaling method (cf. text).