Tracking denudation and sediment production and transport with cosmogenic ¹⁰Be in arid, high‐altitude Himalayan half‐grabens, Zanskar, northern India

Abstract

Understanding the extent to which local factors, including bedrock and structure, govern catchment denudation in mountainous environments as opposed to broader climate or tectonic patterns provides insight into how landscapes evolve as sediment is generated and transported through them, and whether they have approached steady-state equilibrium. We measured beryllium-10 (10 Be) concentrations in 21 sediment samples from glaciated footwall and hanging wall catchments, including a set of nested catchments, and 12 bedrock samples in the Puga and Tso Morari half-grabens located in the high-elevation, arid Zanskar region of northern India. In the Puga half-graben where catchments are underlain by quartzo-feldspathic gneissic bedrock, bedrock along catchment divides is eroding very slowly, about 5m/Ma, due to extreme aridity and 10 Be concentrations in catchment sediments are the highest (~60-90×10 5 atoms/g SiO 2) as colluvium accumulates on hillslopes, decoupled from their ephemeral streams. At Puga, 10 Be concentrations and the average erosion rates of a set of six nested catchments demonstrate that catchment denudation is transport-limited as sediment stagnates on lower slopes before reaching the catchment outlet. In the Tso Morari half-graben, gneissic bedrock is also eroding very slowly but 10 Be concentrations in sediments in catchments underlain by low grade meta-sedimentary rocks, are significantly lower (~10-35×10 5 atoms/g SiO 2). In these arid, high-elevation environments, 10 Be concentrations in catchment sediments have more to do with bedrock weathering and transport times than steady-state denudation rates.