Assessing non‐steady‐state erosion processes using paired ¹⁰Be–²⁶Al in southeastern Tibet

Abstract

Quantifying erosion rates over various spatial and temporal scales across the Tibetan Plateau and its surrounding mountains is crucial to understanding the topographic evolution of the orogen. In this work, we report a new dataset of 10Be‐derived basin‐wide erosion rates from the main tributaries and streams draining the eastern Himalayan syntaxis. The 22 basin‐wide erosion rates ranged from 78 ± 7 m Myear−1 to 3,490 ± 612 m Myear−1 across the study area. 26Al was contemporarily measured to evaluate the impact of sediment storage and non‐steady‐state erosion processes in the syntaxis region. The paired study of 10Be and 26Al reveals that several samples violated the steady‐state erosion assumption and were compatible with the scenario of perturbation of reworked sediments or deeply sourced materials introduced by landslides. For most samples, deep‐sourced materials with higher 26Al/10Be ratios were no longer perturbing the 10Be signals in river sediments. It is possible that the deep‐sourced materials had been wiped out of the basins before the collection of samples in this work. However, the perturbation of reworked sediments was observed over a range of basin scales, limiting the use of a single sediment sample as a representative erosion product for upstream basins. Compared with tectonically stable regions, the incorporation of reworked fluvial sediments, deeply sourced materials or sub‐glacial eroded materials into sampled sediments led to the decoupling between basin‐wide erosion rates and topographic or climatic indices. Caution should be taken when deriving erosion rates from rapidly eroding regions with old, deeply buried sediments such as the eastern Himalayan syntaxis, where calculated erosion rates may be highly overestimated.

Cosmogenic (un-)steadiness revealed by paired-nuclide catchment-wide denudation rates in the formerly half-glaciated Vosges Mountains (NE France)

“…Although this singlenuclide approach relies on several key assumptions, it seems that the current 10 Be's routine approach used to infer CWD rates has somehow overshadowed two of them. First, little attention has been recently paid to cosmogenic dealt with glaciated catchments (Safran et al, 2005;Hippe et al, 2012;Grin et al, 2018;Wittmann et al, 2020;Zhang et al, 2021), but cosmogenic disequilibrium is highlighted not only in currently glaciated catchments of Tibet (Zhang et al, 2021) but also in formerly and marginally glaciated catchments of the Bolivian Andes (Hippe et al, 2012). However, Zhang et al (2021) could only conclude that sediment reworking is a potential issue without further exploration nor quantification on this process.…”

“…First, little attention has been recently paid to cosmogenic dealt with glaciated catchments (Safran et al, 2005;Hippe et al, 2012;Grin et al, 2018;Wittmann et al, 2020;Zhang et al, 2021), but cosmogenic disequilibrium is highlighted not only in currently glaciated catchments of Tibet (Zhang et al, 2021) but also in formerly and marginally glaciated catchments of the Bolivian Andes (Hippe et al, 2012). However, Zhang et al (2021) could only conclude that sediment reworking is a potential issue without further exploration nor quantification on this process. This altogether demonstrates our current lack of understanding in how cosmogenic signals measured in modern stream sediments might be perturbed in (formerly) glaciated catchments.…”