Articles | Volume 5, issue 3
Earth Surf. Dynam., 5, 429–449, 2017
Earth Surf. Dynam., 5, 429–449, 2017

Research article 10 Aug 2017

Research article | 10 Aug 2017

10Be systematics in the Tsangpo-Brahmaputra catchment: the cosmogenic nuclide legacy of the eastern Himalayan syntaxis

Maarten Lupker1,6, Jérôme Lavé2, Christian France-Lanord2, Marcus Christl3, Didier Bourlès4, Julien Carcaillet5, Colin Maden6, Rainer Wieler6, Mustafizur Rahman7, Devojit Bezbaruah8, and Liu Xiaohan9 Maarten Lupker et al.
  • 1Geological Institute, D-ERDW, ETH Zürich, Zürich, 8092, Switzerland
  • 2CRPG, UMR 7358 CNRS–Univ. de Lorraine, Vandoeuvre les Nancy, 54500, France
  • 3Institute of Particle Physics, D-PHYS, ETH Zürich, Zürich, 8093, Switzerland
  • 4CEREGE, UMR 34 UAM-CNRS-IRD, Aix-en-Provence, 13545, France
  • 5ISTerre, Univ. Grenoble Alpes–CNRS, Grenoble, 38000, France
  • 6Institute of Geochemistry and Petrology, D-ERDW, ETH Zürich, Zürich, 8092, Switzerland
  • 7Department of Soil, Water and Environment, Dhaka University, Dhaka, 1000, Bangladesh
  • 8Department of Applied Geology, Dibrugarh University, Dibrugarh, 786004, India
  • 9Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing, China

Abstract. The Tsangpo-Brahmaputra River drains the eastern part of the Himalayan range and flows from the Tibetan Plateau through the eastern Himalayan syntaxis downstream to the Indo-Gangetic floodplain and the Bay of Bengal. As such, it is a unique natural laboratory to study how denudation and sediment production processes are transferred to river detrital signals. In this study, we present a new 10Be data set to constrain denudation rates across the catchment and to quantify the impact of rapid erosion within the syntaxis region on cosmogenic nuclide budgets and signals. The measured 10Be denudation rates span around 2 orders of magnitude across individual catchments (ranging from 0.03 to > 4 mm yr−1) and sharply increase as the Tsangpo-Brahmaputra flows across the eastern Himalaya. The increase in denudation rates, however, occurs  ∼  150 km downstream of the Namche Barwa–Gyala Peri massif (NBGPm), an area which has been previously characterized by extremely high erosion and exhumation rates. We suggest that this downstream lag is mainly due to the physical abrasion of coarse-grained, low 10Be concentration, landslide material produced within the syntaxis that dilutes the upstream high-concentration 10Be flux from the Tibetan Plateau only after abrasion has transferred sediment to the studied sand fraction. A simple abrasion model produces typical lag distances of 50 to 150 km compatible with our observations. Abrasion effects reduce the spatial resolution over which denudation can be constrained in the eastern Himalayan syntaxis. In addition, we also highlight that denudation rate estimates are dependent on the sediment connectivity, storage, and quartz content of the upstream Tibetan Plateau part of the catchment, which tends to lead to an overestimation of downstream denudation rates. While no direct 10Be denudation measurements were made in the syntaxis, the dilution of the upstream 10Be signal, measured in Tsangpo-Brahmaputra sediments, provides constraints on the denudation rates in that region. These denudation estimates range from ca. 2 to 5 mm yr−1 for the entire syntaxis and ca. 4 to 28 mm yr−1 for the NBGPm, which is significantly higher than other large catchments. Overall, 10Be concentrations measured at the outlet of the Tsangpo-Brahmaputra in Bangladesh suggest a sediment flux between 780 and 1430 Mt yr−1 equivalent to a denudation rate between 0.7 and 1.2 mm yr−1 for the entire catchment.

Short summary
We use geochemical approaches (10Be) on river sediments to quantify the erosion rates across the Tsangpo-Brahmaputra (TB) catchment in the eastern Himalayas. Our approach confirms the high erosion rates in the eastern Himalayan syntaxis region and we suggest that the abrasion of landslide material in the syntaxis is a key process in explaining how erosion signals are transferred to the sediment load.