Abstract
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.
Highlights
The large-scale and intense interaction between tectonics and climate make the Himalayan range the largest sediment source to the ocean
This suggests that the occurrence of significant 9Be concentrations in quartz from the Brahmaputra basins is restricted to a few basins on the southern edge of the Tibetan Plateau
A possible explanation for this downstream lag in response to high denudation is the role of abrasion processes of predominantly coarse material that is delivered by landslides within the Namche Barwa–Gyala Peri massif (NBGPm)
Summary
The large-scale and intense interaction between tectonics and climate make the Himalayan range the largest sediment source to the ocean. The Tsangpo-Brahmaputra is a major catchment draining the Himalayan range and the Tibetan Plateau. The extent and nature of this coupling has recently been challenged (Bendick and Ehlers, 2014; Wang et al, 2014; King et al, 2016), there is ample evidence for superimposed rapid exhumation (Burg et al, 1998; Seward and Burg, 2008; Zeitler et al, 2014; Bracciali et al, 2016) and active erosion (Finlayson et al, 2002; Finnegan et al, 2008; Stewart et al, 2008; Enkelmann et al, 2011; Larsen and Montgomery, 2012; Lang et al, 2013) in a focused area around the Namche Barwa–Gyala Peri massif (NBGPm), where the course of the Tsangpo-Brahmaputra is marked by a sharp bend to the south-west (Fig. 1). The wide range of surface processes and rates of landscape evolution across the catchment represent a unique opportunity to study how these denudation signals are transferred through the catchment and integrated into the sedimentary load. The use of TCN represents an opportunity to provide new quantitative constraints on the denudation rates across the catchment in general and the NBGPm in particular
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