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

Abstract

Abstract. The Tsangpo-Brahmaputra River drains the eastern part of the Himalayan range, flowing from the Tibetan Plateau through the eastern Himalayan syntaxis and downstream to the Indo-Gangetic floodplain. 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. 10Be denudation rates span around two orders of magnitude across the catchments (ranging from 0.03 mm/yr to > 4 mm/yr) 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 denudations rates. Taking these effects into account we estimate a denudation rates of ca. 2 to 5 mm/yr for the entire syntaxis and ca. 4 to 28 mm/yr for the NBGPm, which is significantly higher than other to 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 equivalent to a denudation rate between 0.7 and 1.2 mm/yr for the entire catchment.

Cosmogenic nuclide weathering biases: corrections and potential for denudation and weathering rate measurements

“…Other studies have discussed a grain size reduction during sediment transport and weathering-facilitated sediment breakdown in the regolith (e.g. Carretier et al, 2009;Carretier and Regard, 2011;Lukens et al, 2016;Sklar et al, 2017;Lupker et al, 2017). The case presented here is more similar to the latter phenomenon, and in our model, the magnitude of this grain size bias increases with the weathering intensity.…”

Effects of deep‐seated versus shallow hillslope processes on cosmogenic 10Be concentrations in fluvial sand and gravel

“…In another study from the Tsangpo‐Brahmaputra catchment, Lupker et al . () modelled abrasion to explain observed dilution in fluvial [ 10 Be]. They predicted that the effects of abrasion become apparent after 50 to 150 km, which is longer than the majority of our catchments.…”

Outburst floods strongly influence valley evolution in the Tsangpo Gorge, Eastern Himalaya

“…Here we document the geomorphic effects of an outburst ood in the Namche Barwa-Gyala Peri (NBGP) massif of the Eastern Himalayan Syntaxis, which is among the most actively uplifting parts of the mountain belt and has experienced rapid exhumation (> 5 mm/y [11][12][13][14][15][16] ) through the late Cenozoic. The Yarlung River cuts through the Syntaxis between two peaks with elevations of > 7 km, forming a narrow bedrock gorge ~ 5 km deep, among the deepest on Earth 11,17 (Fig.…”