10Be systematics in the Tsangpo-Brahmaputra catchment: the cosmogenic nuclide legacy of the eastern Himalayan syntaxis
Earth Surface Dynamics Discussions
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 <sup>10</sup>Be data set to constrain denudation rates across the catchment and to quantify the impact of rapid erosion within
... apid erosion within the syntaxis region on cosmogenic nuclide budgets and signals. <sup>10</sup>Be denudation rates span around two orders of magnitude across the catchments (ranging from 0.03&thinsp;mm/yr to >&thinsp;4&thinsp;mm/yr) and sharply increase as the Tsangpo-Brahmaputra flows across the eastern Himalaya. The increase in denudation rates however occurs ~&thinsp;150&thinsp;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 <sup>10</sup>Be concentration, landslide material produced within the syntaxis that dilutes the upstream high concentration <sup>10</sup>Be 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&thinsp;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&thinsp;mm/yr for the entire syntaxis and ca. 4 to 28&thinsp;mm/yr for the NBGPm, which is significantly higher than other to other large catchments. Overall, <sup>10</sup>Be concentrations measured at the outlet of the Tsangpo-Brahmaputra in Bangladesh suggest a sediment flux between 780 and 1430&thinsp;Mt/yr equivalent to a denudation rate between 0.7 and 1.2&thinsp;mm/yr for the entire catchment.