Variable Weathering Responses to Li Isotopic Fractionation in Mountainous and Associated Mega Fan Region of the Teesta River Basin, Eastern Himalaya, India: An Insight Into the Impact of Active Tectonics on Weathering Processes

Silicate weathering in the terrestrial environment exerts a dominant control over the atmospheric CO 2 budget over million year timescale. Since last few decades, the possible location of this silicate weathering, that is, whether the mountainous terrain or the floodplain region, remains a topic of debate. The present study delineates the chemical weathering distribution in both the mountain and the floodplain regions through the δ 7 Li compositions of the bed loads in the Teesta river system. The average Li concentrations and its isotopic compositions (δ 7 Li) in pure silicate fractions of the Teesta sediments (15.3 ± 7.2 ppm and −2.0‰ ± 9.6‰, respectively) are depleted than those of the Upper Continental Crust (UCC) (35 ± 11 ppm and 0‰ ± 2‰, respectively). The obtained δ 7 Li compositions represent some of the lightest globally known Li isotopic values for the fluvial sediments, indicating a high magnitude of chemical weathering in the basin. In entire course of the river the δ 7 Li compositions show two distinctive trends where the δ 7 Li values gradually decrease from source to the north of Main Central Thrust (MCT) before, it increases continuously for rest of the river course. The tectonically active Rangeet window lying with the central part of the basin seems to influence the chemical weathering intensity and δ 7 Li compositional variation in a distinct manner. The northern end of the duplex that coincides with MCT, acts as an erosional hotspot owing to higher exhumation and vertical uplift rate with focused precipitation over the region. This rapid fresh rock exposure and associated physical erosion prevent the water‐sediment interaction resulting in a δ 7 Li composition similar to that of the unweathered silicates. On the immediate north of the MCT, although the exhumation rate and rainfall amount is minimal, it has resulted in the erosion rate sufficient for a higher degree of chemical weathering as inferred from the lightest δ 7 Li values. However, in source regions, relatively low temperature and rainfall conditions restrict the chemical weathering. On the south of the MCT, despite the conditions are favourable for chemical weathering, a significant amount of sediments generated along the duplex results in the sediment accumulation at mountainous terrain as well as floodplain with extensive residence time and development of alluvial fan (mega fan). As a consequence of this prolonged sediments‐water interaction within the fan, three distinct processes, that is, (i) the 7 Li exchanges from water to sediment, (ii) clay formation and (iii) dominancy of resistant mineralogy like the quartz, cause an enrichment of the δ 7 Li compositions in particulate phase, either by working separately or unitedly. This highlights the role of tectonics in erosion and weathering distribution in the tectonically active mountain chain like Himalaya.