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Dissolved major ions and Sr concentrations of the Brahmaputra River at Guwahati, India, have been investigated on weekly basis for one year to understand the seasonality in weathering pattern and relative contributions from possible solute sources. Comparison of major ion data sets from present and earlier studies for this location shows no appreciable change during last ~50 years. Elemental concentrations and Ca/Na* (Na* = Na − Cl) ratio of the Brahmaputra covary (inversely) with the water discharge; the degree of seasonality, however, is less pronounced compared to other Himalayan (e.g., Ganga and Salween) rivers. The monthly averaged Ca/Si ratios of the Brahmaputra (3.7 ± 0.2), which is lower by ~2 times than those reported earlier for the Ganga outflow (6 ± 1), show minimal (~6%) seasonal changes. Seasonal variability in Na*/K with higher ratios during non‐monsoon period has been attributed to proportionally higher Na supply from hot springs and/or alkaline salts. The silicate‐derived cations (Cats) and Sr (Srs) have been estimated using an inversion method. Although these estimated values broadly show seasonal changes, the average Cats and Srs values for the monsoon (26 ± 4% [Cats]; 26 ± 6% [Srs]) and non‐monsoon (27 ± 3% (Cats); 24 ± 4% (Srs)) seasons are statistically same. These estimates indicate a weak runoff‐weathering linkage for the Brahmaputra river. Outcomes from this study suggest that the chemical weathering intensity of this basin is more dominated by regional rapid weathering around the eastern syntaxis than the climatic (runoff) parameters.

We present here the most comprehensive study of the thickness and composition (Vp/Vs ratio) of the South India Precambrian crust and the nature of shallower mantle inferred from analysis of teleseismic receiver functions from 70 broad-band seismic stations operated as a part of the India Deep Earth Imaging Experiment (INDEX). South India could be broadly divided into regions with thin crust (32-38 km) and thick crust (38-54 km). Thin crust domains include the East Dharwar Craton (EDC), Cuddapah basin and Madurai/Kerala Khondalite Block. The thicker crust domain includes the Western Dharwar Craton (WDC) and northern part of Southern Granulite Terrain. The WDC shows progressive increase in thickness from 38 km in north to 46-54 km in south, compared to an almost flat Moho beneath the EDC. Compositionally, most of the crustal domains are felsic to intermediate (Vp/Vs ∼ 1.69-1.75) except the mid Archean block in the southern WDC where it is mafic (Vp/Vs > 1.81). Considering erosion depth in the WDC, we argue for Himalaya like ∼70 km thick crust beneath it during the Archean. Variation in crustal thickness does not have a first-order influence on regional topography in South India and suggests significant role for the crustal composition. We also present evidence of mid-lithospheric low velocity at ∼85-100 km beneath South India.

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