Seasonal, weathering and water use controls of silicon cycling along the river flow in two contrasting basins of South India

We present the first study of river water silicon isotopic composition from two contrasting basins in South India, the east flowing Kaveri river and the west flowing Netravathi river. Both rivers originate from the Western Ghats. River water samples were collected from mainstream, tributaries and reservoirs at different locations along the river flow during dry and wet (monsoon) seasons, with an additional post-monsoon sampling in Netravathi. High rainfall in Netravathi basin and upper reaches of Kaveri induce intense weathering in the region, with superposing contribution from anthropogenic controls in downstream Kaveri. The δ 30 Si DSi values range from +0.42 to +1.65‰ for Netravathi river basin and + 0.32 to +2.85‰ for Kaveri river basin. Silicate weathering index (R e ) shows intense weathering associated with monosiallitization (kaolinite-gibbsite formation) in Netravathi basin and relatively moderate weathering with bisiallitization (smectite-kaolinite formation) in Kaveri basin. The seasonal changes in δ 30 Si DSi and R e in each basin shows similar patterns, with a likely higher and heavier contribution of soil water from deeper soil profiles closer to weathering front and bedrock, associated with significant secondary mineral formation during dry season and leaching of superficial soil profiles during the high discharge periods of monsoon. We provide a theoretical framework to estimate relative contribution of silicate weathering vs. anthropogenic processes on riverine δ 30 Si DSi. R e is broadly correlated with δ 30 Si DSi and an isotopic mass balance involving the whole rock composition (excluding the stable quartz and sericite) shows that the δ 30 Si in river water are well explained by silicate weathering and gibbsite-kaolinite formation in the Netravathi and upper upper reaches of Kaveri. However, silicate weathering explains only partially the heavier δ 30 Si DSi signatures in the middle and lower reaches of the Kaveri, and the additional enrichment of about +1.06‰ can be attributed to uptake of silicon and Si-depleted return flow through irrigated agriculture in the basin. This study confirms the major control of pedoclimatic conditions on the δ 30 Si of rivers and provides for the first time an estimation of the impact of human activities on the silicon isotopic signature of rivers.