Impact of sediment–seawater cation exchange on Himalayan chemical weathering fluxes

Abstract. Continental-scale chemical weathering budgets are commonly assessed based on the flux of dissolved elements carried by large rivers to the oceans. However, the interaction between sediments and seawater in estuaries can lead to additional cation exchange fluxes that have been very poorly constrained so far. We constrained the magnitude of cation exchange fluxes from the Ganga–Brahmaputra river system based on cation exchange capacity (CEC) measurements of riverine sediments. CEC values of sediments are variable throughout the river water column as a result of hydrological sorting of minerals with depth that control grain sizes and surface area. The average CEC of the integrated sediment load of the Ganga–Brahmaputra is estimated ca. 6.5 meq 100 g⁻¹. The cationic charge of sediments in the river is dominated by bivalent ions Ca²⁺ (76 %) and Mg²⁺ (16 %) followed by monovalent K⁺ (6 %) and Na⁺ (2 %), and the relative proportion of these ions is constant among all samples and both rivers. Assuming a total exchange of exchangeable Ca²⁺ for marine Na⁺ yields a maximal additional Ca²⁺ flux of 28  ×  10⁹ mol yr⁻¹ of calcium to the ocean, which represents an increase of ca. 6 % of the actual river dissolved Ca²⁺ flux. In the more likely event that only a fraction of the adsorbed riverine Ca²⁺ is exchanged, not only for marine Na⁺ but also Mg²⁺ and K⁺, estuarine cation exchange for the Ganga–Brahmaputra is responsible for an additional Ca²⁺ flux of 23  ×  10⁹ mol yr⁻¹, while ca. 27  ×  10⁹ mol yr⁻¹ of Na⁺, 8  ×  10⁹ mol yr⁻¹ of Mg²⁺ and 4  ×  10⁹ mol yr⁻¹ of K⁺ are re-absorbed in the estuaries. This represents an additional riverine Ca²⁺ flux to the ocean of 5 % compared to the measured dissolved flux. About 15 % of the dissolved Na⁺ flux, 8 % of the dissolved K⁺ flux and 4 % of the Mg²⁺ are reabsorbed by the sediments in the estuaries. The impact of estuarine sediment–seawater cation exchange appears to be limited when evaluated in the context of the long-term carbon cycle and its main effect is the sequestration of a significant fraction of the riverine Na flux to the oceans. The limited exchange fluxes of the Ganga–Brahmaputra relate to the lower than average CEC of its sediment load that do not counterbalance the high sediment flux to the oceans. This can be attributed to the nature of Himalayan river sediment such as low proportion of clays and organic matter.