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

Abstract

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 Ca 2+ (76 %) and Mg 2+ (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 Ca 2+ for marine Na + yields a maximal additional Ca 2+ flux of 28 × 10 9 mol yr −1 of calcium to the ocean, which represents an increase of ca. 6 % of the actual river dissolved Ca 2+ flux. In the more likely event that only a fraction of the adsorbed riverine Ca 2+ is exchanged, not only for marine Na + but also Mg 2+ and K + , estuarine cation exchange for the GangaBrahmaputra is responsible for an additional Ca 2+ flux of 23 × 10 9 mol yr −1 , while ca. 27 × 10 9 mol yr −1 of Na + , 8 × 10 9 mol yr −1 of Mg 2+ and 4 × 10 9 mol yr −1 of K + are re-absorbed in the estuaries. This represents an additional riverine Ca 2+ 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 Mg 2+ 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.

Global silicate weathering flux overestimated because of sediment–water cation exchange

“…In addition, a number of small streams from Svalbard and the Alps were analyzed. The data are compared to the Amazon and the Ganges-Brahmaputra systems (10,11). This sample collection is from a very wide range of catchments, with diverse lithological, climatic, geomorphological, and weathering characteristics.…”

“…Exchangeable ions on the suspended particulate matter (SPM) were determined either by reacting the sediment with calcitesaturated cobalt(III) hexammine chloride (CoHex; SI Appendix) (11) or by using ammonium chloride (NH4Cl). NH4Cl is known to induce the dissolution of carbonate minerals if present (5), but, after filtering the data for exchange equilibrium (discussed below), the CEC and chemical compositions are within uncertainty for NH4Cl and CoHex extractions (SI Appendix, Figs.…”

“…The importance of the cation exchange pool is well recognized in soils and aquifers (4,5), has significant implications for enhanced weathering (6), and has been proposed as an important mechanism for buffering the composition of river waters (7)(8)(9). However, data on the riverine exchange pool are only available for two large river systems [Amazon and Ganges-Brahmaputra (10,11)], despite its significance in providing a source of elements that are immediately bioavailable (12), and their potential for biasing the quantification of silicate weathering (9).…”

River Sediment Geochemistry as a Conservative Mixture of Source Regions: Observations and Predictions From the Cairngorms, UK

“…The elemental composition of river sediments is determined by the chemistry of the eroding substrate (e.g., bedrock and soils) and modified by processes including chemical weathering, cation exchange, and hydrodynamic sorting (e.g., Bouchez et al., 2011, 2012; Lupker et al., 2016). Consequently, river sediment compositions are used to investigate controls on chemical and physical weathering rates (e.g., climate change, tectonics, and geomorphic processes; Gaillardet et al., 1999; Riebe et al., 2003; von Blanckenburg & Wittmann, 2012).…”

Unmixing river sediments for the elemental geochemistry of their source regions

“…An exception to the general rule of unbiased predictions (i.e., residuals distributed around zero) appears to be calcium, which our inversion scheme tends to over-predict relative to G-BASE upstream (Figure 10a-b; Figure S-5). One possible explanation for this could be the adsorption of dissolved calcium cations to the surface of clays in sediments, a process that is observed in rivers globally (Sayles and Mangelsdorf 1979;Lupker et al 2016;Tipper et al 2021).…”