Size distribution of alkali elements in riverbed sediment and its relevance to fractionation of alkali elements during chemical weathering

- The degree of chemical weathering increased with decreasing particle size of riverbed sediment.
- K and Rb are released from primary minerals during chemical weathering, whereas Cs accumulates in the weathering products.
- Rb/K and Cs/K ratios can be used to evaluate the degree of chemical weathering for felsic rocks.
- The transportation of particulates and dissolved fractions results in a further fractionation between the alkali elements.

In this study, we analyzed major elements, Rb and Cs in riverbed sediments after size separation to investigate the release and retention of alkali elements (K, Rb, and Cs) during chemical weathering. The degree of chemical weathering (measured using the chemical index of alteration) increased with decreasing particle size of the sediment, which is consistent with the mineralogy of the size-fractioned sediments. The analytical results from the size-fractioned sediments show that K and Rb are released from primary minerals during chemical weathering, whereas Cs is retained and accumulates in the weathering products (i.e., fine particle fractions), possibly because of a strong fixation by clay minerals. The analytical results show that the durability toward chemical weathering increases in the order of K, Rb, and Cs. As a result, the Rb/K and Cs/K ratios in the river water samples were low relative to the sediments, reflecting ongoing chemical weathering in the catchment basin. The Rb/K and Cs/K ratios of the sediments and soils would increase with progressive chemical weathering. Thus, the degree of fractionation between K, Rb, and Cs can be used to evaluate the degree of chemical weathering for granitic rocks. The behavior of stable Cs and radiocesium isotopes, both of which were distributed more in the fine particles of sediments, was coherent. However, isotope equilibrium was not reached, possibly because of the presence of stable Cs at the inner (nonreactive) sites of the minerals, depending on the particle size. Thus, the Cs isotope ratio of 137Cs/133Cs decreased with increasing particle size. In conclusion, the strong affinity of Cs for the weathering products of clay minerals causes a large fractionation of alkali elements during chemical weathering, and the transportation of particulates and dissolved fractions results in a further fractionation between the alkali elements.