Rubidium isotope fractionation and its controlling mechanism during river transport
Description
Rubidium (Rb) is an alkali metal that is preferentially released during silicate weathering and exhibits strong affinity for secondary clay minerals, giving the Rb isotope system potential as a tracer for continental weathering. Rivers are a crucial channel for transporting continental Rb to the ocean; however, the Rb isotope composition of rivers and the mechanisms of Rb isotope fractionation during river transport remain unclear. This study explores the sources of Rb and the behavior of Rb isotopes during river transport by analyzing hydrochemical parameters, Rb concentrations, and Rb isotopic compositions in river samples collected from the Dongjiang River and the Pearl River Estuary. The results show that dissolved Rb in the Dongjiang River is overwhelmingly derived from silicate weathering (>96%), establishing an initial isotopic baseline of approximately −0.22‰ to −0.26‰ consistent with unweathered regional granite. Along the mainstream, dissolved δ⁸⁷Rb exhibits a non-monotonic downstream pattern, shifting progressively toward heavier values upstream and then reversing to lighter values downstream. XRD analysis reveals that the kaolinite/illite (K/I) mass ratio in riverbed sediments follows a spatially corresponding inverted-V pattern, and shows a significant positive correlation with dissolved δ⁸⁷Rb (r = 0.89, p < 0.01). Both the inflection in K/I ratios and the directional reversal in dissolved δ⁸⁷Rb coincide spatially with the Xinfengjiang Reservoir. This non-monotonic pattern reflects the combined influence of clay mineral assemblage variability and reservoir regulation on dissolved Rb isotope compositions in the basin. In the Pearl River Estuary, dissolved δ⁸⁷Rb behaves conservatively during estuarine mixing, preserving the riverine isotopic signal through to the ocean. These findings demonstrate that dissolved δ⁸⁷Rb holds clear potential as a tracer for continental silicate weathering, while highlighting that in-transit fractionation driven by clay mineral assemblages and reservoir regulation must be explicitly considered in its application.