Evaluation of the Rb/Sr ratio as an indicator of chemical weathering: a case study from five sedimentary sections in Northeast China

Chemical weathering plays a pivotal role in the Earth's surface material cycle and global climate regulation. It not only facilitates the reconstruction of palaeoclimate characteristics but also provides essential insights into Earth's evolutionary history. The Rb/Sr ratio has been widely applied in studies of chemical weathering and palaeoclimate reconstruction. However, its effectiveness and applicability remain limited. This study focuses on five typical terrestrial sedimentary profiles in Northeast China. By comparing the Rb/Sr ratio with the commonly used chemical weathering indices (e.g. CIA, CIW, PIA, CPA, CIX, β, Na/K and K/Al ratios), and integrating element migration characteristics and sedimentological features, we evaluate the potential of the Rb/Sr ratio to signify the degree of chemical weathering. Our findings reveal that the applicability of the Rb/Sr ratio is stage‐dependent: effective under weak‐to‐moderate weathering, limited by depositional or pedogenic overprinting, and breaks down under extreme weathering as Rb becomes mobile. Within systems characterized by stable provenance and dominated by silicate weathering, this ratio can effectively trace relative variations in chemical weathering intensity, such as the fluvial sandy gravel profile and the granite weathering profile. However, their absolute values are constrained by parent rock composition, hindering quantitative cross‐sectional comparisons. In specific depositional environments, these ratios, such as in the palaeosol profiles, deviate from the true silicate weathering progression due to influences from preferential carbonate leaching during the initial weathering stages or dynamic provenance inputs under gleyed pedogenic conditions. Crucially, a threshold effect governs the Rb/Sr ratio. In profiles subjected to extreme‐intensity chemical weathering, such as the continental red bed profile (CIA > 85), the geochemical behaviour of Rb transitions from an immobile to a mobile state. This shift results in the Rb/Sr ratio losing its indicative significance. Consequently, when applying the Rb/Sr ratio for palaeoenvironmental reconstruction, a prior assessment of its geochemical controlling mechanisms is imperative. A multi‐proxy framework must be employed for cross‐validation to ensure the scientific rigour and reliability of interpretations.