Non-monotonic rare earth element redistribution under intense subtropical weathering

Abstract

Rare earth elements (REEs) are widely used as tracers of chemical weathering, yet their redistribution in intensely weathered subtropical systems remains poorly constrained. We investigate REE redistribution in loess-like Quaternary Red Clay (QRC) from mid-subtropical China using geochemical, statistical, and clay mineralogical approaches. Across intensely weathered conditions (CIA > 80), segmented regression and generalized additive models (GAMs) reveal a reproducible non-monotonic ΣREE trajectory with statistically supported breakpoints near CIA ≈ 86 and ≈ 89. Rather than following a simple monotonic depletion trend expected from progressive leaching alone, ΣREE shows a relative stabilization or slight recovery within the CIA 86–89 interval, followed by pronounced depletion under more extreme weathering conditions. Clay mineralogical observations indicate progressive shifts in mineral assemblages, including decreasing proportions of primary minerals and increasing abundance of secondary minerals, providing a plausible context for observed REE fractionation. The reproducibility of this non-monotonic pattern across multiple sections suggests that progressive weathering dominates, whereas variations in sediment input modulate its local expression. These findings demonstrate that REE redistribution in deeply weathered subtropical systems reflects coupled geochemical, mineralogical, and open-system sedimentary controls.