Two-stage heavy metal enrichment during carbonate rock weathering-pedogenesis: Micro-scale evidence from soil-rock...

Carbonate rock weathering could lead to selective enrichment of heavy metals (HMs), posing potential ecological risks, however, the geochemical behavior of HMs and the dominant controls on chemical weathering within the soil-rock systems remain incompletely understood. This study systematically investigated four soil profiles at different pedogenic stages and weathering rinds at the soil-rock interface in a representative karst region of China, focusing on Cd, Cr, Cu, Pb, Zn, and Ni. Bulk concentrations and sequential extraction results showed that both total contents and residual fractions of these HMs increased with pedogenesis. In-situ microanalyses of the soil-rock interface, using LA-ICP-MS and SEM-EDS, revealed marked increases in Al, Si, Fe and HMs, accompanied by Ca depletion, from bedrock to weathering rinds. Redundancy analysis (RDA) indicated that pH and clay minerals exerted key controls on HMs accumulation across pedogenic stages. Integrating macro- and micro-scale evidence, we proposed a two-stage enrichment pattern: during early carbonate dissolution, HMs were concentrated due to Ca leaching and clay mineral formation; subsequently, elevated soil pH facilitated further enrichment, while ongoing pedogenesis promoted their encapsulation or structural incorporation into clays. These findings provide critical insights into heavy-metal dynamics in karst soils and establish a theoretical basis for pollution control and risk assessment in carbonate terrains.