Influence of geomorphological dynamics and associated mineralogical structures on carbon mineralization processes within different surfaces of sand ramps

Sand ramps are recognized as critical archives for reconstructing Late Quaternary environmental changes, including sediment supply, paleoclimate, and soil carbon dynamics. We investigated the influence of geomorphic processes, weathering, and mineralogical dynamics on the mechanisms controlling the sequestration and mineralization of soil organic carbon (SOC) within falling and climbing sand ramps. Analyses including XRD, XRF, SEM, granulometry, physicochemical attributes, and weathering indices were conducted on sand ramps in central Iran. The abundance of carbonate minerals, sand-sized sediments, well-sorted clastic aggregates, and symmetrical grain distribution, combined with high roundness and minimal quartz corrosion, suggests that falling surfaces remain relatively intact with limited evidence of weathering. Despite the presence of carbonate minerals on falling sand ramps, the lower proportion of clay minerals and fine particles resulted in weaker organic matter binding compared to climbing surfaces. Conversely, the higher frequency of secondary clay minerals, poor sediment sorting, high angularity, and the formation of micro-scale features on granites indicate long-term exposure of climbing surfaces to chemical weathering. The elevated presence of illite, kaolinite, montmorillonite, as well as Fe₂O₃ and Al₂O₃ oxides, supported pedogenic processes and promoted microaggregate-associated carbon, rendering SOC less accessible to microbial mineralization. The findings indicate that climbing sand ramps represent a stabilized, pedogenically altered colluvial-aeolian apron overlying the granite, where primary sedimentary structures have been obscured by chemical weathering and clay illuviation. This contrasts with the structurally intact, gravity-dominated falling ramps. These results suggest that localized variations in geomorphic and weathering processes, alongside mineralogical composition, significantly influence the biogeochemical mechanisms controlling SOC mineralization throughout the evolution of sand ramps, offering valuable insights into carbon dynamics in arid and semi-arid regions.