Early indicators of pedogenesis at Harrat Khaybar volcano, Saudi Arabia

The interactive effects of paleoclimates and current conditions on pedogenesis in soils developed on volcanic parent materials (VPMs) is still poorly understood, particularly for soils in dry environments. Eight representative profiles located at Harrat Khaybar, Saudi Arabia and representing different landforms and positions including alluvial plain, alluvial fans, active slopes and lava field were investigated to better understand the effect of climate and basalt lithology on pedogenesis. The morphological and physicochemical soil properties and clay mineralogy composition were evaluated to identify the primary diagnostic horizons and therefore the pedogenic pathways that created the studied soils. All investigated profiles showed the influence of slope processes, resulting in heterogeneous soil profiles. The investigated soils can be described as polygenetic due to evidence of andisolization and the presence of salic and calcic diagnostic horizons. Andisolization implies development under a sub-humid paleoclimate during initial pedogenesis, while calcic and salic diagnostic horizons, are typical of the current arid conditions. As a result, about 63% of the investigated soils were classified as Torrands/Vitric Andosols and 37% as Salids/Solonchaks based on Soil Taxonomy and World Reference Base, respectively. Values of Alₒ, Feₒ, and Siₒ ranged from 0.20 to 0.68%, 0.24 to 3.27%, and 0.23 to 1.03%, respectively, implying the presence of poorly-crystalline fine components (i.e., allophane and ferrihydrite). Vermiculite, kaolinite, and interstratified vermiculite-chlorite were abundant, with illite, palygorskite, and Ca-zeolites as minor crystalline minerals. A combination of paleo/present-climate influenced pedogenesis leading to andisolization and the coexistence of poorly/crystalline minerals and formation of salic and calcic diagnostic horizons. This study demonstrates that andic properties can persist in dry-hot environments, highlighting the critical role of paleo/modern climate interactions as an influential factor driving the pedogenic pathways of VPMs-derived soils.