Determination of chemical weathering and profile development indexes of soils formed on volcanic rocks in the Biga Peninsula, NW Türkiye

Chemical weathering and profile development indexes of soils formed on Ayvacik volcanites (pyroclastic, basalt, andesite, and ignimbrite) in the SW of Biga Peninsula were investigated in this study. Eight soil profiles (two representing each rock type) were excavated on soils formed on pyroclastic, basalt, andesite, and ignimbrite rocks. The profiles were described according to the Soil Survey Manual and classified according to the Soil Taxonomy and the World Reference Base (WRB). Within the study, morphological observations, physicochemical, oxide, and mineralogical analyses were conducted, thin-section interpretations were carried out, and chemical weathering and profile development indexes (PDI) were calculated. In addition, statistical analyses were performed on all data. Chemical index of alteration-CIA, Parker weathering index-WIP, chemical index of weathering-CIW, plagioclase index of alteration-PIA, silica-titania index-STI, bases/R2O3, product index-PWI, Vogt index-V and Ruxton ratio-R were calculated. Relationships between chemical weathering and profile development indexes were tested with multiple statistical evaluations. According to the results of multivariate analysis of covariance (MANCOVA) the effects between chemical weathering indexes and PDI were statistically significant with CIA, WIP, CIW, PWI, and V weathering indexes. In contrast, the impacts of PIA, STI, Bases/R2O3, and R weathering indexes were not significant. These relationships between chemical weathering and PDI were supported by thin section and mineralogical analysis. With the integration of PDI and chemical weathering indexes, WIP was found to be the most suitable weathering index for evaluating soils formed on pyroclastic and basalt rocks, while PWI was found to be suitable for andesite and ignimbrite rocks. These findings provide a robust framework for selecting appropriate weathering indexes tailored to specific volcanic rocks in pedogenetic studies.