Mineralogical analysis using SEM and XRD and a hybrid model for the formation of Amazonian dark earths

Abstract Amazonian dark earths (ADEs) are highly fertile soils whose formation mechanisms remain debated. While human activity is widely recognized as a key factor, growing evidence suggests that natural processes also contribute significantly to their genesis. This study investigates whether the mineral assemblage of ADEs reflects natural provenance inputs and how these interact with anthropogenic modification to produce the characteristic fertility of these soils. Using X-ray diffraction (XRD) and scanning electron microscopy with energy-dispersive spectroscopy (SEM-EDS), we analyzed samples from four archaeological sites in Pará, Brazil. Results reveal a bulk mineralogy dominated by quartz and kaolinite, with site-specific accessory minerals, including zircon, ilmenite, anatase, chromite, and detrital apatite, indicating inheritance from regional geological sources and exogenous alluvial inputs. Notably, evidence of prismatic apatite suggests that part of the phosphorus enrichment in ADEs derives from natural parent material rather than exclusively from human waste. These findings support a proposed hybrid formation model where geological inheritance and alluvial deposition supply mineral-bound nutrients, pedogenesis regulates their release, and human activity enhances stabilization. Although limited by small sample size and the absence of fully quantitative mineralogical data, this study provides a testable framework for future isotopic and provenance research.