The Formation of Phosphate in Archaeological Site: Material Characteristics of Settlement Deposits from the Village Before the Construction of the Southern Wall of Wolseong, Gyeongju

A consolidated grayish-white layer was identified in the excavation site of land formation for settlements located beneath the southern wall of Wolseong, Gyeongju, the royal castle of the Silla Kingdom.Mineralogical and chemical analyses revealed high concentrations of nitrate, sulfate, and chloride within this layer.Although their mineral compositions were similar to that of the surrounding soil samples, significant differences were observed in the microstructure and chemical composition.The upper samples of the consolidated layer contained herbaceous plant remnants in soil minerals, while the lower samples were predominantly composed of the rice husk, which is primarily Si-based.This consolidated layer, interposed between them, exhibited significant enrichment of Fe, P, and Ca within their microstructure, suggesting the presence of amorphous phosphate minerals.Notably, fragments of blackened wood, from the oak family, were distributed throughout the consolidated layer, with the enrichment of Fe, P, and Ca inside the pores of their microstructure.It is thought that the decayed wooden fragments and rice husks have absorbed chemicals contributing to the formation of iron phosphate.Furthermore, the identification of vivianite on the upper wall structure means that this area experienced flooding, low-oxygen conditions, supply of P and Ca, and microbial activity.These factors facilitated the formation of band-shaped consolidated layers of amorphous phosphate minerals, including Ca-Fe phosphates on the rice husk layers.