Soil chemistry and vivianite formation in archaeological contexts: evidence from the Mengxihe Palaeolithic site, China

Abstract

Vivianite, a secondary iron phosphate mineral, has increasingly been recognized as a sensitive indicator of burial environments in archaeological contexts. This study presents a systematic characterization of vivianite from the Mengxihe Paleolithic site in Ziyang, Sichuan Province, China, a rare locality preserving abundant lithic, faunal, and botanical remains. Vivianite associated with acorn shells, bone fragments, and ped surfaces was investigated using scanning electron microscopy with energy-dispersive spectroscopy (SEM-EDS), Raman spectroscopy, and X-ray diffraction (XRD). The crystals occur as fine microcrystalline aggregates composed of irregular short prismatic to pseudocubic crystallites. EDS analyses show an elemental composition dominated by Fe and P, with minor Mn (0.3–1.4 wt%). Raman peak shifts and peak broadening, together with XRD patterns, indicate nanocrystalline vivianite formed under specific local geochemical conditions. Geochemical analyses reveal systematic contrasts between vivianite-bearing and non-vivianite-bearing zones. Sediments proximal to vivianite display higher water-soluble phosphate concentrations, substantially reduced sulfate levels, near-neutral pH, and elevated organic carbon contents relative to distal soils, while grain-size distributions remain comparable. These patterns are consistent with localized redox processes, organic matter degradation, and low sulfate availability influencing vivianite precipitation. From an archaeological perspective, the occurrence of vivianite at Mengxihe provides insights into burial diagenesis and helps explain the heterogeneous preservation of organic remains within the cultural layer. More broadly, the results highlight the potential of vivianite and its associated geochemical signatures as potential proxies for identifying zones of enhanced organic preservation and for reconstructing burial environments at Paleolithic sites.