Sacrificial pit or not? — Deposition history of Pit K4 at the Sanxingdui site

The Sanxingdui site, renowned for the extraordinary assemblage of artifacts recovered from its multiple pits, documents a distinct and enigmatic cultural tradition that flourished in southwestern China approximately 4000−3000 years ago. Nevertheless, the precise ritual or functional nature of these pits remains a topic of ongoing debate. Following the resumption of excavations at Sanxingdui in 2019, we launched a geoarchaeological investigation of the newly exposed Pit K4. By integrating magnetic-susceptibility measurements, chromaticity, elemental analysis, infrared spectroscopy, and soil micromorphology, we examined the combustion residues inside Pit K4 and reconstructed the process of its infill sequence. The results indicate that the combustion residues within Pit K4 were introduced from an external source rather than produced by in situ burning, thereby confirming the pit's function as a “ritual objects-burial pit”. A three-stage depositional sequence is therefore proposed: ritual deposits-addition of external ash/charcoal and soil-post-depositional compaction, staining, leaching, and bioturbation. This study elucidates the accumulation processes of Pit K4 and establishes a geoarchaeological baseline for interpreting the burial and ritual practices of the Sanxingdui civilization, while offering empirical support for understanding the Shang dynasty's control over southern China-particularly its southwestern periphery. • Multi-proxy analysis reveals the depositional history of Sanxingdui Pit K4. • Combustion residues in Pit K4 were not from an in situ burning event. • The pit has a feature for burying ritual objects after an external fire. • This study provides a new framework for analyzing similar burial features.