Hydrodynamic processes and extreme events recognized from the sedimentary records at ancient Port Shuomen in the Oujiang River mouth, Southeast China

Abstract

Identifying extreme events from geological records is fundamental to Paleotempestology. Archaeological excavations at the ancient Port Shuomen on the Southeast Chinese coast have provided high-resolution stratigraphic data for recognizing deposits associated with extreme events. This study examined three profiles from the Port and surficial sediments from modern colluvial and estuarine environments in the lower Oujiang River basin. Analyses including chronology, sedimentology, alkaline earth metal geochemistry, and ancient environmental DNA (eDNA) were performed on the sedimentary records to identify hydrodynamic processes and extreme events during the Song Dynasty (960–1279 CE). Comparisons of alkaline earth metal compositions between the profiles and surficial sediments suggest a reduction in salinity at the Port Shuomen, driven by increased freshwater discharge during the mid- to late-Northern Song Dynasty. High eDNA concentrations and consistent frequency distributions of fragment lengths further suggest rapid channel siltation during a single typhoon season. Following this typhoon season, the Port evolved into tide-dominated regime and experienced five periods of flooding, three of which were characterized by intensified salinity intrusion as evidenced by elevated Sr content. In particular, a mud unit (U10) exhibits the highest Sr content and is interpreted as a depositional record of extreme typhoon events that occurred in the 1160s CE. Furthermore, this study demonstrates that in estuarine environments, the preservation of eDNA is more impacted by sedimentary processes than grain size.