Where was the Yangtze River mouth during the last glacial maximum?

ABSTRACT During the last glacial maximum (LGM), the Yangtze River drained the exposed East China Sea shelf and discharged into the Okinawa Trough. Determining the extent of the ancient Yangtze sediment‐routing system and the precise location of its palaeo‐mouth at the shelf edge is crucial for understanding the Yangtze's source‐to‐sink processes, characterising the architecture and sedimentology of the shelf‐edge delta and reconstructing shelf palaeogeography. Here, an integrated survey of geochemistry, grain size and benthic foraminiferal assemblages was conducted on bottom sediments from the shelf margin. We utilise chemical approaches to discriminate sediment sources, evaluate hydrodynamic‐sorting effects and extract geomorphic information. Sub‐bottom seismic profiles and high‐resolution bathymetric data further aid in identifying shelf‐edge geomorphology and seafloor topography. Our results indicate that shelf‐edge sediments are predominantly derived from the Yangtze River. The influence of fine‐grained Old Yellow River sediments is discernible in the northwestern study area, adjacent to the outer‐shelf mud patch southwest of Cheju Island, marked by a reduced K/Rb ratio. A sharp increase in K/Rb within lowstand shoreface sands reflects mineral sorting due to prolonged wave action. The LGM shorelines are further constrained by an enrichment of porcelaneous benthic foraminifera along the 120‐m isobath. Elevated Zr/Y ratios delineate the spatial extent of the palaeo‐Yangtze sediment‐routing system across the shelf. The inferred probable location of the LGM Yangtze mouth, though lack of precise direct dating, lies between 28.5°N–28.9°N and 126.3°E–126.5°E, tracing the 120 m isobath. This was a large, tide‐dominated estuary with braided channels. A southwestward‐flowing coastal current dominated the shelf edge during this period. Abundant calcareous bioclasts and elevated benthic foraminiferal diversity in the estuarine sediment indicate a high productivity during the LGM. As sea level rose gradually, the palaeo‐mouth retreated northwestward before shifting to another palaeo‐estuary near the 100 m isobath, immediately preceding Meltwater Pulse 1A ~14 600 years ago.