Development of a Middle-Late Holocene subaqueous clinoform in the northern Jiangsu coastal zone, western South Yellow Sea

The northern Jiangsu coastal zone (NJCZ) is part of the western coast of the South Yellow Sea and is located between the mouths of the Yellow and Yangtze rivers. The two rivers and the Huaihe River have discharged into the NJCZ at various times during the Holocene. As a result, multi-stage deltas and clinoforms have developed in the NJCZ, including the old Yellow River delta (OYRD) (1128-1855 CE). An old clinoform has recently been identified just below the OYRD, but its age and origin are uncertain because of limited data. In this study, integrated analyses of a new borehole core SYS-1601 penetrating the clinoform were conducted to document the evolution of the NJCZ and to provide a study case for other coastal regions influenced by major river systems and complex hydrodynamic conditions. Analyses included accelerator mass spectrometry (AMS) 14C and optically stimulated luminescence (OSL) dating, measurements of grain size and Sr-Nd isotopes, and identification of benthic foraminifera and ostracoda. Results show that the evolution of the subaqueous clinoform prior to the development of the OYRD in the western South Yellow Sea was concurrent with and genetically linked to the formation of the Huaihe delta on the NJCZ. The clinoform started to form at the same time as the Middle Ho-locene maximum flooding of the NJCZ at-7.0 ka. Cross-shelf progradation of the clinoform was coeval with progradation of the Huaihe delta and terminated at 1128 CE, when the Yellow River changed course and started to discharge off the Jiangsu coast to form the OYRD. The clinoform exhibits an overall coarsening-upward succession, comprising clayey silt and silt in the lower part grading upwards into silt and sandy silt, and its sediment was sourced largely from the Huaihe and Yangtze rivers. During the development of the clinoform from-7.0 ka to 1128 CE, the Yellow River did not flow into the South Yellow Sea through the Jiangsu coast for a sufficiently long time to contribute a substantial amount of sediment to the formation of the clinoform. The initiation and evolution of the clinoform were controlled mainly by complex interactions of dynamic processes, geomorphological conditions, and sediment supply into the western South Yellow Sea during the Middle-Late Holocene.