Source-to-sink analysis for the mud and sand in the late-Quaternary Qiantang River incised-valley fill and its implications for delta-shelf-estuary dispersal systems globally

The Changjiang (i.e. the Yangtze River) debouches a huge amount of sediment to the East China Sea and exerts a significant effect on the evolution of the down-drift Qiantang River system. This study quantitatively documents the temporal change in the import of Changjiang sand and mud to the late Quaternary Qiantang River incised valley by establishing separate mixing models based on zircon U-Pb ages for the sand and trace-element compositions of the mud fraction. Results indicate that the Qiantang River provided all of the sediment to its valley before ca 8.0 cal kyr bp. However, its sediment proportion decreases upward due to the onset of export of sediment from the Changjiang after its delta had prograded sufficiently. Changjiang mud, a major component (ca 87%) of the Changjiang-derived sediment, is considered to be primarily supplied through the resuspension and landward transportation of sediment from the inner-shelf mud wedge in the East China Sea by flood-tidal currents and waves. Changjiang sand, which arrived in the Qiantang River valley later than the mud, is interpreted to be derived from the reworking of older deposits on the East China Sea shelf and along the northern margin of the Qiantang River estuary. In the later part of the valley fill, Qiantang River sediment is only ca 9% of the Changjiang contribution, although Qiantang River mud has begun to be exported recently. The Changjiang contribution to the Qiantang River system constitutes ca 50% of the total sediment load discharged by the Changjiang during the 8-6 cal kyr bp period, but bp. This study illustrates the need to exercise caution when interpreting compositional data from linked coastal-shelf sediment-dispersal systems with significant along-coast transport, and is expected to be applicable in a general way to other linked river-coastal-shelf systems.