Reconstruction of the Holocene sedimentary–ecological complex in the incised valley of the Yangtze Delta, China

The well-¹⁴C–dated sediment core HM retrieved from the Yangtze River mouth was used to reconstruct the Holocene sedimentary–ecological complex of its incised valley. The complete Holocene sedimentary facies that were exclusively formed in the valley and evidenced by bio-geochemical measurement, showed the Latest Pleistocene fluvial facies to the Early-Holocene marine facies, and the Middle-Late Holocene deltaic facies. The established Holocene stratigraphic framework consists of 3 (I-III) infilling phases, of which phase I demonstrated the most rapidly infilling with coastal sediments into the valley (1.0 cm/yr), driven by the Early Holocene marine transgression. The remarkably low sedimentation rate of 0.1 cm/yr of phase II to the relatively low rate of 0.7 cm/yr of phase III occurred during the Middle-Late Holocene, reflecting sediments transported from river basin to directly fill in the valley along with deltaic depocenter shifted seawards as retrogressive processes. In addition, our microfossil (foraminifera and diatom) spectra displayed 3 distinctive biological stages with related sedimentary–ecological implications. During the Early Holocene, there were remarkably high numbers of diatom communities but few foraminifera, then followed a zone of less abundant microfossils between ca. 9.0–5.0 ka. After this time, richer foraminiferal communities appeared but diatoms were less abundant. The massive sediment infilling in the Early Holocene in the deeper (>60 m) incised valley would suppress benthic foraminiferal development, most likely due to anaerobic conditions in the lower water column. However, the flourishing diatom communities persisted in the brackish marine water associated with sea-level rise. The zone with fewer microfossils corresponded generally with the Middle Holocene sandy ridge formation at the core site, leading to poorer bio–preservation. After ca. 5.0 ka, foraminiferal communities gradually became adapted to the reduced salinity of the estuarine water associated with the deltaic progradation, though this appears not to have supported the diatom communities.