Pollen evidence of enhanced Yellow River provenance into the South Yellow Sea after the Middle Pleistocene Transition

Abstract

Quaternary sediment of the South Yellow Sea (SYS) is usually assumed to record climatic changes, tectonic activities and major river evolutions. However, as an important terrigenous indicator, a long sequence of pollen proxy in the SYS lacks for studies of river provenance and climate changes. In this study, we focused on a 150-m pollen record in the SYS that covered most of the Quaternary period over the past 1.7 Ma. The most significant changes of pollen assemblage were caused by river provenance, that high pollen percentages of broadleaved trees such as Castanea/Castanopsis reflect a dominant provenance of the Yangtze River since ca. 1.8 Ma, with a decline trend during the Middle Pleistocene Transition (MPT) mainly between 1.1 and 0.8 Ma. The sharp increase in coniferous pollen, particularly Picea, originated from the Yellow River drainage areas, suggesting a major alteration in terrigenous pollen source from the Yangtze River to the Yellow River at ∼0.8 Ma, which agrees well with the timing of the final integration of the modern Yellow River. The high pollen quantities of Artemisia and Ephedra also shows the enhanced Yellow River provenance where most landscape was covered by semi-arid open vegetation (e.g. steppe). The alternate appearances of marine foraminifera and dinoflagellate cysts, and freshwater wetland plants (Cyperaceae, Poaceae) through the interglacial intervals from MIS 19/21 demonstrate different estuary sedimentary environments and diverse salinity, associated with the sea-level fluctuations. The pollen concentration suggests an increase in terrigenous materials towards the interglacial MIS 11 and the preliminary delta formation of the Yellow River. We propose that the terrigenous input from the Yellow River accelerated after the MPT, and combined effects of tectonic movement drove the evolution of the river delta plains in eastern China. Our data also reveal some secondary periodic changes in evergreen broadleaved forest during the past 1.8 Ma and cold-tolerant coniferous forest after particularly the MPT, which are generally linked with the 400-ka long eccentricity cycle and the variability of monsoon climate.