Quaternary sedimentary characteristics and paleoclimate evolution on the northern coast of Hangzhou Bay

The northern coast of Hangzhou Bay is distributed with extensive and thick Quaternary strata, and conducting research on these strata can help infer the Quaternary paleoclimate evolutionary characteristics. Based on the data of BZK05 borehole core, the authors in this research established a framework for multi-layered Quaternary stratigraphic correlation and discussed the sedimentary environment and paleoclimate evolution since the Pleistocene, based on the analysis results of grain size, ¹⁴C dating, optically stimulated luminescence dating, paleomagnetism, pollen assemblage, microfossil species identification and abundance statistics, and comprehensive logging. The lithological analysis revealed that a small amount of Early Cretaceous Jinhua Formation mudstone was exposed at the base of BZK05 borehole. The Pleistocene strata of this borehole were divided into five formations from bottom to top through multi-layered stratigraphic correlation, and the depth of 35.9 m in BZK05 borehole marked the boundary between Pleistocene and Holocene strata. The grain size analysis showed that the 0~80 m section exhibited fine-grained unimodal distribution, reflecting a stable sedimentary environment primarily dominated by marine and lacustrine deposits. The 81~150 m section displayed chaotic grain size frequency curves, indicating complex and diverse sedimentary processes. The 151~225 m section showed a strong coarse-grained peak, suggesting sedimentation dominated by alluvial (proluvial) processes, indicating a riverine depositional environment. The 119~232 m section exhibited low and concentral paleomagnetic magnetic susceptibility values, indicating that the sedimentary sources of the lower part of this borehole were generally stable and similar. A total of 115 pollen species were identified, predominantly from woody and herbaceous plant pollen, and 11 pollen zones were delineated based on pollen abundance characteristics. fifteen genera and twenty three species of foraminifera were identified, and most of them were benthic foraminifera, and six stratigraphic intervals were divided. six genera and seven species of Ostracods were identified, which could be classified into three stratigraphic intervals. Based on multiple stratigraphic characteristics such as lithology, logging curves, and microfossil distribution, and combined with comprehensive analysis of sedimentary environment and paleoclimate evolution, the paleoclimate environment recorded in this borehole could be divided into nine warm intervals and six cold intervals. The research findings could provide guiding significance for deepening the Quaternary stratigraphic division, identification of sedimentary environments, and understanding paleoclimatic evolution processes in Yangtze River delta region.