The Middle and Late Holocene climate change in Bohai Bay revealed by diatom proxy

A total of 98 samples from two boreholes in shallow sea area and two oyster reefs in adjacent coastal plain in the northwestern coast of Bohai Bay were collected for diatom analyses and species identification. The ratio of the marine species Thalassionema nitzschioides to the intertidal-coastal species complex Cyclotella striata/stylorum serves as a novel proxy for assessing the strength of marine influence. Chronological data, corrected for the local residence time effect, facilitated the construction of a diatom proxy-based marine influence curve for the study area. This curve delineates the dynamics of marine influence and their correlations with paleo-climate fluctuations and the East Asian monsoon variability, as well as their role in chenier formation. Results include: (1) eight periods of intensified marine influence have been documented since 7 000 a BP in the study area. The peak of each period, as determined by the diatom proxy, corresponds closely to the warm climatic phases and stronger East Asian summer monsoons, suggesting that the peaks marine influence indicate typically the periods of climatic warmth and monsoon activity intensification in the region; (2) a strong correlation exists between the development of cheniers and marine influence, and chenier formation began with the increasing marine influence and terminated at the end of warm periods as marine influence weakens. The climatic changes in the coastal area, as indicated by the diatom proxy, hold significant potential for future related research endeavors.