Middle to late Holocene environmental evolution and sea level change on the west coast of Bohai Bay

Abstract

Bohai Bay is a typical area of global change research in East Asia, due to its unique geographical location and shallow water environment. In this paper, we present the results of a study of grain-size, geochemistry, pollen and algae of a middle to late Holocene sediment core from Haixing wetland, which located on the west coast of Bohai Bay. The results show that: 1) Prior to 6700 cal a B.P., the content of Cl, Sr/Ca ratio were the highest, end-member analysis of grain size suggested EM2 were dominant, which is characteristic of marine sediment and the concentrations of Pediastrum and Typha (indicating a freshwater environment) pollen were lowest, overall indicating the maximum of relative sea level; 2) During 6700–5000 cal a B.P., and the Ti and K contents, EM1 (lacustrine sedimentary component) increased substantially, EM2 decreased, and EM3 (fluvial sedimentary component) began to appear. The concentrations of Pediastrum, Dinophyceae and Typha pollen began to increase. All these indicating that the relative sea level began to fall and the study area became a lagoon environment; 3) During 5000–3500 cal a B.P. the Ti and K content increased further; EM1 was dominant, EM2 was rare, indicating that the relative sea level decreased further and the lagoon gradually became enclosed; 4) After 3500 cal a B.P., the Ti content reached its maximum and the Cl content and Sr/Ca ratio decreased to their lowest levels. EM1 was dominant and EM2 was virtually absent; dinoflagellate cysts almost disappeared and there was large increase of Typha pollen, indicating the complete exclusion of a marine influence during most of this period. However, during 2500–2000 cal a B.P. the Cl and the Sr/Ca ratio increased slightly once again, indicating a transient transgression.