Late Holocene environmental evolution of Qilihai Lagoon, North China, based on a high-resolution multi-proxy sedimentary record

Lagoons are sensitive to the interaction between marine and terrestrial environments, and their evolution is also closely related to climate and sea level changes. Bohai Bay is a representative area of global change research in East Asia, due to its unique geographical location and shallow water environment. Here, we present the results of a study of pollen, algae, and grain-size of a late Holocene sediment core from Qilihai Lagoon in Bohai Bay, which is the largest lagoon in North China. The results show that the Qilihai Lagoon area experienced three stages of environmental change in 3,100-1,000 cal yr BP: during 3,100-1,800 cal yr BP the regional vegetation was broadleaved forest dominated by Quercus with a small amount of Pinus, indicating that the regional climate was warm and wet. The level of the Bohai Sea was high, and the sampling site was a shallow bay environment. During 1,800-1,400 cal yr BP, corresponding to the Dark Ages Cold Period (DACP), coniferous trees and grassland expanded. The sedimentary environment of the sampling area was greatly influenced by terrestrial sediment sources as the sea level fell and sandbars were exposed. We speculate that the modern Qilihai Lagoon formed at ~ 1,800 cal yr BP. The lagoon shrank during the coldest interval of the DACP, and an extensive area of coastal wetland developed around the lagoon. During 1,400-1,000 cal yr BP the regional climate warmed, however, there was little change in the terrestrial vegetation. The lagoon became closed and the sedimentary environment was unstable due to the encroachment of coastal dunes driven. The coastal wetland began to decrease due to the expansion of the lagoon. Our study provides an important supplement for the environmental response research in the East Asian coastal areas, as well as other regions of the world.