Response of late Holocene vegetation to abrupt climatic events on the northwestern coast of the Bay of Bohai, China

Abstract

Coastal wetlands provide valuable ecosystem services and understanding their response to past climatic changes may help predict their possible future responses. In this paper, we obtained records of pollen, algae, sediment grain-size, and other environmental proxies, from a late Holocene sediment core (CFD-E) from the Caofeidian area, on the northwest coast of the Bay of Bohai, China. Our results indicate three major stages of environmental change. During Stage I (3500–2800 cal yr BP), arboreal pollen content was high (mostly >60%), especially for Pinus and Quercus, and the PCA sample scores on Axis 1 were negative indicating that regional vegetation was temperate broadleaved forest and the climate was wet. During Stage II (2800–2350 cal yr BP), the arboreal pollen content decreased substantially (mostly <40%), and the PCA Axis 1 sample scores were positive indicating a decrease in forest vegetation, grassland expansion, and a drier climate. During Stage III (2350–1400 cal yr BP), the arboreal pollen content increased again (mostly >40%), although it remained lower than during Stage I, indicating that forests expanded under a relatively humid climate while the wetland area decreased slightly. Our results also record the 2.8 ka and 2.4 ka events of monsoon weakening, which were characterized by increases in herbaceous pollen (indicating grassland expansion) and the drying of the regional climate. There are several differences in the regional expression of these two climatic events. During the 2.4 ka event, Chenopodiaceae pollen increased substantially (average of 42.0%), indicating the expansion of halophytes; whereas during the 2.8 ka event, an increase in Artemisia indicates the occurrence of a dry climate throughout the region. We suggest that the aridity during the 2.8 ka event was triggered by decreased solar activity and the resulting changes in the El Niño-Southern Oscillation, which weakened the East Asian summer monsoon. However, we suggest that the 2.4 ka event was driven by the weakening of the Atlantic Meridional Overturning Circulation.