Palynology-based reconstruction of Holocene environmental history in the northern Yangtze Delta, China

Abstract

The Holocene vegetation in the Yangtze Delta is sensitive to global/regional climate change, sea-level oscillations, and human impacts, and it represents a valuable archive for deciphering environmental change. In this paper, the vegetation history of the northern Yangtze deltaic coast, and its response to climate/sea-level changes over the past 10.3 ka, have been investigated based on palynological assemblages from a well-dated sediment core (YZ07) in the northern Yangtze delta. Comparison of vegetation successions with regional climatic proxies, relative sea level change, and depositional environments suggests that this region was dominated by mixed subtropical and temperate forests in a warm /wet climate regime during the early Holocene (~10.3–8.7 ka), when it was characterized by tidally-influenced fluvial environments. From ~8.7 ka to 7.5 ka, subtropical and warm temperate pollen taxa decreased significantly while cool-tolerant conifers and grasses became more dominant, indicating a cooler and drier climate. During the mid-Holocene thermal optimum (~7.5–4.3 ka), mixed subtropical and temperate forests expanded markedly in coastal regions, caused by a return to a warm/wet climate during an episode of rising sea-level. At the start of late Holocene (~4.3 to 3.4 ka), subtropical and temperate trees were widely replaced by grasses around coastal environments, indicating a cooling climate. From ~3.4–1.7 ka, mixed subtropical and temperate forests flourished in the delta region, suggesting that climate returned to warm and wet conditions again. From ~1.7–1.3 ka, in delta environments, subtropical and temperate forests contracted under cool/dry climatic conditions. Since ~1.3 ka, subtropical and temperate pollen taxa increased around the marsh to intertidal flat environments, suggesting a warm/wet climate resurgence. The generally warm/wet Holoence climate of the Yangtze Delta region was interrupted by a series of short-term cold events at 8.2 ka, 7.3 ka, 4.2 ka, 3.5 ka and 1.4 ka. These fluctuations are broadly synchronous with regional climate variability and global sea-level oscillations. In addition, the influence of human cultivation on vegetation is recognized since the mid-Holocene (around 5.6 ka), with the appearance of Fagopyrum pollen in the northern Yangtze Delta.