Climatic changes in the western part of the Chinese Loess Plateau during the Last Deglacial and the Holocene: A synthesis of pollen records

The Last Deglacial (∼15,000-∼11,500 cal. BP) in the western part of the Chinese Loess Plateau experienced three sub-stages of vegetation changes: a steppe period (15,000 to ∼13,600 cal. BP), a forest-steppe period (∼13,600 to ∼12,400 cal. BP), and again a steppe period (∼12,400 cal. BP and ∼11,500 cal. BP). The Holocene climate can be divided into three stages. The Early Holocene (∼11,500-∼7000 cal. BP) was the Holocene Climatic Optimum. The Middle Holocene (∼7000-∼3000 cal. BP) was considerably drier and colder than the Early Holocene. The Late Holocene (past ∼3000 years) was a period of further and more dramatic climate deterioration. The pollen data documented five abrupt dry and cool events of the Early Holocene. The five abrupt events (dry and cool) were chronologically correspondent with the North Atlantic “Bond Events”. To explain the climate changes during the last Deglacial and the Holocene, three mechanisms can be invoked: solar insolation, North Atlantic Deep Water (NADW), and solar activity. First, the Holocene Climatic Optimum and the drying and cooling trend of the past ∼7000 years were the delayed climatic responses to the insolation. Second, slowdown or shutdown of NADW formation resulted from ice melt-water injection into the North Atlantic Ocean during the Last Deglacial warming produced those deglacial cold events (e.g., Older Dryas and Younger Dryas). Third, the Holocene “Bond Events” were solar activity-driven, i.e., atmospheric convection shifted away from western Pacific warm pool and the Asian monsoons were thus weakened during lower solar-activity intervals.