Stratigraphic evidence of a Megahumid climate between 10,000 and 4000 years B.P. in the western part of the Chinese Loess Plateau

摘要

Our field surveys show that a wetland/swamp layer is widely distributed in the western part of the Chinese Loess Plateau. The grayish-blue and aquatic mollusk-enriched layer at Sujianwan section (typical in the major valleys) is dated between ～ 10,000 and 4000 cal. years B.P. A wetland/swamp sub-layer and a pedogenically altered wetland/swamp sub-layer bracket a middle complex of wetland/swamp and fluvial alternating couplets at Dadiwan section (typical in the branch valleys). The middle complex formed between ～ 8000 and 6000 cal. years B.P. and contains abundant aquatic mollusks, as well as the highest percentage of the tree and shrub pollens. We propose that four mechanisms might have shared the responsibility for generating and maintaining the Megehumid climate in the western part of the Chinese Loess Plateau. They are (1) the insolation peak (7% more than today's) between 12,000 and 8000 years B.P., (2) the increased late summer insolation about 6000 cal. years B.P. in the Northern Hemisphere, (3) the shift of the long-term El Nino-like system towards the Asian side of the Pacific, and (4) positive vegetation feedbacks under a wet and warm climate.