Rapid retreat of the East Asian summer monsoon in the middle Holocene and a millennial weak monsoon interval at 9 ka in northern China

摘要

Knowledge of hydroclimatic dynamics in the East Asian monsoon region during the Holocene was hindered by few absolutely-dated and decadally-resolved proxy records in northern China. Here we present replicated carbonate δ18O records of six stalagmites with sub-decadal to multi-decadal resolutions from the Lianhua cave to reveal a detailed evolution of the East Asian Summer Monsoon (EASM) intensity in northern China since 11.5 thousand years before present (ka BP, before 1950 CE). Our composite record shows that solar forcing dominated hydroclimatic changes regionally, including an intensified monsoon at the Holocene Optimum from the termination of Younger Dryas to 6.5 ka BP, and a subsequent multi-millennial weakening monsoon, that agrees with cave records in central and southern China. However, the EASM has retreated southwards more rapidly than the Indian summer monsoon after ∼6.5 ka BP, resulting in aridity conditions occurring at 4.0 ka BP in northern China, which is almost 2000-year earlier than that in central and southern China. This north–south asynchroneity is likely related to the different regional responses among the coupling of the EASM, Indian summer monsoon, the solar forcing, and the differences in thermal forcing due to complex geographical configurations. In addition, a relative enrichment of 1‰ in 18O data of the Lianhua record from 9.5 to 8.1 ka BP shows that the Holocene Optimum was punctuated by a millennial-long weakening monsoon interval, which is not registered among previous cave records in central and southern China. The fresh water-induced cold climate conditions in the North Atlantic region could create stronger East Asian winter monsoon, and induce a weakened EASM and a southward shift of rain belt in northern China. Therefore, it shall not be surprised that there are strong heterogeneities among regional hydroclimatic conditions across monsoonal China, given the complex interplay between external and internal forcing mechanisms over the entire Holocene.