Spatial and temporal characteristics of the precipitation response to the 4.2 ka event in the Asian summer monsoon region

The 4.2 ka event was the most abrupt climatic event during the transition between the middle and late Holocene. It had a profound influence on the regional ecological environment and human cultural development and was characterized by the rapid onset of aridification in the mid- and low-latitudes regions of the Northern Hemisphere. However, the nature, structure and spatial expression of the 4.2 ka event in the Asian summer monsoon (ASM) region are controversial. We produced a detailed record of regional vegetation change during the interval of 5000–3500 cal yr BP, based on a high-resolution (~10 yr) pollen record from Yazihai Lake, on the margin of the region influenced by the ASM. The results indicate that an interval of climatic aridification, correlative with the 4.2 ka event, occurred during 4340–3880 cal yr BP, with the duration of 460 yr. However, the structure of the event is relatively complex, with arid conditions during 4340–4280 cal yr BP, wetter conditions during 4280–4150 cal yr BP, and arid conditions during 4150–3880 cal yr BP. A comprehensive comparison of the results from Yazihai Lake with existing high-resolution and well-dated paleoclimate records from the ASM region show that the precipitation response to the 4.2 ka event was spatially and temporally synchronous, with a consistent of timing, duration, and structural characteristics in the northern and southwestern parts of the ASM region; whereas the opposite response occurred in the southeastern part, where the precipitation increased. We suggest that the driving mechanism of the 4.2 ka event was related to sea-air interactions in the low-latitude region. Sea surface temperature (SST) anomalies and the variation of the El Niño-Southern Oscillation (ENSO) in the tropical Pacific region was the main cause, which led to a weaker ASM circulation and a southward shift of the monsoon rainbelt. These changes resulted in decreased monsoonal precipitation and a dry climate in the northern and southwestern parts of the ASM region, but to increased precipitation in the southeastern part.